<!-- doc/src/sgml/protocol.sgml -->

<chapter id="protocol">
 <title>Frontend/Backend Protocol</title>

 <indexterm zone="protocol">
  <primary>protocol</primary>
  <secondary>frontend-backend</secondary>
 </indexterm>

 <para>
  <productname>PostgreSQL</productname> uses a message-based protocol
  for communication between frontends and backends (clients and servers).
  The protocol is supported over <acronym>TCP/IP</acronym> and also over
  Unix-domain sockets.  Port number 5432 has been registered with IANA as
  the customary TCP port number for servers supporting this protocol, but
  in practice any non-privileged port number can be used.
 </para>

 <para>
  This document describes version 3.2 of the protocol, introduced in
  <productname>PostgreSQL</productname> version 18. The server and the libpq
  client library are backwards compatible with protocol version 3.0,
  implemented in <productname>PostgreSQL</productname> 7.4 and later.
 </para>

  <para>
   In order to serve multiple clients efficiently, the server launches
   a new <quote>backend</quote> process for each client.
   In the current implementation, a new child
   process is created immediately after an incoming connection is detected.
   This is transparent to the protocol, however.  For purposes of the
   protocol, the terms <quote>backend</quote> and <quote>server</quote> are
   interchangeable; likewise <quote>frontend</quote> and <quote>client</quote>
   are interchangeable.
  </para>

 <sect1 id="protocol-overview">
  <title>Overview</title>

  <para>
   The protocol has separate phases for startup and normal operation.
   In the startup phase, the frontend opens a connection to the server
   and authenticates itself to the satisfaction of the server.  (This might
   involve a single message, or multiple messages depending on the
   authentication method being used.)  If all goes well, the server then sends
   status information to the frontend, and finally enters normal operation.
   Except for the initial startup-request message, this part of the
   protocol is driven by the server.
  </para>

  <para>
   During normal operation, the frontend sends queries and
   other commands to the backend, and the backend sends back query results
   and other responses.  There are a few cases (such as <command>NOTIFY</command>)
   wherein the
   backend will send unsolicited messages, but for the most part this portion
   of a session is driven by frontend requests.
  </para>

  <para>
   Termination of the session is normally by frontend choice, but can be
   forced by the backend in certain cases.  In any case, when the backend
   closes the connection, it will roll back any open (incomplete) transaction
   before exiting.
  </para>

  <para>
   Within normal operation, SQL commands can be executed through either of
   two sub-protocols.  In the <quote>simple query</quote> protocol, the frontend
   just sends a textual query string, which is parsed and immediately
   executed by the backend.  In the <quote>extended query</quote> protocol,
   processing of queries is separated into multiple steps: parsing,
   binding of parameter values, and execution.  This offers flexibility
   and performance benefits, at the cost of extra complexity.
  </para>

  <para>
   Normal operation has additional sub-protocols for special operations
   such as <command>COPY</command>.
  </para>

 <sect2 id="protocol-message-concepts">
  <title>Messaging Overview</title>

  <para>
   All communication is through a stream of messages.  The first byte of a
   message identifies the message type, and the next four bytes specify the
   length of the rest of the message (this length count includes itself, but
   not the message-type byte).  The remaining contents of the message are
   determined by the message type.  For historical reasons, the very first
   message sent by the client (the startup message) has no initial
   message-type byte.
  </para>

  <para>
   To avoid losing synchronization with the message stream, both servers and
   clients typically read an entire message into a buffer (using the byte
   count) before attempting to process its contents.  This allows easy
   recovery if an error is detected while processing the contents.  In
   extreme situations (such as not having enough memory to buffer the
   message), the receiver can use the byte count to determine how much
   input to skip before it resumes reading messages.
  </para>

  <para>
   Conversely, both servers and clients must take care never to send an
   incomplete message.  This is commonly done by marshaling the entire message
   in a buffer before beginning to send it.  If a communications failure
   occurs partway through sending or receiving a message, the only sensible
   response is to abandon the connection, since there is little hope of
   recovering message-boundary synchronization.
  </para>
 </sect2>

  <sect2 id="protocol-query-concepts">
   <title>Extended Query Overview</title>

   <para>
    In the extended-query protocol, execution of SQL commands is divided
    into multiple steps.  The state retained between steps is represented
    by two types of objects: <firstterm>prepared statements</firstterm> and
    <firstterm>portals</firstterm>.  A prepared statement represents the result of
    parsing and semantic analysis of a textual query string.
    A prepared statement is not in itself ready to execute, because it might
    lack specific values for <firstterm>parameters</firstterm>.  A portal represents
    a ready-to-execute or already-partially-executed statement, with any
    missing parameter values filled in.  (For <command>SELECT</command> statements,
    a portal is equivalent to an open cursor, but we choose to use a different
    term since cursors don't handle non-<command>SELECT</command> statements.)
   </para>

   <para>
    The overall execution cycle consists of a <firstterm>parse</firstterm> step,
    which creates a prepared statement from a textual query string; a
    <firstterm>bind</firstterm> step, which creates a portal given a prepared
    statement and values for any needed parameters; and an
    <firstterm>execute</firstterm> step that runs a portal's query.  In the case of
    a query that returns rows (<command>SELECT</command>, <command>SHOW</command>, etc.),
    the execute step can be told to fetch only
    a limited number of rows, so that multiple execute steps might be needed
    to complete the operation.
   </para>

   <para>
    The backend can keep track of multiple prepared statements and portals
    (but note that these exist only within a session, and are never shared
    across sessions).  Existing prepared statements and portals are
    referenced by names assigned when they were created.  In addition,
    an <quote>unnamed</quote> prepared statement and portal exist.  Although these
    behave largely the same as named objects, operations on them are optimized
    for the case of executing a query only once and then discarding it,
    whereas operations on named objects are optimized on the expectation
    of multiple uses.
   </para>
  </sect2>

  <sect2 id="protocol-format-codes">
   <title>Formats and Format Codes</title>

   <para>
    Data of a particular data type might be transmitted in any of several
    different <firstterm>formats</firstterm>.  As of <productname>PostgreSQL</productname> 7.4
    the only supported formats are <quote>text</quote> and <quote>binary</quote>,
    but the protocol makes provision for future extensions.  The desired
    format for any value is specified by a <firstterm>format code</firstterm>.
    Clients can specify a format code for each transmitted parameter value
    and for each column of a query result.  Text has format code zero,
    binary has format code one, and all other format codes are reserved
    for future definition.
   </para>

   <para>
    The text representation of values is whatever strings are produced
    and accepted by the input/output conversion functions for the
    particular data type.  In the transmitted representation, there is
    no trailing null character; the frontend must add one to received
    values if it wants to process them as C strings.
    (The text format does not allow embedded nulls, by the way.)
   </para>

   <para>
    Binary representations for integers use network byte order (most
    significant byte first).  For other data types consult the documentation
    or source code to learn about the binary representation.  Keep in mind
    that binary representations for complex data types might change across
    server versions; the text format is usually the more portable choice.
   </para>
  </sect2>

  <sect2 id="protocol-versions">
   <title>Protocol Versions</title>

   <para>
    The current, latest version of the protocol is version 3.2. However, for
    backwards compatibility with old server versions and middleware that don't
    support the version negotiation yet, libpq still uses protocol version 3.0
    by default.
   </para>

   <para>
    A single server can support multiple protocol versions.  The initial
    startup-request message tells the server which protocol version the client
    is attempting to use.  If the major version requested by the client is not
    supported by the server, the connection will be rejected (for example,
    this would occur if the client requested protocol version 4.0, which does
    not exist as of this writing).  If the minor version requested by the
    client is not supported by the server (e.g., the client requests version
    3.2, but the server supports only 3.0), the server may either reject the
    connection or may respond with a NegotiateProtocolVersion message
    containing the highest minor protocol version which it supports.  The
    client may then choose either to continue with the connection using the
    specified protocol version or to abort the connection.
   </para>

   <para>
    The protocol negotiation was introduced in
    <productname>PostgreSQL</productname> version 9.3.21. Earlier versions
    would reject the connection if the client requested a minor version that
    was not supported by the server.
   </para>

   <para>
    <xref linkend="protocol-versions-table"/> shows the currently supported
    protocol versions.
   </para>

   <table id="protocol-versions-table">
    <title>Protocol Versions</title>

    <tgroup cols="3">
     <thead>
      <row>
       <entry>Version</entry>
       <entry>Supported by</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
      <entry>3.2</entry>
      <entry>PostgreSQL 18 and later</entry>
      <entry>Current latest version. The secret key used in query
        cancellation was enlarged from 4 bytes to a variable length field. The
        BackendKeyData message was changed to accommodate that, and the CancelRequest
        message was redefined to have a variable length payload.
      </entry>
      </row>
      <row>
      <entry>3.1</entry>
      <entry>-</entry>
      <entry>Reserved. Version 3.1 has not been used by any PostgreSQL
        version, but it was skipped because old versions of the popular
        pgbouncer application had a bug in the protocol negotiation which made
        it incorrectly claim that it supported version 3.1.
      </entry>
      </row>
      <row>
      <entry>3.0</entry>
      <entry>PostgreSQL 7.4 and later</entry>
      </row>
      <row>
      <entry>2.0</entry>
      <entry>up to PostgreSQL 13</entry>
      <entry>See previous releases of
      the <productname>PostgreSQL</productname> documentation for
      details</entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </sect2>
 </sect1>

 <sect1 id="protocol-flow">
  <title>Message Flow</title>

  <para>
   This section describes the message flow and the semantics of each
   message type.  (Details of the exact representation of each message
   appear in <xref linkend="protocol-message-formats"/>.)  There are
   several different sub-protocols depending on the state of the
   connection: start-up, query, function call,
   <command>COPY</command>, and termination.  There are also special
   provisions for asynchronous operations (including notification
   responses and command cancellation), which can occur at any time
   after the start-up phase.
  </para>

  <sect2 id="protocol-flow-start-up">
   <title>Start-up</title>

   <para>
    To begin a session, a frontend opens a connection to the server and sends
    a startup message.  This message includes the names of the user and of the
    database the user wants to connect to; it also identifies the particular
    protocol version to be used.  (Optionally, the startup message can include
    additional settings for run-time parameters.)
    The server then uses this information and
    the contents of its configuration files (such as
    <filename>pg_hba.conf</filename>) to determine
    whether the connection is provisionally acceptable, and what additional
    authentication is required (if any).
   </para>

   <para>
    The server then sends an appropriate authentication request message,
    to which the frontend must reply with an appropriate authentication
    response message (such as a password).
    For all authentication methods except GSSAPI, SSPI and SASL, there is at
    most one request and one response. In some methods, no response
    at all is needed from the frontend, and so no authentication request
    occurs. For GSSAPI, SSPI and SASL, multiple exchanges of packets may be
    needed to complete the authentication.
   </para>

   <para>
    The authentication cycle ends with the server either rejecting the
    connection attempt (ErrorResponse), or sending AuthenticationOk.
   </para>

   <para>
    The possible messages from the server in this phase are:

    <variablelist>
     <varlistentry>
      <term>ErrorResponse</term>
      <listitem>
       <para>
        The connection attempt has been rejected.
        The server then immediately closes the connection.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationOk</term>
      <listitem>
       <para>
        The authentication exchange is successfully completed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationKerberosV5</term>
      <listitem>
       <para>
        The frontend must now take part in a Kerberos V5
        authentication dialog (not described here, part of the
        Kerberos specification) with the server.  If this is
        successful, the server responds with an AuthenticationOk,
        otherwise it responds with an ErrorResponse. This is no
        longer supported.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationCleartextPassword</term>
      <listitem>
       <para>
        The frontend must now send a PasswordMessage containing the
        password in clear-text form.  If
        this is the correct password, the server responds with an
        AuthenticationOk, otherwise it responds with an ErrorResponse.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationMD5Password</term>
      <listitem>
       <para>
        The frontend must now send a PasswordMessage containing the
        password (with user name) encrypted via MD5, then encrypted
        again using the 4-byte random salt specified in the
        AuthenticationMD5Password message.  If this is the correct
        password, the server responds with an AuthenticationOk,
        otherwise it responds with an ErrorResponse.  The actual
        PasswordMessage can be computed in SQL as <literal>concat('md5',
        md5(concat(md5(concat(password, username)), random-salt)))</literal>.
        (Keep in mind the <function>md5()</function> function returns its
        result as a hex string.)
       </para>
        <warning>
         <para>
          Support for MD5-encrypted passwords is deprecated and will be removed
          in a future release of <productname>PostgreSQL</productname>.  Refer
          to <xref linkend="auth-password"/> for details about migrating to
          another password type.
         </para>
        </warning>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationGSS</term>
      <listitem>
       <para>
        The frontend must now initiate a GSSAPI negotiation. The frontend
        will send a GSSResponse message with the first part of the GSSAPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSSPI</term>
      <listitem>
       <para>
        The frontend must now initiate an SSPI negotiation. The frontend
        will send a GSSResponse with the first part of the SSPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </para>
      </listitem>

     </varlistentry>
     <varlistentry>
      <term>AuthenticationGSSContinue</term>
      <listitem>
       <para>
        This message contains the response data from the previous step
        of GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI
        or a previous AuthenticationGSSContinue). If the GSSAPI
        or SSPI data in this message
        indicates more data is needed to complete the authentication,
        the frontend must send that data as another GSSResponse message. If
        GSSAPI or SSPI authentication is completed by this message, the server
        will next send AuthenticationOk to indicate successful authentication
        or ErrorResponse to indicate failure.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSASL</term>
      <listitem>
       <para>
        The frontend must now initiate a SASL negotiation, using one of the
        SASL mechanisms listed in the message. The frontend will send a
        SASLInitialResponse with the name of the selected mechanism, and the
        first part of the SASL data stream in response to this. If further
        messages are needed, the server will respond with
        AuthenticationSASLContinue. See <xref linkend="sasl-authentication"/>
        for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSASLContinue</term>
      <listitem>
       <para>
        This message contains challenge data from the previous step of SASL
        negotiation (AuthenticationSASL, or a previous
        AuthenticationSASLContinue). The frontend must respond with a
        SASLResponse message.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSASLFinal</term>
      <listitem>
       <para>
        SASL authentication has completed with additional mechanism-specific
        data for the client. The server will next send AuthenticationOk to
        indicate successful authentication, or an ErrorResponse to indicate
        failure. This message is sent only if the SASL mechanism specifies
        additional data to be sent from server to client at completion.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NegotiateProtocolVersion</term>
      <listitem>
       <para>
        The server does not support the minor protocol version requested
        by the client, but does support an earlier version of the protocol;
        this message indicates the highest supported minor version.  This
        message will also be sent if the client requested unsupported protocol
        options (i.e., beginning with <literal>_pq_.</literal>) in the
        startup packet.
       </para>
       <para>
        After this message, the authentication will continue using the version
        indicated by the server.  If the client does not support the older
        version, it should immediately close the connection.  If the server
        does not send this message, it supports the client's requested
        protocol version and all the protocol options.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    If the frontend does not support the authentication method
    requested by the server, then it should immediately close the
    connection.
   </para>

   <para>
    After having received AuthenticationOk, the frontend must wait
    for further messages from the server.  In this phase a backend process
    is being started, and the frontend is just an interested bystander.
    It is still possible for the startup attempt
    to fail (ErrorResponse) or the server to decline support for the requested
    minor protocol version (NegotiateProtocolVersion), but in the normal case
    the backend will send some ParameterStatus messages, BackendKeyData, and
    finally ReadyForQuery.
   </para>

   <para>
    During this phase the backend will attempt to apply any additional
    run-time parameter settings that were given in the startup message.
    If successful, these values become session defaults.  An error causes
    ErrorResponse and exit.
   </para>

   <para>
    The possible messages from the backend in this phase are:

    <variablelist>
     <varlistentry>
      <term>BackendKeyData</term>
      <listitem>
       <para>
        This message provides secret-key data that the frontend must
        save if it wants to be able to issue cancel requests later.
        The frontend should not respond to this message, but should
        continue listening for a ReadyForQuery message.
       </para>
       <para>
        The <productname>PostgreSQL</productname> server will always send this
        message, but some third party backend implementations of the protocol
        that don't support query cancellation are known not to.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ParameterStatus</term>
      <listitem>
       <para>
        This message informs the frontend about the current (initial)
         setting of backend parameters, such as <xref
         linkend="guc-client-encoding"/> or <xref linkend="guc-datestyle"/>.
         The frontend can ignore this message, or record the settings
         for its future use; see <xref linkend="protocol-async"/> for
         more details.  The frontend should not respond to this
         message, but should continue listening for a ReadyForQuery
         message.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ReadyForQuery</term>
      <listitem>
       <para>
        Start-up is completed.  The frontend can now issue commands.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ErrorResponse</term>
      <listitem>
       <para>
        Start-up failed.  The connection is closed after sending this
        message.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NoticeResponse</term>
      <listitem>
       <para>
        A warning message has been issued.  The frontend should
        display the message but continue listening for ReadyForQuery
        or ErrorResponse.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The ReadyForQuery message is the same one that the backend will
    issue after each command cycle.  Depending on the coding needs of
    the frontend, it is reasonable to consider ReadyForQuery as
    starting a command cycle, or to consider ReadyForQuery as ending the
    start-up phase and each subsequent command cycle.
   </para>
  </sect2>

  <sect2 id="protocol-flow-simple-query">
   <title>Simple Query</title>

   <para>
    A simple query cycle is initiated by the frontend sending a Query message
    to the backend.  The message includes an SQL command (or commands)
    expressed as a text string.
    The backend then sends one or more response
    messages depending on the contents of the query command string,
    and finally a ReadyForQuery response message.  ReadyForQuery
    informs the frontend that it can safely send a new command.
    (It is not actually necessary for the frontend to wait for
    ReadyForQuery before issuing another command, but the frontend must
    then take responsibility for figuring out what happens if the earlier
    command fails and already-issued later commands succeed.)
   </para>

   <para>
    The possible response messages from the backend are:

    <variablelist>
     <varlistentry>
      <term>CommandComplete</term>
      <listitem>
       <para>
        An SQL command completed normally.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CopyInResponse</term>
      <listitem>
       <para>
        The backend is ready to copy data from the frontend to a
        table; see <xref linkend="protocol-copy"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CopyOutResponse</term>
      <listitem>
       <para>
        The backend is ready to copy data from a table to the
        frontend; see <xref linkend="protocol-copy"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>RowDescription</term>
      <listitem>
       <para>
        Indicates that rows are about to be returned in response to
        a <command>SELECT</command>, <command>FETCH</command>, etc. query.
        The contents of this message describe the column layout of the rows.
        This will be followed by a DataRow message for each row being returned
        to the frontend.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>DataRow</term>
      <listitem>
       <para>
        One of the set of rows returned by
        a <command>SELECT</command>, <command>FETCH</command>, etc. query.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>EmptyQueryResponse</term>
      <listitem>
       <para>
        An empty query string was recognized.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ErrorResponse</term>
      <listitem>
       <para>
        An error has occurred.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ReadyForQuery</term>
      <listitem>
       <para>
        Processing of the query string is complete.  A separate
        message is sent to indicate this because the query string might
        contain multiple SQL commands.  (CommandComplete marks the
        end of processing one SQL command, not the whole string.)
        ReadyForQuery will always be sent, whether processing
        terminates successfully or with an error.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NoticeResponse</term>
      <listitem>
       <para>
        A warning message has been issued in relation to the query.
        Notices are in addition to other responses, i.e., the backend
        will continue processing the command.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    The response to a <command>SELECT</command> query (or other queries that
    return row sets, such as <command>EXPLAIN</command> or <command>SHOW</command>)
    normally consists of RowDescription, zero or more
    DataRow messages, and then CommandComplete.
    <command>COPY</command> to or from the frontend invokes special protocol
    as described in <xref linkend="protocol-copy"/>.
    All other query types normally produce only
    a CommandComplete message.
   </para>

   <para>
    Since a query string could contain several queries (separated by
    semicolons), there might be several such response sequences before the
    backend finishes processing the query string.  ReadyForQuery is issued
    when the entire string has been processed and the backend is ready to
    accept a new query string.
   </para>

   <para>
    If a completely empty (no contents other than whitespace) query string
    is received, the response is EmptyQueryResponse followed by ReadyForQuery.
   </para>

   <para>
    In the event of an error, ErrorResponse is issued followed by
    ReadyForQuery.  All further processing of the query string is aborted by
    ErrorResponse (even if more queries remained in it).  Note that this
    might occur partway through the sequence of messages generated by an
    individual query.
   </para>

   <para>
    In simple Query mode, the format of retrieved values is always text,
    except when the given command is a <command>FETCH</command> from a cursor
    declared with the <literal>BINARY</literal> option.  In that case, the
    retrieved values are in binary format.  The format codes given in
    the RowDescription message tell which format is being used.
   </para>

   <para>
    A frontend must be prepared to accept ErrorResponse and
    NoticeResponse messages whenever it is expecting any other type of
    message.  See also <xref linkend="protocol-async"/> concerning messages
    that the backend might generate due to outside events.
   </para>

   <para>
    Recommended practice is to code frontends in a state-machine style
    that will accept any message type at any time that it could make sense,
    rather than wiring in assumptions about the exact sequence of messages.
   </para>

   <sect3 id="protocol-flow-multi-statement">
    <title>Multiple Statements in a Simple Query</title>

    <para>
     When a simple Query message contains more than one SQL statement
     (separated by semicolons), those statements are executed as a single
     transaction, unless explicit transaction control commands are included
     to force a different behavior.  For example, if the message contains
<programlisting>
INSERT INTO mytable VALUES(1);
SELECT 1/0;
INSERT INTO mytable VALUES(2);
</programlisting>
     then the divide-by-zero failure in the <command>SELECT</command> will force
     rollback of the first <command>INSERT</command>.  Furthermore, because
     execution of the message is abandoned at the first error, the second
     <command>INSERT</command> is never attempted at all.
    </para>

    <para>
     If instead the message contains
<programlisting>
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELECT 1/0;
</programlisting>
     then the first <command>INSERT</command> is committed by the
     explicit <command>COMMIT</command> command.  The second <command>INSERT</command>
     and the <command>SELECT</command> are still treated as a single transaction,
     so that the divide-by-zero failure will roll back the
     second <command>INSERT</command>, but not the first one.
    </para>

    <para>
     This behavior is implemented by running the statements in a
     multi-statement Query message in an <firstterm>implicit transaction
     block</firstterm> unless there is some explicit transaction block for them to
     run in.  The main difference between an implicit transaction block and
     a regular one is that an implicit block is closed automatically at the
     end of the Query message, either by an implicit commit if there was no
     error, or an implicit rollback if there was an error.  This is similar
     to the implicit commit or rollback that happens for a statement
     executed by itself (when not in a transaction block).
    </para>

    <para>
     If the session is already in a transaction block, as a result of
     a <command>BEGIN</command> in some previous message, then the Query message
     simply continues that transaction block, whether the message contains
     one statement or several.  However, if the Query message contains
     a <command>COMMIT</command> or <command>ROLLBACK</command> closing the existing
     transaction block, then any following statements are executed in an
     implicit transaction block.
     Conversely, if a <command>BEGIN</command> appears in a multi-statement Query
     message, then it starts a regular transaction block that will only be
     terminated by an explicit <command>COMMIT</command> or <command>ROLLBACK</command>,
     whether that appears in this Query message or a later one.
     If the <command>BEGIN</command> follows some statements that were executed as
     an implicit transaction block, those statements are not immediately
     committed; in effect, they are retroactively included into the new
     regular transaction block.
    </para>

    <para>
     A <command>COMMIT</command> or <command>ROLLBACK</command> appearing in an implicit
     transaction block is executed as normal, closing the implicit block;
     however, a warning will be issued since a <command>COMMIT</command>
     or <command>ROLLBACK</command> without a previous <command>BEGIN</command> might
     represent a mistake.  If more statements follow, a new implicit
     transaction block will be started for them.
    </para>

    <para>
     Savepoints are not allowed in an implicit transaction block, since
     they would conflict with the behavior of automatically closing the
     block upon any error.
    </para>

    <para>
     Remember that, regardless of any transaction control commands that may
     be present, execution of the Query message stops at the first error.
     Thus for example given
<programlisting>
BEGIN;
SELECT 1/0;
ROLLBACK;
</programlisting>
     in a single Query message, the session will be left inside a failed
     regular transaction block, since the <command>ROLLBACK</command> is not
     reached after the divide-by-zero error.  Another <command>ROLLBACK</command>
     will be needed to restore the session to a usable state.
    </para>

    <para>
     Another behavior of note is that initial lexical and syntactic
     analysis is done on the entire query string before any of it is
     executed.  Thus simple errors (such as a misspelled keyword) in later
     statements can prevent execution of any of the statements.  This
     is normally invisible to users since the statements would all roll
     back anyway when done as an implicit transaction block.  However,
     it can be visible when attempting to do multiple transactions within a
     multi-statement Query.  For instance, if a typo turned our previous
     example into
<programlisting>
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELCT 1/0;<!-- this typo is intentional -->
</programlisting>
     then none of the statements would get run, resulting in the visible
     difference that the first <command>INSERT</command> is not committed.
     Errors detected at semantic analysis or later, such as a misspelled
     table or column name, do not have this effect.
    </para>

    <para>
     Lastly, note that all the statements within the Query message will
     observe the same value of <function>statement_timestamp()</function>,
     since that timestamp is updated only upon receipt of the Query
     message.  This will result in them all observing the same
     value of <function>transaction_timestamp()</function> as well,
     except in cases where the query string ends a previously-started
     transaction and begins a new one.
    </para>
   </sect3>
  </sect2>

  <sect2 id="protocol-flow-ext-query">
   <title>Extended Query</title>

   <para>
    The extended query protocol breaks down the above-described simple
    query protocol into multiple steps.  The results of preparatory
    steps can be re-used multiple times for improved efficiency.
    Furthermore, additional features are available, such as the possibility
    of supplying data values as separate parameters instead of having to
    insert them directly into a query string.
   </para>

   <para>
    In the extended protocol, the frontend first sends a Parse message,
    which contains a textual query string, optionally some information
    about data types of parameter placeholders, and the
    name of a destination prepared-statement object (an empty string
    selects the unnamed prepared statement).  The response is
    either ParseComplete or ErrorResponse.  Parameter data types can be
    specified by OID; if not given, the parser attempts to infer the
    data types in the same way as it would do for untyped literal string
    constants.
   </para>

   <note>
    <para>
     A parameter data type can be left unspecified by setting it to zero,
     or by making the array of parameter type OIDs shorter than the
     number of parameter symbols (<literal>$</literal><replaceable>n</replaceable>)
     used in the query string.  Another special case is that a parameter's
     type can be specified as <type>void</type> (that is, the OID of the
     <type>void</type> pseudo-type).  This is meant to allow parameter symbols
     to be used for function parameters that are actually OUT parameters.
     Ordinarily there is no context in which a <type>void</type> parameter
     could be used, but if such a parameter symbol appears in a function's
     parameter list, it is effectively ignored.  For example, a function
     call such as <literal>foo($1,$2,$3,$4)</literal> could match a function with
     two IN and two OUT arguments, if <literal>$3</literal> and <literal>$4</literal>
     are specified as having type <type>void</type>.
    </para>
   </note>

   <note>
    <para>
     The query string contained in a Parse message cannot include more
     than one SQL statement; else a syntax error is reported.  This
     restriction does not exist in the simple-query protocol, but it
     does exist in the extended protocol, because allowing prepared
     statements or portals to contain multiple commands would complicate
     the protocol unduly.
    </para>
   </note>

   <para>
    If successfully created, a named prepared-statement object lasts till
    the end of the current session, unless explicitly destroyed.  An unnamed
    prepared statement lasts only until the next Parse statement specifying
    the unnamed statement as destination is issued.  (Note that a simple
    Query message also destroys the unnamed statement.)  Named prepared
    statements must be explicitly closed before they can be redefined by
    another Parse message, but this is not required for the unnamed statement.
    Named prepared statements can also be created and accessed at the SQL
    command level, using <command>PREPARE</command> and <command>EXECUTE</command>.
   </para>

   <para>
    Once a prepared statement exists, it can be readied for execution using a
    Bind message.  The Bind message gives the name of the source prepared
    statement (empty string denotes the unnamed prepared statement), the name
    of the destination portal (empty string denotes the unnamed portal), and
    the values to use for any parameter placeholders present in the prepared
    statement.  The
    supplied parameter set must match those needed by the prepared statement.
    (If you declared any <type>void</type> parameters in the Parse message,
    pass NULL values for them in the Bind message.)
    Bind also specifies the format to use for any data returned
    by the query; the format can be specified overall, or per-column.
    The response is either BindComplete or ErrorResponse.
   </para>

   <note>
    <para>
     The choice between text and binary output is determined by the format
     codes given in Bind, regardless of the SQL command involved.  The
     <literal>BINARY</literal> attribute in cursor declarations is irrelevant when
     using extended query protocol.
    </para>
   </note>

   <para>
    Query planning typically occurs when the Bind message is processed.
    If the prepared statement has no parameters, or is executed repeatedly,
    the server might save the created plan and re-use it during subsequent
    Bind messages for the same prepared statement.  However, it will do so
    only if it finds that a generic plan can be created that is not much
    less efficient than a plan that depends on the specific parameter values
    supplied.  This happens transparently so far as the protocol is concerned.
   </para>

   <para>
    If successfully created, a named portal object lasts till the end of the
    current transaction, unless explicitly destroyed.  An unnamed portal is
    destroyed at the end of the transaction, or as soon as the next Bind
    statement specifying the unnamed portal as destination is issued.  (Note
    that a simple Query message also destroys the unnamed portal.)  Named
    portals must be explicitly closed before they can be redefined by another
    Bind message, but this is not required for the unnamed portal.
    Named portals can also be created and accessed at the SQL
    command level, using <command>DECLARE CURSOR</command> and <command>FETCH</command>.
   </para>

   <para>
    Once a portal exists, it can be executed using an Execute message.
    The Execute message specifies the portal name (empty string denotes the
    unnamed portal) and
    a maximum result-row count (zero meaning <quote>fetch all rows</quote>).
    The result-row count is only meaningful for portals
    containing commands that return row sets; in other cases the command is
    always executed to completion, and the row count is ignored.
    The possible
    responses to Execute are the same as those described above for queries
    issued via simple query protocol, except that Execute doesn't cause
    ReadyForQuery or RowDescription to be issued.
   </para>

   <para>
    If Execute terminates before completing the execution of a portal
    (due to reaching a nonzero result-row count), it will send a
    PortalSuspended message; the appearance of this message tells the frontend
    that another Execute should be issued against the same portal to
    complete the operation.  The CommandComplete message indicating
    completion of the source SQL command is not sent until
    the portal's execution is completed.  Therefore, an Execute phase is
    always terminated by the appearance of exactly one of these messages:
    CommandComplete, EmptyQueryResponse (if the portal was created from
    an empty query string), ErrorResponse, or PortalSuspended.
   </para>

   <para>
    At completion of each series of extended-query messages, the frontend
    should issue a Sync message.  This parameterless message causes the
    backend to close the current transaction if it's not inside a
    <command>BEGIN</command>/<command>COMMIT</command> transaction block (<quote>close</quote>
    meaning to commit if no error, or roll back if error).  Then a
    ReadyForQuery response is issued.  The purpose of Sync is to provide
    a resynchronization point for error recovery.  When an error is detected
    while processing any extended-query message, the backend issues
    ErrorResponse, then reads and discards messages until a Sync is reached,
    then issues ReadyForQuery and returns to normal message processing.
    (But note that no skipping occurs if an error is detected
    <emphasis>while</emphasis> processing Sync &mdash; this ensures that there is one
    and only one ReadyForQuery sent for each Sync.)
   </para>

   <note>
    <para>
     Sync does not cause a transaction block opened with <command>BEGIN</command>
     to be closed.  It is possible to detect this situation since the
     ReadyForQuery message includes transaction status information.
    </para>
   </note>

   <para>
    In addition to these fundamental, required operations, there are several
    optional operations that can be used with extended-query protocol.
   </para>

   <para>
    The Describe message (portal variant) specifies the name of an existing
    portal (or an empty string for the unnamed portal).  The response is a
    RowDescription message describing the rows that will be returned by
    executing the portal; or a NoData message if the portal does not contain a
    query that will return rows; or ErrorResponse if there is no such portal.
   </para>

   <para>
    The Describe message (statement variant) specifies the name of an existing
    prepared statement (or an empty string for the unnamed prepared
    statement).  The response is a ParameterDescription message describing the
    parameters needed by the statement, followed by a RowDescription message
    describing the rows that will be returned when the statement is eventually
    executed (or a NoData message if the statement will not return rows).
    ErrorResponse is issued if there is no such prepared statement.  Note that
    since Bind has not yet been issued, the formats to be used for returned
    columns are not yet known to the backend; the format code fields in the
    RowDescription message will be zeroes in this case.
   </para>

   <tip>
    <para>
     In most scenarios the frontend should issue one or the other variant
     of Describe before issuing Execute, to ensure that it knows how to
     interpret the results it will get back.
    </para>
   </tip>

   <para>
    The Close message closes an existing prepared statement or portal
    and releases resources.  It is not an error to issue Close against
    a nonexistent statement or portal name.  The response is normally
    CloseComplete, but could be ErrorResponse if some difficulty is
    encountered while releasing resources.  Note that closing a prepared
    statement implicitly closes any open portals that were constructed
    from that statement.
   </para>

   <para>
    The Flush message does not cause any specific output to be generated,
    but forces the backend to deliver any data pending in its output
    buffers.  A Flush must be sent after any extended-query command except
    Sync, if the frontend wishes to examine the results of that command before
    issuing more commands.  Without Flush, messages returned by the backend
    will be combined into the minimum possible number of packets to minimize
    network overhead.
   </para>

   <note>
    <para>
     The simple Query message is approximately equivalent to the series Parse,
     Bind, portal Describe, Execute, Close, Sync, using the unnamed prepared
     statement and portal objects and no parameters.  One difference is that
     it will accept multiple SQL statements in the query string, automatically
     performing the bind/describe/execute sequence for each one in succession.
     Another difference is that it will not return ParseComplete, BindComplete,
     CloseComplete, or NoData messages.
    </para>
   </note>
  </sect2>

  <sect2 id="protocol-flow-pipelining">
   <title>Pipelining</title>

   <indexterm zone="protocol-flow-pipelining">
    <primary>pipelining</primary>
    <secondary>protocol specification</secondary>
   </indexterm>

   <para>
    Use of the extended query protocol
    allows <firstterm>pipelining</firstterm>, which means sending a series
    of queries without waiting for earlier ones to complete.  This reduces
    the number of network round trips needed to complete a given series of
    operations.  However, the user must carefully consider the required
    behavior if one of the steps fails, since later queries will already
    be in flight to the server.
   </para>

   <para>
    One way to deal with that is to make the whole query series be a
    single transaction, that is wrap it in <command>BEGIN</command> ...
    <command>COMMIT</command>.  However, this does not help if one wishes
    for some of the commands to commit independently of others.
   </para>

   <para>
    The extended query protocol provides another way to manage this
    concern, which is to omit sending Sync messages between steps that
    are dependent.  Since, after an error, the backend will skip command
    messages until it finds Sync, this allows later commands in a pipeline
    to be skipped automatically when an earlier one fails, without the
    client having to manage that explicitly with <command>BEGIN</command>
    and <command>COMMIT</command>.  Independently-committable segments
    of the pipeline can be separated by Sync messages.
   </para>

   <para>
    If the client has not issued an explicit <command>BEGIN</command>,
    then an implicit transaction block is started and each Sync ordinarily
    causes an implicit <command>COMMIT</command> if the preceding step(s)
    succeeded, or an implicit <command>ROLLBACK</command> if they failed.
    This implicit transaction block will only be detected by the server
    when the first command ends without a sync.  There are a few DDL
    commands (such as <command>CREATE DATABASE</command>) that cannot be
    executed inside a transaction block. If one of these is executed in a
    pipeline, it will fail unless it is the first command after a Sync.
    Furthermore, upon success it will force an immediate commit to preserve
    database consistency. Thus a Sync immediately following one of these
    commands has no effect except to respond with ReadyForQuery.
   </para>

   <para>
    When using this method, completion of the pipeline must be determined
    by counting ReadyForQuery messages and waiting for that to reach the
    number of Syncs sent.  Counting command completion responses is
    unreliable, since some of the commands may be skipped and thus not
    produce a completion message.
   </para>
  </sect2>

  <sect2 id="protocol-flow-function-call">
   <title>Function Call</title>

   <para>
    The Function Call sub-protocol allows the client to request a direct
    call of any function that exists in the database's
    <structname>pg_proc</structname> system catalog.  The client must have
    execute permission for the function.
   </para>

   <note>
    <para>
     The Function Call sub-protocol is a legacy feature that is probably best
     avoided in new code.  Similar results can be accomplished by setting up
     a prepared statement that does <literal>SELECT function($1, ...)</literal>.
     The Function Call cycle can then be replaced with Bind/Execute.
    </para>
   </note>

   <para>
    A Function Call cycle is initiated by the frontend sending a
    FunctionCall message to the backend.  The backend then sends one
    or more response messages depending on the results of the function
    call, and finally a ReadyForQuery response message.  ReadyForQuery
    informs the frontend that it can safely send a new query or
    function call.
   </para>

   <para>
    The possible response messages from the backend are:

    <variablelist>
     <varlistentry>
      <term>ErrorResponse</term>
      <listitem>
       <para>
        An error has occurred.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>FunctionCallResponse</term>
      <listitem>
       <para>
        The function call was completed and returned the result given
        in the message.
        (Note that the Function Call protocol can only handle a single
        scalar result, not a row type or set of results.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ReadyForQuery</term>
      <listitem>
       <para>
        Processing of the function call is complete.  ReadyForQuery
        will always be sent, whether processing terminates
        successfully or with an error.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NoticeResponse</term>
      <listitem>
       <para>
        A warning message has been issued in relation to the function
        call.  Notices are in addition to other responses, i.e., the
        backend will continue processing the command.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect2>

  <sect2 id="protocol-copy">
   <title>COPY Operations</title>

   <para>
    The <command>COPY</command> command allows high-speed bulk data transfer
    to or from the server.  Copy-in and copy-out operations each switch
    the connection into a distinct sub-protocol, which lasts until the
    operation is completed.
   </para>

   <para>
    Copy-in mode (data transfer to the server) is initiated when the
    backend executes a <command>COPY FROM STDIN</command> SQL statement.  The backend
    sends a CopyInResponse message to the frontend.  The frontend should
    then send zero or more CopyData messages, forming a stream of input
    data.  (The message boundaries are not required to have anything to do
    with row boundaries, although that is often a reasonable choice.)
    The frontend can terminate the copy-in mode by sending either a CopyDone
    message (allowing successful termination) or a CopyFail message (which
    will cause the <command>COPY</command> SQL statement to fail with an
    error).  The backend then reverts to the command-processing mode it was
    in before the <command>COPY</command> started, which will be either simple or
    extended query protocol.  It will next send either CommandComplete
    (if successful) or ErrorResponse (if not).
   </para>

   <para>
    In the event of a backend-detected error during copy-in mode (including
    receipt of a CopyFail message), the backend will issue an ErrorResponse
    message.  If the <command>COPY</command> command was issued via an extended-query
    message, the backend will now discard frontend messages until a Sync
    message is received, then it will issue ReadyForQuery and return to normal
    processing.  If the <command>COPY</command> command was issued in a simple
    Query message, the rest of that message is discarded and ReadyForQuery
    is issued.  In either case, any subsequent CopyData, CopyDone, or CopyFail
    messages issued by the frontend will simply be dropped.
   </para>

   <para>
    The backend will ignore Flush and Sync messages received during copy-in
    mode.  Receipt of any other non-copy message type constitutes an error
    that will abort the copy-in state as described above.  (The exception for
    Flush and Sync is for the convenience of client libraries that always
    send Flush or Sync after an Execute message, without checking whether
    the command to be executed is a <command>COPY FROM STDIN</command>.)
   </para>

   <para>
    Copy-out mode (data transfer from the server) is initiated when the
    backend executes a <command>COPY TO STDOUT</command> SQL statement.  The backend
    sends a CopyOutResponse message to the frontend, followed by
    zero or more CopyData messages (always one per row), followed by CopyDone.
    The backend then reverts to the command-processing mode it was
    in before the <command>COPY</command> started, and sends CommandComplete.
    The frontend cannot abort the transfer (except by closing the connection
    or issuing a Cancel request),
    but it can discard unwanted CopyData and CopyDone messages.
   </para>

   <para>
    In the event of a backend-detected error during copy-out mode,
    the backend will issue an ErrorResponse message and revert to normal
    processing.  The frontend should treat receipt of ErrorResponse as
    terminating the copy-out mode.
   </para>

   <para>
    It is possible for NoticeResponse and ParameterStatus messages to be
    interspersed between CopyData messages; frontends must handle these cases,
    and should be prepared for other asynchronous message types as well (see
    <xref linkend="protocol-async"/>).  Otherwise, any message type other than
    CopyData or CopyDone may be treated as terminating copy-out mode.
   </para>

   <para>
    There is another Copy-related mode called copy-both, which allows
    high-speed bulk data transfer to <emphasis>and</emphasis> from the server.
    Copy-both mode is initiated when a backend in walsender mode
    executes a <command>START_REPLICATION</command> statement.  The
    backend sends a CopyBothResponse message to the frontend.  Both
    the backend and the frontend may then send CopyData messages
    until either end sends a CopyDone message. After the client
    sends a CopyDone message, the connection goes from copy-both mode to
    copy-out mode, and the client may not send any more CopyData messages.
    Similarly, when the server sends a CopyDone message, the connection
    goes into copy-in mode, and the server may not send any more CopyData
    messages. After both sides have sent a CopyDone message, the copy mode
    is terminated, and the backend reverts to the command-processing mode.
    In the event of a backend-detected error during copy-both mode,
    the backend will issue an ErrorResponse message, discard frontend messages
    until a Sync message is received, and then issue ReadyForQuery and return
    to normal processing.  The frontend should treat receipt of ErrorResponse
    as terminating the copy in both directions; no CopyDone should be sent
    in this case.  See <xref linkend="protocol-replication"/> for more
    information on the subprotocol transmitted over copy-both mode.
   </para>

   <para>
    The CopyInResponse, CopyOutResponse and CopyBothResponse messages
    include fields that inform the frontend of the number of columns
    per row and the format codes being used for each column.  (As of
    the present implementation, all columns in a given <command>COPY</command>
    operation will use the same format, but the message design does not
    assume this.)
   </para>

  </sect2>

  <sect2 id="protocol-async">
   <title>Asynchronous Operations</title>

   <para>
    There are several cases in which the backend will send messages that
    are not specifically prompted by the frontend's command stream.
    Frontends must be prepared to deal with these messages at any time,
    even when not engaged in a query.
    At minimum, one should check for these cases before beginning to
    read a query response.
   </para>

   <para>
    It is possible for NoticeResponse messages to be generated due to
    outside activity; for example, if the database administrator commands
    a <quote>fast</quote> database shutdown, the backend will send a NoticeResponse
    indicating this fact before closing the connection.  Accordingly,
    frontends should always be prepared to accept and display NoticeResponse
    messages, even when the connection is nominally idle.
   </para>

   <para>
    ParameterStatus messages will be generated whenever the active
    value changes for any of the parameters the backend believes the
    frontend should know about.  Most commonly this occurs in response
    to a <command>SET</command> SQL command executed by the frontend, and
    this case is effectively synchronous &mdash; but it is also possible
    for parameter status changes to occur because the administrator
    changed a configuration file and then sent the
    <systemitem>SIGHUP</systemitem> signal to the server.  Also,
    if a <command>SET</command> command is rolled back, an appropriate
    ParameterStatus message will be generated to report the current
    effective value.
   </para>

   <para>
    At present there is a hard-wired set of parameters for which
    ParameterStatus will be generated.  They are:
    <simplelist type="vert" columns="2">
     <member><varname>application_name</varname></member>
     <member><varname>client_encoding</varname></member>
     <member><varname>DateStyle</varname></member>
     <member><varname>default_transaction_read_only</varname></member>
     <member><varname>in_hot_standby</varname></member>
     <member><varname>integer_datetimes</varname></member>
     <member><varname>IntervalStyle</varname></member>
     <member><varname>is_superuser</varname></member>
     <member><varname>scram_iterations</varname></member>
     <member><varname>search_path</varname></member>
     <member><varname>server_encoding</varname></member>
     <member><varname>server_version</varname></member>
     <member><varname>session_authorization</varname></member>
     <member><varname>standard_conforming_strings</varname></member>
     <member><varname>TimeZone</varname></member>
    </simplelist>
    (<varname>default_transaction_read_only</varname> and
    <varname>in_hot_standby</varname> were not reported by releases before
    14; <varname>scram_iterations</varname> was not reported by releases
    before 16; <varname>search_path</varname> was not reported by releases
    before 18.)
    Note that
    <varname>server_version</varname>,
    <varname>server_encoding</varname> and
    <varname>integer_datetimes</varname>
    are pseudo-parameters that cannot change after startup.
    This set might change in the future, or even become configurable.
    Accordingly, a frontend should simply ignore ParameterStatus for
    parameters that it does not understand or care about.
   </para>

   <para>
    If a frontend issues a <command>LISTEN</command> command, then the
    backend will send a NotificationResponse message (not to be
    confused with NoticeResponse!)  whenever a
    <command>NOTIFY</command> command is executed for the same
    channel name.
   </para>

   <note>
    <para>
     At present, NotificationResponse can only be sent outside a
     transaction, and thus it will not occur in the middle of a
     command-response series, though it might occur just before ReadyForQuery.
     It is unwise to design frontend logic that assumes that, however.
     Good practice is to be able to accept NotificationResponse at any
     point in the protocol.
    </para>
   </note>
  </sect2>

  <sect2 id="protocol-flow-canceling-requests">
   <title>Canceling Requests in Progress</title>

   <para>
    During the processing of a query, the frontend might request
    cancellation of the query.  The cancel request is not sent
    directly on the open connection to the backend for reasons of
    implementation efficiency: we don't want to have the backend
    constantly checking for new input from the frontend during query
    processing.  Cancel requests should be relatively infrequent, so
    we make them slightly cumbersome in order to avoid a penalty in
    the normal case.
   </para>

   <para>
    To issue a cancel request, the frontend opens a new connection to
    the server and sends a CancelRequest message, rather than the
    StartupMessage message that would ordinarily be sent across a new
    connection.  The server will process this request and then close
    the connection.  For security reasons, no direct reply is made to
    the cancel request message.
   </para>

   <para>
    A CancelRequest message will be ignored unless it contains the
    same key data (PID and secret key) passed to the frontend during
    connection start-up.  If the request matches the PID and secret
    key for a currently executing backend, the processing of the
    current query is aborted.  (In the existing implementation, this is
    done by sending a special signal to the backend process that is
    processing the query.)
   </para>

   <para>
    The cancellation signal might or might not have any effect &mdash; for
    example, if it arrives after the backend has finished processing
    the query, then it will have no effect.  If the cancellation is
    effective, it results in the current command being terminated
    early with an error message.
   </para>

   <para>
    The upshot of all this is that for reasons of both security and
    efficiency, the frontend has no direct way to tell whether a
    cancel request has succeeded.  It must continue to wait for the
    backend to respond to the query.  Issuing a cancel simply improves
    the odds that the current query will finish soon, and improves the
    odds that it will fail with an error message instead of
    succeeding.
   </para>

   <para>
    Since the cancel request is sent across a new connection to the
    server and not across the regular frontend/backend communication
    link, it is possible for the cancel request to be issued by any
    process, not just the frontend whose query is to be canceled.
    This might provide additional flexibility when building
    multiple-process applications.  It also introduces a security
    risk, in that unauthorized persons might try to cancel queries.
    The security risk is addressed by requiring a dynamically
    generated secret key to be supplied in cancel requests.
   </para>
  </sect2>

  <sect2 id="protocol-flow-termination">
   <title>Termination</title>

   <para>
    The normal, graceful termination procedure is that the frontend
    sends a Terminate message and immediately closes the connection.
    On receipt of this message, the backend closes the connection and
    terminates.
   </para>

   <para>
    In rare cases (such as an administrator-commanded database shutdown)
    the backend might disconnect without any frontend request to do so.
    In such cases the backend will attempt to send an error or notice message
    giving the reason for the disconnection before it closes the connection.
   </para>

   <para>
    Other termination scenarios arise from various failure cases, such as core
    dump at one end or the other, loss of the communications link, loss of
    message-boundary synchronization, etc.  If either frontend or backend sees
    an unexpected closure of the connection, it should clean
    up and terminate.  The frontend has the option of launching a new backend
    by recontacting the server if it doesn't want to terminate itself.
    Closing the connection is also advisable if an unrecognizable message type
    is received, since this probably indicates loss of message-boundary sync.
   </para>

   <para>
    For either normal or abnormal termination, any open transaction is
    rolled back, not committed.  One should note however that if a
    frontend disconnects while a non-<command>SELECT</command> query
    is being processed, the backend will probably finish the query
    before noticing the disconnection.  If the query is outside any
    transaction block (<command>BEGIN</command> ... <command>COMMIT</command>
    sequence) then its results might be committed before the
    disconnection is recognized.
   </para>
  </sect2>

  <sect2 id="protocol-flow-ssl">
   <title><acronym>SSL</acronym> Session Encryption</title>

   <para>
    If <productname>PostgreSQL</productname> was built with
    <acronym>SSL</acronym> support, frontend/backend communications
    can be encrypted using <acronym>SSL</acronym>.  This provides
    communication security in environments where attackers might be
    able to capture the session traffic. For more information on
    encrypting <productname>PostgreSQL</productname> sessions with
    <acronym>SSL</acronym>, see <xref linkend="ssl-tcp"/>.
   </para>

   <para>
    To initiate an <acronym>SSL</acronym>-encrypted connection, the
    frontend initially sends an SSLRequest message rather than a
    StartupMessage.  The server then responds with a single byte
    containing <literal>S</literal> or <literal>N</literal>, indicating that it is
    willing or unwilling to perform <acronym>SSL</acronym>,
    respectively.  The frontend might close the connection at this point
    if it is dissatisfied with the response.  To continue after
    <literal>S</literal>, perform an <acronym>SSL</acronym> startup handshake
    (not described here, part of the <acronym>SSL</acronym>
    specification) with the server.  If this is successful, continue
    with sending the usual StartupMessage.  In this case the
    StartupMessage and all subsequent data will be
    <acronym>SSL</acronym>-encrypted.  To continue after
    <literal>N</literal>, send the usual StartupMessage and proceed without
    encryption.
    (Alternatively, it is permissible to issue a GSSENCRequest message
    after an <literal>N</literal> response to try to
    use <acronym>GSSAPI</acronym> encryption instead
    of <acronym>SSL</acronym>.)
   </para>

   <para>
    The frontend should also be prepared to handle an ErrorMessage
    response to SSLRequest from the server. The frontend should not display
    this error message to the user/application, since the server has not been
    authenticated
    (<ulink url="https://www.postgresql.org/support/security/CVE-2024-10977/">CVE-2024-10977</ulink>).
    In this case the connection must
    be closed, but the frontend might choose to open a fresh connection
    and proceed without requesting <acronym>SSL</acronym>.
   </para>

   <para>
    When <acronym>SSL</acronym> encryption can be performed, the server
    is expected to send only the single <literal>S</literal> byte and then
    wait for the frontend to initiate an <acronym>SSL</acronym> handshake.
    If additional bytes are available to read at this point, it likely
    means that a man-in-the-middle is attempting to perform a
    buffer-stuffing attack
    (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
    Frontends should be coded either to read exactly one byte from the
    socket before turning the socket over to their SSL library, or to
    treat it as a protocol violation if they find they have read additional
    bytes.
   </para>

   <para>
     Likewise the server expects the client to not begin
     the <acronym>SSL</acronym> negotiation until it receives the server's
     single byte response to the <acronym>SSL</acronym> request.  If the
     client begins the <acronym>SSL</acronym> negotiation immediately without
     waiting for the server response to be received it can reduce connection
     latency by one round-trip.  However this comes at the cost of not being
     able to handle the case where the server sends a negative response to the
     <acronym>SSL</acronym> request.  In that case instead of continuing with either GSSAPI or an
     unencrypted connection or a protocol error the server will simply
     disconnect.
   </para>

   <para>
    An initial SSLRequest can also be used in a connection that is being
    opened to send a CancelRequest message.
   </para>

   <para>
     A second alternate way to initiate <acronym>SSL</acronym> encryption is
     available.  The server will recognize connections which immediately
     begin <acronym>SSL</acronym> negotiation without any previous SSLRequest
     packets.  Once the <acronym>SSL</acronym> connection is established the
     server will expect a normal startup-request packet and continue
     negotiation over the encrypted channel.  In this case any other requests
     for encryption will be refused.  This method is not preferred for general
     purpose tools as it cannot negotiate the best connection encryption
     available or handle unencrypted connections.  However it is useful for
     environments where both the server and client are controlled together.
     In that case it avoids one round trip of latency and allows the use of
     network tools that depend on standard <acronym>SSL</acronym> connections.
     When using <acronym>SSL</acronym> connections in this style the client is
     required to use the ALPN extension defined
     by <ulink url="https://tools.ietf.org/html/rfc7301">RFC 7301</ulink> to
     protect against protocol confusion attacks.
     The <productname>PostgreSQL</productname> protocol is "postgresql" as
     registered
     at <ulink url="https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids">IANA
     TLS ALPN Protocol IDs</ulink> registry.
   </para>

   <para>
    While the protocol itself does not provide a way for the server to
    force <acronym>SSL</acronym> encryption, the administrator can
    configure the server to reject unencrypted sessions as a byproduct
    of authentication checking.
   </para>
  </sect2>

  <sect2 id="protocol-flow-gssapi">
   <title><acronym>GSSAPI</acronym> Session Encryption</title>

   <para>
    If <productname>PostgreSQL</productname> was built with
    <acronym>GSSAPI</acronym> support, frontend/backend communications
    can be encrypted using <acronym>GSSAPI</acronym>.  This provides
    communication security in environments where attackers might be
    able to capture the session traffic. For more information on
    encrypting <productname>PostgreSQL</productname> sessions with
    <acronym>GSSAPI</acronym>, see <xref linkend="gssapi-enc"/>.
   </para>

   <para>
    To initiate a <acronym>GSSAPI</acronym>-encrypted connection, the
    frontend initially sends a GSSENCRequest message rather than a
    StartupMessage.  The server then responds with a single byte
    containing <literal>G</literal> or <literal>N</literal>, indicating that it
    is willing or unwilling to perform <acronym>GSSAPI</acronym> encryption,
    respectively.  The frontend might close the connection at this point
    if it is dissatisfied with the response.  To continue after
    <literal>G</literal>, using the GSSAPI C bindings as discussed in
    <ulink url="https://datatracker.ietf.org/doc/html/rfc2744">RFC 2744</ulink>
    or equivalent, perform a <acronym>GSSAPI</acronym> initialization by
    calling <function>gss_init_sec_context()</function> in a loop and sending
    the result to the server, starting with an empty input and then with each
    result from the server, until it returns no output.  When sending the
    results of <function>gss_init_sec_context()</function> to the server,
    prepend the length of the message as a four byte integer in network byte
    order.
    To continue after
    <literal>N</literal>, send the usual StartupMessage and proceed without
    encryption.
    (Alternatively, it is permissible to issue an SSLRequest message
    after an <literal>N</literal> response to try to
    use <acronym>SSL</acronym> encryption instead
    of <acronym>GSSAPI</acronym>.)
   </para>

   <para>
    The frontend should also be prepared to handle an ErrorMessage
    response to GSSENCRequest from the server.  The frontend should not display
    this error message to the user/application, since the server has not been
    authenticated
    (<ulink url="https://www.postgresql.org/support/security/CVE-2024-10977/">CVE-2024-10977</ulink>).
    In this case the connection must be closed, but the frontend might choose
    to open a fresh connection and proceed without requesting
    <acronym>GSSAPI</acronym> encryption.
   </para>

   <para>
    When <acronym>GSSAPI</acronym> encryption can be performed, the server
    is expected to send only the single <literal>G</literal> byte and then
    wait for the frontend to initiate a <acronym>GSSAPI</acronym> handshake.
    If additional bytes are available to read at this point, it likely
    means that a man-in-the-middle is attempting to perform a
    buffer-stuffing attack
    (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
    Frontends should be coded either to read exactly one byte from the
    socket before turning the socket over to their GSSAPI library, or to
    treat it as a protocol violation if they find they have read additional
    bytes.
   </para>

   <para>
    An initial GSSENCRequest can also be used in a connection that is being
    opened to send a CancelRequest message.
   </para>

   <para>
    Once <acronym>GSSAPI</acronym> encryption has been successfully
    established, use <function>gss_wrap()</function> to
    encrypt the usual StartupMessage and all subsequent data, prepending the
    length of the result from <function>gss_wrap()</function> as a four byte
    integer in network byte order to the actual encrypted payload.  Note that
    the server will only accept encrypted packets from the client which are less
    than 16kB; <function>gss_wrap_size_limit()</function> should be used by the
    client to determine the size of the unencrypted message which will fit
    within this limit and larger messages should be broken up into multiple
    <function>gss_wrap()</function> calls.  Typical segments are 8kB of
    unencrypted data, resulting in encrypted packets of slightly larger than 8kB
    but well within the 16kB maximum.  The server can be expected to not send
    encrypted packets of larger than 16kB to the client.
   </para>

   <para>
    While the protocol itself does not provide a way for the server to
    force <acronym>GSSAPI</acronym> encryption, the administrator can
    configure the server to reject unencrypted sessions as a byproduct
    of authentication checking.
   </para>
  </sect2>
 </sect1>

 <sect1 id="sasl-authentication">
  <title>SASL Authentication</title>

  <para>
   <firstterm>SASL</firstterm> is a framework for authentication in connection-oriented
   protocols. At the moment, <productname>PostgreSQL</productname> implements three
   SASL authentication mechanisms: SCRAM-SHA-256, SCRAM-SHA-256-PLUS, and
   OAUTHBEARER. More might be added in the future. The below steps illustrate how SASL
   authentication is performed in general, while the next subsections give
   more details on particular mechanisms.
  </para>

  <procedure>
   <title>SASL Authentication Message Flow</title>

   <step id="sasl-auth-begin">
    <para>
     To begin a SASL authentication exchange, the server sends an
     AuthenticationSASL message. It includes a list of SASL authentication
     mechanisms that the server can accept, in the server's preferred order.
    </para>
   </step>

   <step id="sasl-auth-initial-response">
    <para>
     The client selects one of the supported mechanisms from the list, and sends
     a SASLInitialResponse message to the server. The message includes the name
     of the selected mechanism, and an optional Initial Client Response, if the
     selected mechanism uses that.
    </para>
   </step>

   <step id="sasl-auth-continue">
    <para>
     One or more server-challenge and client-response message will follow. Each
     server-challenge is sent in an AuthenticationSASLContinue message, followed
     by a response from client in a SASLResponse message. The particulars of
     the messages are mechanism specific.
    </para>
   </step>

   <step id="sasl-auth-end">
    <para>
     Finally, when the authentication exchange is completed successfully, the
     server sends an optional AuthenticationSASLFinal message, followed
     immediately by an AuthenticationOk message. The AuthenticationSASLFinal
     contains additional server-to-client data, whose content is particular to the
     selected authentication mechanism. If the authentication mechanism doesn't
     use additional data that's sent at completion, the AuthenticationSASLFinal
     message is not sent.
    </para>
   </step>
  </procedure>

  <para>
   On error, the server can abort the authentication at any stage, and send an
   ErrorMessage.
  </para>

  <sect2 id="sasl-scram-sha-256">
   <title>SCRAM-SHA-256 Authentication</title>

   <para>
    <literal>SCRAM-SHA-256</literal>, and its variant with channel
    binding <literal>SCRAM-SHA-256-PLUS</literal>, are password-based
    authentication mechanisms. They are described in
    detail in <ulink url="https://datatracker.ietf.org/doc/html/rfc7677">RFC 7677</ulink>
    and <ulink url="https://datatracker.ietf.org/doc/html/rfc5802">RFC 5802</ulink>.
   </para>

   <para>
    When SCRAM-SHA-256 is used in PostgreSQL, the server will ignore the user name
    that the client sends in the <structname>client-first-message</structname>. The user name
    that was already sent in the startup message is used instead.
    <productname>PostgreSQL</productname> supports multiple character encodings, while SCRAM
    dictates UTF-8 to be used for the user name, so it might be impossible to
    represent the PostgreSQL user name in UTF-8.
   </para>

   <para>
    The SCRAM specification dictates that the password is also in UTF-8, and is
    processed with the <firstterm>SASLprep</firstterm> algorithm.
    <productname>PostgreSQL</productname>, however, does not require UTF-8 to be used for
    the password. When a user's password is set, it is processed with SASLprep
    as if it was in UTF-8, regardless of the actual encoding used. However, if
    it is not a legal UTF-8 byte sequence, or it contains UTF-8 byte sequences
    that are prohibited by the SASLprep algorithm, the raw password will be used
    without SASLprep processing, instead of throwing an error. This allows the
    password to be normalized when it is in UTF-8, but still allows a non-UTF-8
    password to be used, and doesn't require the system to know which encoding
    the password is in.
   </para>

   <para>
    <firstterm>Channel binding</firstterm> is supported in PostgreSQL builds with
    SSL support. The SASL mechanism name for SCRAM with channel binding is
    <literal>SCRAM-SHA-256-PLUS</literal>.  The channel binding type used by
    PostgreSQL is <literal>tls-server-end-point</literal>.
   </para>

   <para>
    In <acronym>SCRAM</acronym> without channel binding, the server chooses
    a random number that is transmitted to the client to be mixed with the
    user-supplied password in the transmitted password hash.  While this
    prevents the password hash from being successfully retransmitted in
    a later session, it does not prevent a fake server between the real
    server and client from passing through the server's random value
    and successfully authenticating.
   </para>

   <para>
    <acronym>SCRAM</acronym> with channel binding prevents such
    man-in-the-middle attacks by mixing the signature of the server's
    certificate into the transmitted password hash. While a fake server can
    retransmit the real server's certificate, it doesn't have access to the
    private key matching that certificate, and therefore cannot prove it is
    the owner, causing SSL connection failure.
   </para>

   <procedure>
    <title>Example</title>
    <step id="scram-begin">
     <para>
      The server sends an AuthenticationSASL message. It includes a list of
      SASL authentication mechanisms that the server can accept.
      This will be <literal>SCRAM-SHA-256-PLUS</literal>
      and <literal>SCRAM-SHA-256</literal> if the server is built with SSL
      support, or else just the latter.
     </para>
    </step>

    <step id="scram-client-first">
     <para>
      The client responds by sending a SASLInitialResponse message, which
      indicates the chosen mechanism, <literal>SCRAM-SHA-256</literal> or
      <literal>SCRAM-SHA-256-PLUS</literal>. (A client is free to choose either
      mechanism, but for better security it should choose the channel-binding
      variant if it can support it.) In the Initial Client response field, the
      message contains the SCRAM <structname>client-first-message</structname>.
      The <structname>client-first-message</structname> also contains the channel
      binding type chosen by the client.
     </para>
    </step>

    <step id="scram-server-first">
     <para>
      Server sends an AuthenticationSASLContinue message, with a SCRAM
      <structname>server-first-message</structname> as the content.
     </para>
    </step>

    <step id="scram-client-final">
     <para>
      Client sends a SASLResponse message, with SCRAM
      <structname>client-final-message</structname> as the content.
     </para>
    </step>

    <step id="scram-server-final">
     <para>
      Server sends an AuthenticationSASLFinal message, with the SCRAM
      <structname>server-final-message</structname>, followed immediately by
      an AuthenticationOk message.
     </para>
    </step>
   </procedure>
  </sect2>

  <sect2 id="sasl-oauthbearer">
   <title>OAUTHBEARER Authentication</title>

   <para>
    <literal>OAUTHBEARER</literal> is a token-based mechanism for federated
    authentication. It is described in detail in
    <ulink url="https://datatracker.ietf.org/doc/html/rfc7628">RFC 7628</ulink>.
   </para>

   <para>
    A typical exchange differs depending on whether or not the client already
    has a bearer token cached for the current user. If it does not, the exchange
    will take place over two connections: the first "discovery" connection to
    obtain OAuth metadata from the server, and the second connection to send
    the token after the client has obtained it. (libpq does not currently
    implement a caching method as part of its builtin flow, so it uses the
    two-connection exchange.)
   </para>

   <para>
    This mechanism is client-initiated, like SCRAM. The client initial response
    consists of the standard "GS2" header used by SCRAM, followed by a list of
    <literal>key=value</literal> pairs. The only key currently supported by
    the server is <literal>auth</literal>, which contains the bearer token.
    <literal>OAUTHBEARER</literal> additionally specifies three optional
    components of the client initial response (the <literal>authzid</literal> of
    the GS2 header, and the <structfield>host</structfield> and
    <structfield>port</structfield> keys) which are currently ignored by the
    server.
   </para>

   <para>
    <literal>OAUTHBEARER</literal> does not support channel binding, and there
    is no "OAUTHBEARER-PLUS" mechanism. This mechanism does not make use of
    server data during a successful authentication, so the
    AuthenticationSASLFinal message is not used in the exchange.
   </para>

   <procedure>
    <title>Example</title>
    <step>
     <para>
      During the first exchange, the server sends an AuthenticationSASL message
      with the <literal>OAUTHBEARER</literal> mechanism advertised.
     </para>
    </step>

    <step>
     <para>
      The client responds by sending a SASLInitialResponse message which
      indicates the <literal>OAUTHBEARER</literal> mechanism. Assuming the
      client does not already have a valid bearer token for the current user,
      the <structfield>auth</structfield> field is empty, indicating a discovery
      connection.
     </para>
    </step>

    <step>
     <para>
      Server sends an AuthenticationSASLContinue message containing an error
      <literal>status</literal> alongside a well-known URI and scopes that the
      client should use to conduct an OAuth flow.
     </para>
    </step>

    <step>
     <para>
      Client sends a SASLResponse message containing the empty set (a single
      <literal>0x01</literal> byte) to finish its half of the discovery
      exchange.
     </para>
    </step>

    <step>
     <para>
      Server sends an ErrorMessage to fail the first exchange.
     </para>
     <para>
      At this point, the client conducts one of many possible OAuth flows to
      obtain a bearer token, using any metadata that it has been configured with
      in addition to that provided by the server. (This description is left
      deliberately vague; <literal>OAUTHBEARER</literal> does not specify or
      mandate any particular method for obtaining a token.)
     </para>
     <para>
      Once it has a token, the client reconnects to the server for the final
      exchange:
     </para>
    </step>

    <step>
     <para>
      The server once again sends an AuthenticationSASL message with the
      <literal>OAUTHBEARER</literal> mechanism advertised.
     </para>
    </step>

    <step>
     <para>
      The client responds by sending a SASLInitialResponse message, but this
      time the <structfield>auth</structfield> field in the message contains the
      bearer token that was obtained during the client flow.
     </para>
    </step>

    <step>
     <para>
      The server validates the token according to the instructions of the
      token provider. If the client is authorized to connect, it sends an
      AuthenticationOk message to end the SASL exchange.
     </para>
    </step>
   </procedure>
  </sect2>
 </sect1>

 <sect1 id="protocol-replication">
  <title>Streaming Replication Protocol</title>

  <para>
   To initiate streaming replication, the frontend sends the
   <literal>replication</literal> parameter in the startup message. A Boolean
   value of <literal>true</literal> (or <literal>on</literal>,
   <literal>yes</literal>, <literal>1</literal>) tells the backend to go into
   physical replication walsender mode, wherein a small set of replication
   commands, shown below, can be issued instead of SQL statements.
  </para>

  <para>
   Passing <literal>database</literal> as the value for the
   <literal>replication</literal> parameter instructs the backend to go into
   logical replication walsender mode, connecting to the database specified in
   the <literal>dbname</literal> parameter.  In logical replication walsender
   mode, the replication commands shown below as well as normal SQL commands can
   be issued.
  </para>

  <para>
   In either physical replication or logical replication walsender mode, only the
   simple query protocol can be used.
  </para>

  <para>
   For the purpose of testing replication commands, you can make a replication
   connection via <application>psql</application> or any other
   <application>libpq</application>-using tool with a connection string including
   the <literal>replication</literal> option,
   e.g.:
<programlisting>
psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
</programlisting>
   However, it is often more useful to use
   <xref linkend="app-pgreceivewal"/> (for physical replication) or
   <xref linkend="app-pgrecvlogical"/> (for logical replication).
  </para>

  <para>
   Replication commands are logged in the server log when
   <xref linkend="guc-log-replication-commands"/> is enabled.
  </para>

  <para>
   The commands accepted in replication mode are:

   <variablelist>
    <varlistentry id="protocol-replication-identify-system">
     <term><literal>IDENTIFY_SYSTEM</literal>
      <indexterm><primary>IDENTIFY_SYSTEM</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to identify itself. Server replies with a result
       set of a single row, containing four fields:
      </para>

      <variablelist>
       <varlistentry>
        <term><literal>systemid</literal> (<type>text</type>)</term>
        <listitem>
         <para>
          The unique system identifier identifying the cluster. This
          can be used to check that the base backup used to initialize the
          standby came from the same cluster.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>timeline</literal> (<type>int8</type>)</term>
        <listitem>
         <para>
          Current timeline ID. Also useful to check that the standby is
          consistent with the primary.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>xlogpos</literal> (<type>text</type>)</term>
        <listitem>
         <para>
          Current WAL flush location. Useful to get a known location in the
          write-ahead log where streaming can start.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>dbname</literal> (<type>text</type>)</term>
        <listitem>
         <para>
          Database connected to or null.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-show">
     <term><literal>SHOW</literal> <replaceable class="parameter">name</replaceable>
      <indexterm><primary>SHOW</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to send the current setting of a run-time parameter.
       This is similar to the SQL command <xref linkend="sql-show"/>.
      </para>

      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">name</replaceable></term>
        <listitem>
         <para>
          The name of a run-time parameter. Available parameters are documented
          in <xref linkend="runtime-config"/>.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-timeline-history">
     <term><literal>TIMELINE_HISTORY</literal> <replaceable class="parameter">tli</replaceable>
      <indexterm><primary>TIMELINE_HISTORY</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to send over the timeline history file for timeline
       <replaceable class="parameter">tli</replaceable>.  Server replies with a
       result set of a single row, containing two fields.  While the fields
       are labeled as <type>text</type>, they effectively return raw bytes,
       with no encoding conversion:
      </para>

      <variablelist>
       <varlistentry>
        <term><literal>filename</literal> (<type>text</type>)</term>
        <listitem>
         <para>
          File name of the timeline history file, e.g., <filename>00000002.history</filename>.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>content</literal> (<type>text</type>)</term>
        <listitem>
         <para>
          Contents of the timeline history file.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-create-replication-slot" xreflabel="CREATE_REPLICATION_SLOT">
     <term><literal>CREATE_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> [ <literal>TEMPORARY</literal> ] { <literal>PHYSICAL</literal> | <literal>LOGICAL</literal> <replaceable class="parameter">output_plugin</replaceable> } [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ]
      <indexterm><primary>CREATE_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Create a physical or logical replication
       slot. See <xref linkend="streaming-replication-slots"/> for more about
       replication slots.
      </para>

      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
         <para>
          The name of the slot to create. Must be a valid replication slot
          name (see <xref linkend="streaming-replication-slots-manipulation"/>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="parameter">output_plugin</replaceable></term>
        <listitem>
         <para>
          The name of the output plugin used for logical decoding
          (see <xref linkend="logicaldecoding-output-plugin"/>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>TEMPORARY</literal></term>
        <listitem>
         <para>
          Specify that this replication slot is a temporary one. Temporary
          slots are not saved to disk and are automatically dropped on error
          or when the session has finished.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>The following options are supported:</para>

      <variablelist>
       <varlistentry>
        <term><literal>TWO_PHASE [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
        <listitem>
         <para>
          If true, this logical replication slot supports decoding of two-phase
          commit. With this option, commands related to two-phase commit such as
          <literal>PREPARE TRANSACTION</literal>, <literal>COMMIT PREPARED</literal>
          and <literal>ROLLBACK PREPARED</literal> are decoded and transmitted.
          The transaction will be decoded and transmitted at
          <literal>PREPARE TRANSACTION</literal> time.
          The default is false.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>RESERVE_WAL [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
        <listitem>
         <para>
          If true, this physical replication slot reserves <acronym>WAL</acronym>
          immediately.  Otherwise, <acronym>WAL</acronym> is only reserved upon
          connection from a streaming replication client.
          The default is false.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>SNAPSHOT { 'export' | 'use' | 'nothing' }</literal></term>
        <listitem>
         <para>
          Decides what to do with the snapshot created during logical slot
          initialization. <literal>'export'</literal>, which is the default,
          will export the snapshot for use in other sessions. This option can't
          be used inside a transaction.  <literal>'use'</literal> will use the
          snapshot for the current transaction executing the command. This
          option must be used in a transaction, and
          <literal>CREATE_REPLICATION_SLOT</literal> must be the first command
          run in that transaction.  Finally, <literal>'nothing'</literal> will
          just use the snapshot for logical decoding as normal but won't do
          anything else with it.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>FAILOVER [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
        <listitem>
         <para>
          If true, the slot is enabled to be synced to the standbys
          so that logical replication can be resumed after failover.
          The default is false.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>
       In response to this command, the server will send a one-row result set
       containing the following fields:

       <variablelist>
        <varlistentry>
         <term><literal>slot_name</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The name of the newly-created replication slot.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>consistent_point</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The WAL location at which the slot became consistent.  This is the
           earliest location from which streaming can start on this replication
           slot.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>snapshot_name</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The identifier of the snapshot exported by the command.  The
           snapshot is valid until a new command is executed on this connection
           or the replication connection is closed.  Null if the created slot
           is physical.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>output_plugin</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The name of the output plugin used by the newly-created replication
           slot.  Null if the created slot is physical.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-create-replication-slot-legacy">
     <term><literal>CREATE_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> [ <literal>TEMPORARY</literal> ] { <literal>PHYSICAL</literal> [ <literal>RESERVE_WAL</literal> ] | <literal>LOGICAL</literal> <replaceable class="parameter">output_plugin</replaceable> [ <literal>EXPORT_SNAPSHOT</literal> | <literal>NOEXPORT_SNAPSHOT</literal> | <literal>USE_SNAPSHOT</literal> | <literal>TWO_PHASE</literal> ] }
     </term>
     <listitem>
      <para>
       For compatibility with older releases, this alternative syntax for
       the <literal>CREATE_REPLICATION_SLOT</literal> command is still supported.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-alter-replication-slot" xreflabel="ALTER_REPLICATION_SLOT">
     <term><literal>ALTER_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> ( <replaceable class="parameter">option</replaceable> [, ...] )
      <indexterm><primary>ALTER_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Change the definition of a replication slot.
       See <xref linkend="streaming-replication-slots"/> for more about
       replication slots. This command is currently only supported for logical
       replication slots.
      </para>

      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
         <para>
          The name of the slot to alter. Must be a valid replication slot
          name (see <xref linkend="streaming-replication-slots-manipulation"/>).
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>The following options are supported:</para>

      <variablelist>
       <varlistentry>
        <term><literal>TWO_PHASE [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
        <listitem>
         <para>
          If true, this logical replication slot supports decoding of two-phase
          commit. With this option, commands related to two-phase commit such as
          <literal>PREPARE TRANSACTION</literal>, <literal>COMMIT PREPARED</literal>
          and <literal>ROLLBACK PREPARED</literal> are decoded and transmitted.
          The transaction will be decoded and transmitted at
          <literal>PREPARE TRANSACTION</literal> time.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      <variablelist>
       <varlistentry>
        <term><literal>FAILOVER [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
        <listitem>
         <para>
          If true, the slot is enabled to be synced to the standbys
          so that logical replication can be resumed after failover.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-read-replication-slot">
     <term><literal>READ_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable>
      <indexterm><primary>READ_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Read some information associated with a replication slot. Returns a tuple
       with <literal>NULL</literal> values if the replication slot does not
       exist. This command is currently only supported for physical replication
       slots.
      </para>

      <para>
       In response to this command, the server will return a one-row result set,
       containing the following fields:
       <variablelist>
        <varlistentry>
         <term><literal>slot_type</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The replication slot's type, either <literal>physical</literal> or
           <literal>NULL</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>restart_lsn</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The replication slot's <literal>restart_lsn</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>restart_tli</literal> (<type>int8</type>)</term>
         <listitem>
          <para>
           The timeline ID associated with <literal>restart_lsn</literal>,
           following the current timeline history.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-start-replication">
     <term><literal>START_REPLICATION</literal> [ <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> ] [ <literal>PHYSICAL</literal> ] <replaceable class="parameter">XXX/XXX</replaceable> [ <literal>TIMELINE</literal> <replaceable class="parameter">tli</replaceable> ]
      <indexterm><primary>START_REPLICATION</primary></indexterm>
     </term>
     <listitem>
      <para>
       Instructs server to start streaming WAL, starting at
       WAL location <replaceable class="parameter">XXX/XXX</replaceable>.
       If <literal>TIMELINE</literal> option is specified,
       streaming starts on timeline <replaceable class="parameter">tli</replaceable>;
       otherwise, the server's current timeline is selected. The server can
       reply with an error, for example if the requested section of WAL has already
       been recycled. On success, the server responds with a CopyBothResponse
       message, and then starts to stream WAL to the frontend.
      </para>

      <para>
       If a slot's name is provided
       via <replaceable class="parameter">slot_name</replaceable>, it will be updated
       as replication progresses so that the server knows which WAL segments,
       and if <varname>hot_standby_feedback</varname> is on which transactions,
       are still needed by the standby.
      </para>

      <para>
       If the client requests a timeline that's not the latest but is part of
       the history of the server, the server will stream all the WAL on that
       timeline starting from the requested start point up to the point where
       the server switched to another timeline. If the client requests
       streaming at exactly the end of an old timeline, the server skips COPY
       mode entirely.
      </para>

      <para>
       After streaming all the WAL on a timeline that is not the latest one,
       the server will end streaming by exiting the COPY mode. When the client
       acknowledges this by also exiting COPY mode, the server sends a result
       set with one row and two columns, indicating the next timeline in this
       server's history. The first column is the next timeline's ID (type <type>int8</type>), and the
       second column is the WAL location where the switch happened (type <type>text</type>). Usually,
       the switch position is the end of the WAL that was streamed, but there
       are corner cases where the server can send some WAL from the old
       timeline that it has not itself replayed before promoting. Finally, the
       server sends two CommandComplete messages (one that ends the CopyData
       and the other ends the <literal>START_REPLICATION</literal> itself), and
       is ready to accept a new command.
      </para>

      <para>
       WAL data is sent as a series of CopyData messages;
       see <xref linkend="protocol-message-types"/> and <xref
       linkend="protocol-message-formats"/> for details.
       (This allows other information to be intermixed; in particular the server can send
       an ErrorResponse message if it encounters a failure after beginning
       to stream.)  The payload of each CopyData message from server to the
       client contains a message of one of the following formats:
      </para>

      <variablelist>
       <varlistentry id="protocol-replication-xlogdata">
        <term>XLogData (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('w')</term>
           <listitem>
            <para>
             Identifies the message as WAL data.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The starting point of the WAL data in this message.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The current end of WAL on the server.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The server's system clock at the time of transmission, as
             microseconds since midnight on 2000-01-01.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Byte<replaceable>n</replaceable></term>
           <listitem>
            <para>
             A section of the WAL data stream.
            </para>

            <para>
             A single WAL record is never split across two XLogData messages.
             When a WAL record crosses a WAL page boundary, and is therefore
             already split using continuation records, it can be split at the page
             boundary. In other words, the first main WAL record and its
             continuation records can be sent in different XLogData messages.
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>

       <varlistentry id="protocol-replication-primary-keepalive-message">
        <term>Primary keepalive message (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('k')</term>
           <listitem>
            <para>
             Identifies the message as a sender keepalive.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The current end of WAL on the server.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The server's system clock at the time of transmission, as
             microseconds since midnight on 2000-01-01.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Byte1</term>
           <listitem>
            <para>
             1 means that the client should reply to this message as soon as
             possible, to avoid a timeout disconnect. 0 otherwise.
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>
       The receiving process can send replies back to the sender at any time,
       using one of the following message formats (also in the payload of a
       CopyData message):
      </para>

      <variablelist>
       <varlistentry id="protocol-replication-standby-status-update">
        <term>Standby status update (F)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('r')</term>
           <listitem>
            <para>
             Identifies the message as a receiver status update.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The location of the last WAL byte + 1 received and written to disk
             in the standby.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The location of the last WAL byte + 1 flushed to disk in
             the standby.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The location of the last WAL byte + 1 applied in the standby.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The client's system clock at the time of transmission, as
             microseconds since midnight on 2000-01-01.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Byte1</term>
           <listitem>
            <para>
             If 1, the client requests the server to reply to this message
             immediately. This can be used to ping the server, to test if
             the connection is still healthy.
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>

       <varlistentry id="protocol-replication-hot-standby-feedback-message">
        <term>Hot standby feedback message (F)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('h')</term>
           <listitem>
            <para>
             Identifies the message as a hot standby feedback message.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem>
            <para>
             The client's system clock at the time of transmission, as
             microseconds since midnight on 2000-01-01.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int32</term>
           <listitem>
            <para>
             The standby's current global <literal>xmin</literal>, excluding the
             <literal>catalog_xmin</literal> from any replication slots. If both
             this value and the following <literal>catalog_xmin</literal>
             are 0, this is treated as a notification that hot standby feedback
             will no longer be sent on this connection. Later non-zero messages
             may reinitiate the feedback mechanism.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int32</term>
           <listitem>
            <para>
             The epoch of the global <literal>xmin</literal> xid on the standby.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int32</term>
           <listitem>
            <para>
             The lowest <literal>catalog_xmin</literal> of any replication
             slots on the standby. Set to 0 if no <literal>catalog_xmin</literal>
             exists on the standby or if hot standby feedback is being
             disabled.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term>Int32</term>
           <listitem>
            <para>
             The epoch of the <literal>catalog_xmin</literal> xid on the standby.
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-start-replication-slot-logical">
     <term><literal>START_REPLICATION</literal> <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <literal>LOGICAL</literal> <replaceable class="parameter">XXX/XXX</replaceable> [ ( <replaceable>option_name</replaceable> [ <replaceable>option_value</replaceable> ] [, ...] ) ]</term>
     <listitem>
      <para>
       Instructs server to start streaming WAL for logical replication,
       starting at either WAL location <replaceable
       class="parameter">XXX/XXX</replaceable> or the slot's
       <literal>confirmed_flush_lsn</literal> (see <xref
       linkend="view-pg-replication-slots"/>), whichever is greater. This
       behavior makes it easier for clients to avoid updating their local LSN
       status when there is no data to process. However, starting at a
       different LSN than requested might not catch certain kinds of client
       errors; so the client may wish to check that
       <literal>confirmed_flush_lsn</literal> matches its expectations before
       issuing <literal>START_REPLICATION</literal>.
      </para>

      <para>
       The server can reply with an error, for example if the
       slot does not exist. On success, the server responds with a CopyBothResponse
       message, and then starts to stream WAL to the frontend.
      </para>

      <para>
       The messages inside the CopyBothResponse messages are of the same format
       documented for <literal>START_REPLICATION ... PHYSICAL</literal>, including
       two CommandComplete messages.
      </para>

      <para>
       The output plugin associated with the selected slot is used
       to process the output for streaming.
      </para>

      <variablelist>
       <varlistentry>
        <term><literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
         <para>
          The name of the slot to stream changes from. This parameter is required,
          and must correspond to an existing logical replication slot created
          with <literal>CREATE_REPLICATION_SLOT</literal> in
          <literal>LOGICAL</literal> mode.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="parameter">XXX/XXX</replaceable></term>
        <listitem>
         <para>
          The WAL location to begin streaming at.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="parameter">option_name</replaceable></term>
        <listitem>
         <para>
          The name of an option passed to the slot's logical decoding output
          plugin.  See <xref linkend="protocol-logical-replication"/> for
          options that are accepted by the standard (<literal>pgoutput</literal>)
          plugin.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="parameter">option_value</replaceable></term>
        <listitem>
         <para>
          Optional value, in the form of a string constant, associated with the
          specified option.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-drop-replication-slot">
     <term>
      <literal>DROP_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <optional> <literal>WAIT</literal> </optional>
      <indexterm><primary>DROP_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Drops a replication slot, freeing any reserved server-side resources.
      </para>

      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
         <para>
          The name of the slot to drop.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>WAIT</literal></term>
        <listitem>
         <para>
          This option causes the command to wait if the slot is active until
          it becomes inactive, instead of the default behavior of raising an
          error.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-upload-manifest">
     <term>
      <literal>UPLOAD_MANIFEST</literal>
      <indexterm><primary>UPLOAD_MANIFEST</primary></indexterm>
     </term>
     <listitem>
      <para>
       Uploads a backup manifest in preparation for taking an incremental
       backup.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="protocol-replication-base-backup" xreflabel="BASE_BACKUP">
     <term><literal>BASE_BACKUP</literal> [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ]
      <indexterm><primary>BASE_BACKUP</primary></indexterm>
     </term>
     <listitem>
      <para>
       Instructs the server to start streaming a base backup.
       The system will automatically be put in backup mode before the backup
       is started, and taken out of it when the backup is complete. The
       following options are accepted:

       <variablelist>
        <varlistentry>
         <term><literal>LABEL</literal> <replaceable>'label'</replaceable></term>
         <listitem>
          <para>
           Sets the label of the backup. If none is specified, a backup label
           of <literal>base backup</literal> will be used. The quoting rules
           for the label are the same as a standard SQL string with
           <xref linkend="guc-standard-conforming-strings"/> turned on.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>TARGET</literal> <replaceable>'target'</replaceable></term>
         <listitem>
          <para>
           Tells the server where to send the backup.  If the target is
           <literal>client</literal>, which is the default, the backup data is
           sent to the client. If it is <literal>server</literal>, the backup
           data is written to the server at the pathname specified by the
           <literal>TARGET_DETAIL</literal> option. If it is
           <literal>blackhole</literal>, the backup data is not sent
           anywhere; it is simply discarded.
          </para>

          <para>
           The <literal>server</literal> target requires superuser privilege or
           being granted the <literal>pg_write_server_files</literal> role.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>TARGET_DETAIL</literal> <replaceable>'detail'</replaceable></term>
         <listitem>
          <para>
           Provides additional information about the backup target.
          </para>

          <para>
           Currently, this option can only be used when the backup target is
           <literal>server</literal>. It specifies the server directory
           to which the backup should be written.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>PROGRESS [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
          <para>
           If set to true, request information required to generate a progress
           report. This will send back an approximate size in the header of each
           tablespace, which can be used to calculate how far along the stream
           is done. This is calculated by enumerating all the file sizes once
           before the transfer is even started, and might as such have a
           negative impact on the performance.  In particular, it might take
           longer before the first data
           is streamed. Since the database files can change during the backup,
           the size is only approximate and might both grow and shrink between
           the time of approximation and the sending of the actual files.
           The default is false.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>CHECKPOINT { 'fast' | 'spread' }</literal></term>
         <listitem>
          <para>
           Sets the type of checkpoint to be performed at the beginning of the
           base backup. The default is <literal>spread</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>WAL [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
          <para>
           If set to true, include the necessary WAL segments in the backup.
           This will include all the files between start and stop backup in the
           <filename>pg_wal</filename> directory of the base directory tar
           file. The default is false.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>WAIT [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
          <para>
           If set to true, the backup will wait until the last required WAL
           segment has been archived, or emit a warning if WAL archiving is
           not enabled. If false, the backup will neither wait nor warn,
           leaving the client responsible for ensuring the required log is
           available. The default is true.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>COMPRESSION</literal> <replaceable>'method'</replaceable></term>
         <listitem>
          <para>
           Instructs the server to compress the backup using the specified
           method. Currently, the supported methods are <literal>gzip</literal>,
           <literal>lz4</literal>, and <literal>zstd</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>COMPRESSION_DETAIL</literal> <replaceable>detail</replaceable></term>
         <listitem>
          <para>
           Specifies details for the chosen compression method. This should only
           be used in conjunction with the <literal>COMPRESSION</literal>
           option.  If the value is an integer, it specifies the compression
           level.  Otherwise, it should be a comma-separated list of items,
           each of the form <replaceable>keyword</replaceable> or
           <replaceable>keyword=value</replaceable>. Currently, the supported
           keywords are <literal>level</literal>, <literal>long</literal> and
           <literal>workers</literal>.
          </para>

          <para>
           The <literal>level</literal> keyword sets the compression level.
           For <literal>gzip</literal> the compression level should be an
           integer between <literal>1</literal> and <literal>9</literal>
           (default <literal>Z_DEFAULT_COMPRESSION</literal> or
           <literal>-1</literal>), for <literal>lz4</literal> an integer
           between 1 and 12 (default <literal>0</literal> for fast compression
           mode), and for <literal>zstd</literal> an integer between
           <literal>ZSTD_minCLevel()</literal> (usually <literal>-131072</literal>)
           and <literal>ZSTD_maxCLevel()</literal> (usually <literal>22</literal>),
           (default <literal>ZSTD_CLEVEL_DEFAULT</literal> or
           <literal>3</literal>).
          </para>

          <para>
           The <literal>long</literal> keyword enables long-distance matching
           mode, for improved compression ratio, at the expense of higher memory
           use.  Long-distance mode is supported only for
           <literal>zstd</literal>.
          </para>

          <para>
           The <literal>workers</literal> keyword sets the number of threads
           that should be used for parallel compression. Parallel compression
           is supported only for <literal>zstd</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MAX_RATE</literal> <replaceable>rate</replaceable></term>
         <listitem>
          <para>
           Limit (throttle) the maximum amount of data transferred from server
           to client per unit of time.  The expected unit is kilobytes per second.
           If this option is specified, the value must either be equal to zero
           or it must fall within the range from 32 kB through 1 GB (inclusive).
           If zero is passed or the option is not specified, no restriction is
           imposed on the transfer.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>TABLESPACE_MAP [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
          <para>
           If true, include information about symbolic links present in the
           directory <filename>pg_tblspc</filename> in a file named
           <filename>tablespace_map</filename>. The tablespace map file includes
           each symbolic link name as it exists in the directory
           <filename>pg_tblspc/</filename> and the full path of that symbolic link.
           The default is false.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>VERIFY_CHECKSUMS [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
          <para>
           If true, checksums are verified during a base backup if they are
           enabled. If false, this is skipped. The default is true.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MANIFEST</literal> <replaceable>manifest_option</replaceable></term>
         <listitem>
          <para>
           When this option is specified with a value of <literal>yes</literal>
           or <literal>force-encode</literal>, a backup manifest is created
           and sent along with the backup.  The manifest is a list of every
           file present in the backup with the exception of any WAL files that
           may be included. It also stores the size, last modification time, and
           optionally a checksum for each file.
           A value of <literal>force-encode</literal> forces all filenames
           to be hex-encoded; otherwise, this type of encoding is performed only
           for files whose names are non-UTF8 octet sequences.
           <literal>force-encode</literal> is intended primarily for testing
           purposes, to be sure that clients which read the backup manifest
           can handle this case. For compatibility with previous releases,
           the default is <literal>MANIFEST 'no'</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MANIFEST_CHECKSUMS</literal> <replaceable>checksum_algorithm</replaceable></term>
         <listitem>
          <para>
           Specifies the checksum algorithm that should be applied to each file included
           in the backup manifest. Currently, the available
           algorithms are <literal>NONE</literal>, <literal>CRC32C</literal>,
           <literal>SHA224</literal>, <literal>SHA256</literal>,
           <literal>SHA384</literal>, and <literal>SHA512</literal>.
           The default is <literal>CRC32C</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>INCREMENTAL</literal></term>
         <listitem>
          <para>
           Requests an incremental backup. The
           <literal>UPLOAD_MANIFEST</literal> command must be executed
           before running a base backup with this option.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>

      <para>
       When the backup is started, the server will first send two
       ordinary result sets, followed by one or more CopyOutResponse
       results.
      </para>

      <para>
       The first ordinary result set contains the starting position of the
       backup, in a single row with two columns. The first column contains
       the start position given in XLogRecPtr format, and the second column
       contains the corresponding timeline ID.
      </para>

      <para>
       The second ordinary result set has one row for each tablespace.
       The fields in this row are:

       <variablelist>
        <varlistentry>
         <term><literal>spcoid</literal> (<type>oid</type>)</term>
         <listitem>
          <para>
           The OID of the tablespace, or null if it's the base
           directory.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>spclocation</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The full path of the tablespace directory, or null
           if it's the base directory.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>size</literal> (<type>int8</type>)</term>
         <listitem>
          <para>
           The approximate size of the tablespace, in kilobytes (1024 bytes),
           if progress report has been requested; otherwise it's null.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>

      <para>
       After the second regular result set, a CopyOutResponse will be sent.
       The payload of each CopyData message will contain a message in one of
       the following formats:
      </para>

      <variablelist>
       <varlistentry>
        <term>new archive (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('n')</term>
           <listitem><para>
            Identifies the message as indicating the start of a new archive.
            There will be one archive for the main data directory and one
            for each additional tablespace; each will use tar format
            (following the <quote>ustar interchange format</quote> specified
            in the POSIX 1003.1-2008 standard).
           </para></listitem>
          </varlistentry>

          <varlistentry>
           <term>String</term>
           <listitem><para>
            The file name for this archive.
           </para></listitem>
          </varlistentry>

          <varlistentry>
           <term>String</term>
           <listitem><para>
            For the main data directory, an empty string. For other
            tablespaces, the full path to the directory from which this
            archive was created.
           </para></listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term>manifest (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('m')</term>
           <listitem><para>
            Identifies the message as indicating the start of the backup
            manifest.
           </para></listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term>archive or manifest data (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('d')</term>
           <listitem><para>
            Identifies the message as containing archive or manifest data.
           </para></listitem>
          </varlistentry>

          <varlistentry>
           <term>Byte<replaceable>n</replaceable></term>
           <listitem><para>
            Data bytes.
           </para></listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term>progress report (B)</term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>Byte1('p')</term>
           <listitem><para>
            Identifies the message as a progress report.
           </para></listitem>
          </varlistentry>

          <varlistentry>
           <term>Int64</term>
           <listitem><para>
            The number of bytes from the current tablespace for which
            processing has been completed.
           </para></listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>
       After the CopyOutResponse, or all such responses, have been sent, a
       final ordinary result set will be sent, containing the WAL end position
       of the backup, in the same format as the start position.
      </para>

      <para>
       The tar archive for the data directory and each tablespace will contain
       all files in the directories, regardless of whether they are
       <productname>PostgreSQL</productname> files or other files added to the same
       directory. The only excluded files are:

       <itemizedlist spacing="compact" mark="bullet">
        <listitem>
         <para>
          <filename>postmaster.pid</filename>
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>postmaster.opts</filename>
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_internal.init</filename> (found in multiple directories)
         </para>
        </listitem>
        <listitem>
         <para>
          Various temporary files and directories created during the operation
          of the PostgreSQL server, such as any file or directory beginning
          with <filename>pgsql_tmp</filename> and temporary relations.
         </para>
        </listitem>
        <listitem>
         <para>
          Unlogged relations, except for the init fork which is required to
          recreate the (empty) unlogged relation on recovery.
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_wal</filename>, including subdirectories. If the backup is run
          with WAL files included, a synthesized version of <filename>pg_wal</filename> will be
          included, but it will only contain the files necessary for the
          backup to work, not the rest of the contents.
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_dynshmem</filename>, <filename>pg_notify</filename>,
          <filename>pg_replslot</filename>, <filename>pg_serial</filename>,
          <filename>pg_snapshots</filename>, <filename>pg_stat_tmp</filename>, and
          <filename>pg_subtrans</filename> are copied as empty directories (even if
          they are symbolic links).
         </para>
        </listitem>
        <listitem>
         <para>
          Files other than regular files and directories, such as symbolic
          links (other than for the directories listed above) and special
          device and operating system files, are skipped.  (Symbolic links
          in <filename>pg_tblspc</filename> are maintained.)
         </para>
        </listitem>
       </itemizedlist>
       Owner, group, and file mode are set if the underlying file system on
       the server supports it.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   In all the above commands,
   when specifying a parameter of type <type>boolean</type> the
   <replaceable class="parameter">value</replaceable> part can be omitted,
   which is equivalent to specifying <literal>TRUE</literal>.
  </para>
 </sect1>

<sect1 id="protocol-logical-replication">
 <title>Logical Streaming Replication Protocol</title>

 <para>
  This section describes the logical replication protocol, which is the message
  flow started by the <literal>START_REPLICATION</literal>
  <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable>
  <literal>LOGICAL</literal> replication command.
 </para>

 <para>
  The logical streaming replication protocol builds on the primitives of
  the physical streaming replication protocol.
 </para>

 <para>
  <productname>PostgreSQL</productname> logical decoding supports output
  plugins.  <literal>pgoutput</literal> is the standard one used for
  the built-in logical replication.
 </para>

 <sect2 id="protocol-logical-replication-params">
  <title>Logical Streaming Replication Parameters</title>

  <para>
   Using the <literal>START_REPLICATION</literal> command,
   <literal>pgoutput</literal> accepts the following options:

   <variablelist>
    <varlistentry>
     <term>
      proto_version
     </term>
     <listitem>
      <para>
       Protocol version. Currently versions <literal>1</literal>, <literal>2</literal>,
       <literal>3</literal>, and <literal>4</literal> are supported.  A valid
       version is required.
      </para>
      <para>
       Version <literal>2</literal> is supported only for server version 14
       and above, and it allows streaming of large in-progress transactions.
      </para>
      <para>
       Version <literal>3</literal> is supported only for server version 15
       and above, and it allows streaming of two-phase commits.
      </para>
      <para>
       Version <literal>4</literal> is supported only for server version 16
       and above, and it allows streams of large in-progress transactions to
       be applied in parallel.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      publication_names
     </term>
     <listitem>
      <para>
       Comma-separated list of publication names for which to subscribe
       (receive changes). The individual publication names are treated
       as standard objects names and can be quoted the same as needed.
       At least one publication name is required.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      binary
     </term>
     <listitem>
      <para>
       Boolean option to use binary transfer mode.  Binary mode is faster
       than the text mode but slightly less robust.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      messages
     </term>
     <listitem>
      <para>
       Boolean option to enable sending the messages that are written
       by <function>pg_logical_emit_message</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      streaming
     </term>
     <listitem>
      <para>
       Option to enable streaming of in-progress transactions. Valid values are
       <literal>off</literal> (the default), <literal>on</literal> and
       <literal>parallel</literal>. The setting <literal>parallel</literal>
       enables sending extra information with some messages to be used for
       parallelization. Minimum protocol version 2 is required to turn it
       <literal>on</literal>.  Minimum protocol version 4 is required for the
       <literal>parallel</literal> value.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      two_phase
     </term>
     <listitem>
      <para>
       Boolean option to enable two-phase transactions.   Minimum protocol
       version 3 is required to turn it on.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      origin
     </term>
     <listitem>
      <para>
       Option to send changes by their origin.  Possible values are
       <literal>none</literal> to only send the changes that have no origin
       associated, or <literal>any</literal>
       to send the changes regardless of their origin.  This can be used
       to avoid loops (infinite replication of the same data) among
       replication nodes.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </para>
 </sect2>

 <sect2 id="protocol-logical-messages">
  <title>Logical Replication Protocol Messages</title>

  <para>
   The individual protocol messages are discussed in the following
   subsections. Individual messages are described in
   <xref linkend="protocol-logicalrep-message-formats"/>.
  </para>

  <para>
   All top-level protocol messages begin with a message type byte.
   While represented in code as a character, this is a signed byte with no
   associated encoding.
  </para>

  <para>
   Since the streaming replication protocol supplies a message length there
   is no need for top-level protocol messages to embed a length in their
   header.
  </para>

 </sect2>

 <sect2 id="protocol-logical-messages-flow">
  <title>Logical Replication Protocol Message Flow</title>

  <para>
   With the exception of the <literal>START_REPLICATION</literal> command and
   the replay progress messages, all information flows only from the backend
   to the frontend.
  </para>

  <para>
   The logical replication protocol sends individual transactions one by one.
   This means that all messages between a pair of Begin and Commit messages
   belong to the same transaction. Similarly, all messages between a pair of
   Begin Prepare and Prepare messages belong to the same transaction.
   It also sends changes of large in-progress transactions between a pair of
   Stream Start and Stream Stop messages. The last stream of such a transaction
   contains a Stream Commit or Stream Abort message.
  </para>

  <para>
   Every sent transaction contains zero or more DML messages (Insert,
   Update, Delete). In case of a cascaded setup it can also contain Origin
   messages. The origin message indicates that the transaction originated on
   different replication node. Since a replication node in the scope of logical
   replication protocol can be pretty much anything, the only identifier
   is the origin name. It's downstream's responsibility to handle this as
   needed (if needed). The Origin message is always sent before any DML
   messages in the transaction.
  </para>

  <para>
   Every DML message contains a relation OID, identifying the publisher's
   relation that was acted on.  Before the first DML message for a given
   relation OID, a Relation message will be sent, describing the schema of
   that relation.  Subsequently, a new Relation message will be sent if
   the relation's definition has changed since the last Relation message
   was sent for it.  (The protocol assumes that the client is capable of
   remembering this metadata for as many relations as needed.)
  </para>

  <para>
   Relation messages identify column types by their OIDs.  In the case
   of a built-in type, it is assumed that the client can look up that
   type OID locally, so no additional data is needed.  For a non-built-in
   type OID, a Type message will be sent before the Relation message,
   to provide the type name associated with that OID.  Thus, a client that
   needs to specifically identify the types of relation columns should
   cache the contents of Type messages, and first consult that cache to
   see if the type OID is defined there.  If not, look up the type OID
   locally.
  </para>
 </sect2>
</sect1>

 <sect1 id="protocol-message-types">
  <title>Message Data Types</title>

  <para>
   This section describes the base data types used in messages.
  </para>

  <variablelist>
   <varlistentry>
    <term>Int<replaceable>n</replaceable>(<replaceable>i</replaceable>)</term>
    <listitem>
     <para>
      An <replaceable>n</replaceable>-bit integer in network byte
      order (most significant byte first).
      If <replaceable>i</replaceable> is specified it
      is the exact value that will appear, otherwise the value
      is variable.  Eg. Int16, Int32(42).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>Int<replaceable>n</replaceable>[<replaceable>k</replaceable>]</term>
    <listitem>
     <para>
      An array of <replaceable>k</replaceable>
      <replaceable>n</replaceable>-bit integers, each in network
      byte order.  The array length <replaceable>k</replaceable>
      is always determined by an earlier field in the message.
      Eg. Int16[M].
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>String(<replaceable>s</replaceable>)</term>
    <listitem>
     <para>
      A null-terminated string (C-style string).  There is no
      specific length limitation on strings.
      If <replaceable>s</replaceable> is specified it is the exact
      value that will appear, otherwise the value is variable.
      Eg. String, String("user").
     </para>

     <note>
      <para>
       <emphasis>There is no predefined limit</emphasis> on the length of a string
       that can be returned by the backend.  Good coding strategy for a frontend
       is to use an expandable buffer so that anything that fits in memory can be
       accepted.  If that's not feasible, read the full string and discard trailing
       characters that don't fit into your fixed-size buffer.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>Byte<replaceable>n</replaceable>(<replaceable>c</replaceable>)</term>
    <listitem>
     <para>
      Exactly <replaceable>n</replaceable> bytes.  If the field
      width <replaceable>n</replaceable> is not a constant, it is
      always determinable from an earlier field in the message.
      If <replaceable>c</replaceable> is specified it is the exact
      value.  Eg. Byte2, Byte1('\n').
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect1>

 <sect1 id="protocol-message-formats">
  <title>Message Formats</title>

  <para>
   This section describes the detailed format of each message.  Each is marked to
   indicate that it can be sent by a frontend (F), a backend (B), or both
   (F &amp; B).
   Notice that although each message includes a byte count at the beginning,
   most messages are defined so that the message end can be found without
   reference to the byte count.  This is for historical reasons, as the
   original, now-obsolete protocol version 2 did not have an explicit length
   field.  It also aids validity checking though.
  </para>

  <variablelist>
   <varlistentry id="protocol-message-formats-AuthenticationOk">
    <term>AuthenticationOk (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(0)</term>
       <listitem>
        <para>
         Specifies that the authentication was successful.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationKerberosV5">
    <term>AuthenticationKerberosV5 (B)</term>
    <listitem>

     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(2)</term>
       <listitem>
        <para>
         Specifies that Kerberos V5 authentication is required.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationCleartextPassword">
    <term>AuthenticationCleartextPassword (B)</term>
    <listitem>

     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(3)</term>
       <listitem>
        <para>
         Specifies that a clear-text password is required.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationMD5Password">
    <term>AuthenticationMD5Password (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(12)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(5)</term>
       <listitem>
        <para>
         Specifies that an MD5-encrypted password is required.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte4</term>
       <listitem>
        <para>
         The salt to use when encrypting the password.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationGSS">
    <term>AuthenticationGSS (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(7)</term>
       <listitem>
        <para>
         Specifies that GSSAPI authentication is required.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationGSSContinue">
    <term>AuthenticationGSSContinue (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Specifies that this message contains GSSAPI or SSPI data.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         GSSAPI or SSPI authentication data.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationSSPI">
    <term>AuthenticationSSPI (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(9)</term>
       <listitem>
        <para>
         Specifies that SSPI authentication is required.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationSASL">
    <term>AuthenticationSASL (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(10)</term>
       <listitem>
        <para>
         Specifies that SASL authentication is required.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The message body is a list of SASL authentication mechanisms, in the
      server's order of preference. A zero byte is required as terminator after
      the last authentication mechanism name. For each mechanism, there is the
      following:

      <variablelist>
       <varlistentry>
        <term>String</term>
        <listitem>
         <para>
          Name of a SASL authentication mechanism.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationSASLContinue">
    <term>AuthenticationSASLContinue (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(11)</term>
       <listitem>
        <para>
         Specifies that this message contains a SASL challenge.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         SASL data, specific to the SASL mechanism being used.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-AuthenticationSASLFinal">
    <term>AuthenticationSASLFinal (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as an authentication request.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(12)</term>
       <listitem>
        <para>
         Specifies that SASL authentication has completed.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         SASL outcome "additional data", specific to the SASL mechanism
         being used.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-BackendKeyData">
    <term>BackendKeyData (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('K')</term>
       <listitem>
        <para>
         Identifies the message as cancellation key data.
         The frontend must save these values if it wishes to be
         able to issue CancelRequest messages later.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The process ID of this backend.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The secret key of this backend. This field extends to the end of the
         message, indicated by the length field.
        </para>
        <para>
          The minimum and maximum key length are 4 and 256 bytes, respectively. The
          <productname>PostgreSQL</productname> server only sends keys up to
          32 bytes, but the larger maximum size allows for future server
          versions, as well as connection poolers and other middleware, to use
          longer keys. One possible use case is augmenting the server's key
          with extra information. Middleware is therefore also encouraged to
          not use up all of the bytes, in case multiple middleware
          applications are layered on top of each other, each of which may
          wrap the key with extra data.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
     <para>
      Before protocol version 3.2, the secret key was always 4 bytes long.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Bind">
    <term>Bind (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('B')</term>
       <listitem>
        <para>
         Identifies the message as a Bind command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the destination portal
         (an empty string selects the unnamed portal).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the source prepared statement
         (an empty string selects the unnamed prepared statement).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of parameter format codes that follow
         (denoted <replaceable>C</replaceable> below).
         This can be zero to indicate that there are no parameters
         or that the parameters all use the default format (text);
         or one, in which case the specified format code is applied
         to all parameters; or it can equal the actual number of
         parameters.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>C</replaceable>]</term>
       <listitem>
        <para>
         The parameter format codes.  Each must presently be
         zero (text) or one (binary).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of parameter values that follow (possibly zero).
         This must match the number of parameters needed by the query.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Next, the following pair of fields appear for each parameter:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The length of the parameter value, in bytes (this count
         does not include itself).  Can be zero.
         As a special case, -1 indicates a NULL parameter value.
         No value bytes follow in the NULL case.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The value of the parameter, in the format indicated by the
         associated format code.
         <replaceable>n</replaceable> is the above length.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      After the last parameter, the following fields appear:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of result-column format codes that follow
         (denoted <replaceable>R</replaceable> below).
         This can be zero to indicate that there are no result columns
         or that the result columns should all use the default format
         (text);
         or one, in which case the specified format code is applied
         to all result columns (if any); or it can equal the actual
         number of result columns of the query.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>R</replaceable>]</term>
       <listitem>
        <para>
         The result-column format codes.  Each must presently be
         zero (text) or one (binary).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-BindComplete">
    <term>BindComplete (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('2')</term>
       <listitem>
        <para>
         Identifies the message as a Bind-complete indicator.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CancelRequest">
    <term>CancelRequest (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(80877102)</term>
       <listitem>
        <para>
         The cancel request code.  The value is chosen to contain
         <literal>1234</literal> in the most significant 16 bits, and <literal>5678</literal> in the
         least significant 16 bits.  (To avoid confusion, this code
         must not be the same as any protocol version number.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The process ID of the target backend.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The secret key for the target backend. This field extends to the end of the
         message, indicated by the length field. The maximum key length is 256 bytes.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
     <para>
      Before protocol version 3.2, the secret key was always 4 bytes long.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Close">
    <term>Close (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('C')</term>
       <listitem>
        <para>
         Identifies the message as a Close command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1</term>
       <listitem>
        <para>
         '<literal>S</literal>' to close a prepared statement; or
         '<literal>P</literal>' to close a portal.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the prepared statement or portal to close
         (an empty string selects the unnamed prepared statement
         or portal).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CloseComplete">
    <term>CloseComplete (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('3')</term>
       <listitem>
        <para>
         Identifies the message as a Close-complete indicator.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CommandComplete">
    <term>CommandComplete (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('C')</term>
       <listitem>
        <para>
         Identifies the message as a command-completed response.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
       <para>
        The command tag.  This is usually a single
        word that identifies which SQL command was completed.
       </para>

       <para>
        For an <command>INSERT</command> command, the tag is
        <literal>INSERT <replaceable>oid</replaceable>
        <replaceable>rows</replaceable></literal>, where
        <replaceable>rows</replaceable> is the number of rows
        inserted. <replaceable>oid</replaceable> used to be the object ID
        of the inserted row if <replaceable>rows</replaceable> was 1
        and the target table had OIDs, but OIDs system columns are
        not supported anymore; therefore <replaceable>oid</replaceable>
        is always 0.
       </para>

       <para>
        For a <command>DELETE</command> command, the tag is
        <literal>DELETE <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows deleted.
       </para>

       <para>
        For an <command>UPDATE</command> command, the tag is
        <literal>UPDATE <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows updated.
       </para>

       <para>
        For a <command>MERGE</command> command, the tag is
        <literal>MERGE <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows inserted,
        updated, or deleted.
       </para>

       <para>
        For a <command>SELECT</command> or <command>CREATE TABLE AS</command>
        command, the tag is <literal>SELECT <replaceable>rows</replaceable></literal>
        where <replaceable>rows</replaceable> is the number of rows retrieved.
       </para>

       <para>
        For a <command>MOVE</command> command, the tag is
        <literal>MOVE <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows the
        cursor's position has been changed by.
       </para>

       <para>
        For a <command>FETCH</command> command, the tag is
        <literal>FETCH <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows that
        have been retrieved from the cursor.
       </para>

       <para>
        For a <command>COPY</command> command, the tag is
        <literal>COPY <replaceable>rows</replaceable></literal> where
        <replaceable>rows</replaceable> is the number of rows copied.
        (Note: the row count appears only in
        <productname>PostgreSQL</productname> 8.2 and later.)
       </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyData">
    <term>CopyData (F &amp; B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('d')</term>
       <listitem>
        <para>
         Identifies the message as <command>COPY</command> data.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         Data that forms part of a <command>COPY</command> data stream.  Messages sent
         from the backend will always correspond to single data rows,
         but messages sent by frontends might divide the data stream
         arbitrarily.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyDone">
    <term>CopyDone (F &amp; B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('c')</term>
       <listitem>
        <para>
         Identifies the message as a <command>COPY</command>-complete indicator.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyFail">
    <term>CopyFail (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('f')</term>
       <listitem>
        <para>
         Identifies the message as a <command>COPY</command>-failure indicator.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         An error message to report as the cause of failure.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyInResponse">
    <term>CopyInResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('G')</term>
       <listitem>
        <para>
         Identifies the message as a Start Copy In response.
         The frontend must now send copy-in data (if not
         prepared to do so, send a CopyFail message).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         0 indicates the overall <command>COPY</command> format is textual (rows
         separated by newlines, columns separated by separator
         characters, etc.).
         1 indicates the overall copy format is binary (similar
         to DataRow format).
         See <xref linkend="sql-copy"/>
         for more information.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of columns in the data to be copied
         (denoted <replaceable>N</replaceable> below).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>N</replaceable>]</term>
       <listitem>
        <para>
         The format codes to be used for each column.
         Each must presently be zero (text) or one (binary).
         All must be zero if the overall copy format is textual.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyOutResponse">
    <term>CopyOutResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('H')</term>
       <listitem>
        <para>
         Identifies the message as a Start Copy Out response.
         This message will be followed by copy-out data.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         0 indicates the overall <command>COPY</command> format
         is textual (rows separated by newlines, columns
         separated by separator characters, etc.). 1 indicates
         the overall copy format is binary (similar to DataRow
         format). See <xref linkend="sql-copy"/> for more information.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of columns in the data to be copied
         (denoted <replaceable>N</replaceable> below).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>N</replaceable>]</term>
       <listitem>
        <para>
         The format codes to be used for each column.
         Each must presently be zero (text) or one (binary).
         All must be zero if the overall copy format is textual.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-CopyBothResponse">
    <term>CopyBothResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('W')</term>
       <listitem>
        <para>
         Identifies the message as a Start Copy Both response.
         This message is used only for Streaming Replication.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         0 indicates the overall <command>COPY</command> format
         is textual (rows separated by newlines, columns
         separated by separator characters, etc.). 1 indicates
         the overall copy format is binary (similar to DataRow
         format). See <xref linkend="sql-copy"/> for more information.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of columns in the data to be copied
         (denoted <replaceable>N</replaceable> below).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>N</replaceable>]</term>
       <listitem>
        <para>
         The format codes to be used for each column.
         Each must presently be zero (text) or one (binary).
         All must be zero if the overall copy format is textual.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-DataRow">
    <term>DataRow (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('D')</term>
       <listitem>
        <para>
         Identifies the message as a data row.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of column values that follow (possibly zero).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Next, the following pair of fields appear for each column:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The length of the column value, in bytes (this count
         does not include itself).  Can be zero.
         As a special case, -1 indicates a NULL column value.
         No value bytes follow in the NULL case.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The value of the column, in the format indicated by the
         associated format code.
         <replaceable>n</replaceable> is the above length.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Describe">
    <term>Describe (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('D')</term>
       <listitem>
        <para>
         Identifies the message as a Describe command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1</term>
       <listitem>
        <para>
         '<literal>S</literal>' to describe a prepared statement; or
         '<literal>P</literal>' to describe a portal.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the prepared statement or portal to describe
         (an empty string selects the unnamed prepared statement
         or portal).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-EmptyQueryResponse">
    <term>EmptyQueryResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('I')</term>
       <listitem>
        <para>
         Identifies the message as a response to an empty query string.
         (This substitutes for CommandComplete.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-ErrorResponse">
    <term>ErrorResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('E')</term>
       <listitem>
        <para>
         Identifies the message as an error.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The message body consists of one or more identified fields,
      followed by a zero byte as a terminator.  Fields can appear in
      any order.  For each field there is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>Byte1</term>
       <listitem>
        <para>
         A code identifying the field type; if zero, this is
         the message terminator and no string follows.
         The presently defined field types are listed in
         <xref linkend="protocol-error-fields"/>.
         Since more field types might be added in future,
         frontends should silently ignore fields of unrecognized
         type.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The field value.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Execute">
    <term>Execute (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('E')</term>
       <listitem>
        <para>
         Identifies the message as an Execute command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the portal to execute
         (an empty string selects the unnamed portal).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Maximum number of rows to return, if portal contains
         a query that returns rows (ignored otherwise).  Zero
         denotes <quote>no limit</quote>.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Flush">
    <term>Flush (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('H')</term>
       <listitem>
        <para>
         Identifies the message as a Flush command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-FunctionCall">
    <term>FunctionCall (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('F')</term>
       <listitem>
        <para>
         Identifies the message as a function call.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Specifies the object ID of the function to call.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of argument format codes that follow
         (denoted <replaceable>C</replaceable> below).
         This can be zero to indicate that there are no arguments
         or that the arguments all use the default format (text);
         or one, in which case the specified format code is applied
         to all arguments; or it can equal the actual number of
         arguments.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16[<replaceable>C</replaceable>]</term>
       <listitem>
        <para>
         The argument format codes.  Each must presently be
         zero (text) or one (binary).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         Specifies the number of arguments being supplied to the
         function.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Next, the following pair of fields appear for each argument:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The length of the argument value, in bytes (this count
         does not include itself).  Can be zero.
         As a special case, -1 indicates a NULL argument value.
         No value bytes follow in the NULL case.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The value of the argument, in the format indicated by the
         associated format code.
         <replaceable>n</replaceable> is the above length.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      After the last argument, the following field appears:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The format code for the function result. Must presently be
         zero (text) or one (binary).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-FunctionCallResponse">
    <term>FunctionCallResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('V')</term>
       <listitem>
        <para>
         Identifies the message as a function call result.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The length of the function result value, in bytes (this count
         does not include itself).  Can be zero.
         As a special case, -1 indicates a NULL function result.
         No value bytes follow in the NULL case.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The value of the function result, in the format indicated by
         the associated format code.
         <replaceable>n</replaceable> is the above length.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-GSSENCRequest">
    <term>GSSENCRequest (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(80877104)</term>
       <listitem>
        <para>
         The <acronym>GSSAPI</acronym> Encryption request code.  The value is chosen to contain
         <literal>1234</literal> in the most significant 16 bits, and <literal>5680</literal> in the
         least significant 16 bits.  (To avoid confusion, this code
         must not be the same as any protocol version number.)
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-GSSResponse">
    <term>GSSResponse (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('p')</term>
       <listitem>
        <para>
         Identifies the message as a GSSAPI or SSPI response. Note that
         this is also used for SASL and password response messages.
         The exact message type can be deduced from the context.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         GSSAPI/SSPI specific message data.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-NegotiateProtocolVersion">
    <term>NegotiateProtocolVersion (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('v')</term>
       <listitem>
        <para>
         Identifies the message as a protocol version negotiation
         message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Newest minor protocol version supported by the server
         for the major protocol version requested by the client.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Number of protocol options not recognized by the server.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Then, for protocol option not recognized by the server, there
      is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The option name.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-NoData">
    <term>NoData (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('n')</term>
       <listitem>
        <para>
         Identifies the message as a no-data indicator.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-NoticeResponse">
    <term>NoticeResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('N')</term>
       <listitem>
        <para>
         Identifies the message as a notice.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The message body consists of one or more identified fields,
      followed by a zero byte as a terminator.  Fields can appear in
      any order.  For each field there is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>Byte1</term>
       <listitem>
        <para>
         A code identifying the field type; if zero, this is
         the message terminator and no string follows.
         The presently defined field types are listed in
         <xref linkend="protocol-error-fields"/>.
         Since more field types might be added in future,
         frontends should silently ignore fields of unrecognized
         type.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The field value.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-NotificationResponse">
    <term>NotificationResponse (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('A')</term>
       <listitem>
        <para>
         Identifies the message as a notification response.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The process ID of the notifying backend process.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the channel that the notify has been raised on.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The <quote>payload</quote> string passed from the notifying process.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-ParameterDescription">
    <term>ParameterDescription (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('t')</term>
       <listitem>
        <para>
         Identifies the message as a parameter description.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of parameters used by the statement
         (can be zero).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Then, for each parameter, there is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Specifies the object ID of the parameter data type.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-ParameterStatus">
    <term>ParameterStatus (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('S')</term>
       <listitem>
        <para>
         Identifies the message as a run-time parameter status report.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the run-time parameter being reported.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The current value of the parameter.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Parse">
    <term>Parse (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('P')</term>
       <listitem>
        <para>
         Identifies the message as a Parse command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The name of the destination prepared statement
         (an empty string selects the unnamed prepared statement).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The query string to be parsed.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The number of parameter data types specified
         (can be zero).  Note that this is not an indication of
         the number of parameters that might appear in the
         query string, only the number that the frontend wants to
         prespecify types for.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Then, for each parameter, there is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Specifies the object ID of the parameter data type.
         Placing a zero here is equivalent to leaving the type
         unspecified.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-ParseComplete">
    <term>ParseComplete (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('1')</term>
       <listitem>
        <para>
         Identifies the message as a Parse-complete indicator.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-PasswordMessage">
    <term>PasswordMessage (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('p')</term>
       <listitem>
        <para>
         Identifies the message as a password response. Note that
         this is also used for GSSAPI, SSPI and SASL response messages.
         The exact message type can be deduced from the context.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The password (encrypted, if requested).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-PortalSuspended">
    <term>PortalSuspended (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('s')</term>
       <listitem>
        <para>
         Identifies the message as a portal-suspended indicator.
         Note this only appears if an Execute message's row-count limit
         was reached.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Query">
    <term>Query (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('Q')</term>
       <listitem>
        <para>
         Identifies the message as a simple query.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The query string itself.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-ReadyForQuery">
    <term>ReadyForQuery (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('Z')</term>
       <listitem>
        <para>
         Identifies the message type.  ReadyForQuery is sent
         whenever the backend is ready for a new query cycle.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(5)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1</term>
       <listitem>
        <para>
         Current backend transaction status indicator.
         Possible values are '<literal>I</literal>' if idle (not in
         a transaction block); '<literal>T</literal>' if in a transaction
         block; or '<literal>E</literal>' if in a failed transaction
         block (queries will be rejected until block is ended).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-RowDescription">
    <term>RowDescription (B)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('T')</term>
       <listitem>
        <para>
         Identifies the message as a row description.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         Specifies the number of fields in a row (can be zero).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Then, for each field, there is the following:
     </para>

     <variablelist>
      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The field name.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         If the field can be identified as a column of a specific
         table, the object ID of the table; otherwise zero.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         If the field can be identified as a column of a specific
         table, the attribute number of the column; otherwise zero.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The object ID of the field's data type.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The data type size (see <varname>pg_type.typlen</varname>).
         Note that negative values denote variable-width types.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         The type modifier (see <varname>pg_attribute.atttypmod</varname>).
         The meaning of the modifier is type-specific.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         The format code being used for the field.  Currently will
         be zero (text) or one (binary).  In a RowDescription
         returned from the statement variant of Describe, the
         format code is not yet known and will always be zero.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-SASLInitialResponse">
    <term>SASLInitialResponse (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('p')</term>
       <listitem>
        <para>
         Identifies the message as an initial SASL response. Note that
         this is also used for GSSAPI, SSPI and password response messages.
         The exact message type is deduced from the context.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Name of the SASL authentication mechanism that the client
         selected.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of SASL mechanism specific "Initial Client Response" that
         follows, or -1 if there is no Initial Response.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         SASL mechanism specific "Initial Response".
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-SASLResponse">
    <term>SASLResponse (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('p')</term>
       <listitem>
        <para>
         Identifies the message as a SASL response. Note that
         this is also used for GSSAPI, SSPI and password response messages.
         The exact message type can be deduced from the context.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         SASL mechanism specific message data.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-SSLRequest">
    <term>SSLRequest (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Int32(8)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(80877103)</term>
       <listitem>
        <para>
         The <acronym>SSL</acronym> request code.  The value is chosen to contain
         <literal>1234</literal> in the most significant 16 bits, and <literal>5679</literal> in the
         least significant 16 bits.  (To avoid confusion, this code
         must not be the same as any protocol version number.)
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-StartupMessage">
    <term>StartupMessage (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(196610)</term>
       <listitem>
        <para>
         The protocol version number.  The most significant 16 bits are
         the major version number (3 for the protocol described here).
         The least significant 16 bits are the minor version number
         (2 for the protocol described here).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The protocol version number is followed by one or more pairs of
      parameter name and value strings.  A zero byte is required as a
      terminator after the last name/value pair.
      Parameters can appear in any
      order.  <literal>user</literal> is required, others are optional.
      Each parameter is specified as:
     </para>

     <variablelist>
      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The parameter name.  Currently recognized names are:

         <variablelist>
          <varlistentry>
           <term><literal>user</literal></term>
           <listitem>
            <para>
             The database user name to connect as.  Required;
             there is no default.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term><literal>database</literal></term>
           <listitem>
            <para>
             The database to connect to.  Defaults to the user name.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term><literal>options</literal></term>
           <listitem>
            <para>
             Command-line arguments for the backend.  (This is
             deprecated in favor of setting individual run-time
             parameters.)  Spaces within this string are
             considered to separate arguments, unless escaped with
             a backslash (<literal>\</literal>); write <literal>\\</literal> to
             represent a literal backslash.
            </para>
           </listitem>
          </varlistentry>

          <varlistentry>
           <term><literal>replication</literal></term>
           <listitem>
            <para>
             Used to connect in streaming replication mode, where
             a small set of replication commands can be issued
             instead of SQL statements. Value can be
             <literal>true</literal>, <literal>false</literal>, or
             <literal>database</literal>, and the default is
             <literal>false</literal>. See
             <xref linkend="protocol-replication"/> for details.
            </para>
           </listitem>
          </varlistentry>
         </variablelist>

         In addition to the above, other parameters may be listed.
         Parameter names beginning with <literal>_pq_.</literal> are
         reserved for use as protocol extensions, while others are
         treated as run-time parameters to be set at backend start
         time.  Such settings will be applied during backend start
         (after parsing the command-line arguments if any) and will
         act as session defaults.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The parameter value.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Sync">
    <term>Sync (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('S')</term>
       <listitem>
        <para>
         Identifies the message as a Sync command.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-message-formats-Terminate">
    <term>Terminate (F)</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('X')</term>
       <listitem>
        <para>
         Identifies the message as a termination.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32(4)</term>
       <listitem>
        <para>
         Length of message contents in bytes, including self.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect1>

 <sect1 id="protocol-error-fields">
  <title>Error and Notice Message Fields</title>

  <para>
   This section describes the fields that can appear in ErrorResponse and
   NoticeResponse messages.  Each field type has a single-byte identification
   token.  Note that any given field type should appear at most once per
   message.
  </para>

  <variablelist>
   <varlistentry>
    <term><literal>S</literal></term>
    <listitem>
     <para>
      Severity: the field contents are
      <literal>ERROR</literal>, <literal>FATAL</literal>, or
      <literal>PANIC</literal> (in an error message), or
      <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
      <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message),
      or a localized translation of one of these.  Always present.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>V</literal></term>
    <listitem>
     <para>
      Severity: the field contents are
      <literal>ERROR</literal>, <literal>FATAL</literal>, or
      <literal>PANIC</literal> (in an error message), or
      <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
      <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message).
      This is identical to the <literal>S</literal> field except
      that the contents are never localized.  This is present only in
      messages generated by <productname>PostgreSQL</productname> versions 9.6
      and later.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>C</literal></term>
    <listitem>
     <para>
      Code: the SQLSTATE code for the error (see <xref
      linkend="errcodes-appendix"/>).  Not localizable.  Always present.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>M</literal></term>
    <listitem>
     <para>
      Message: the primary human-readable error message.
      This should be accurate but terse (typically one line).
      Always present.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>D</literal></term>
    <listitem>
     <para>
      Detail: an optional secondary error message carrying more
      detail about the problem.  Might run to multiple lines.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>H</literal></term>
    <listitem>
     <para>
      Hint: an optional suggestion what to do about the problem.
      This is intended to differ from Detail in that it offers advice
      (potentially inappropriate) rather than hard facts.
      Might run to multiple lines.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>P</literal></term>
    <listitem>
     <para>
      Position: the field value is a decimal ASCII integer, indicating
      an error cursor position as an index into the original query string.
      The first character has index 1, and positions are measured in
      characters not bytes.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>p</literal></term>
    <listitem>
     <para>
      Internal position: this is defined the same as the <literal>P</literal>
      field, but it is used when the cursor position refers to an internally
      generated command rather than the one submitted by the client.
      The <literal>q</literal> field will always appear when this field appears.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>q</literal></term>
    <listitem>
     <para>
      Internal query: the text of a failed internally-generated command.
      This could be, for example, an SQL query issued by a PL/pgSQL function.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>W</literal></term>
    <listitem>
     <para>
      Where: an indication of the context in which the error occurred.
      Presently this includes a call stack traceback of active
      procedural language functions and internally-generated queries.
      The trace is one entry per line, most recent first.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>s</literal></term>
    <listitem>
     <para>
      Schema name: if the error was associated with a specific database
      object, the name of the schema containing that object, if any.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>t</literal></term>
    <listitem>
     <para>
      Table name: if the error was associated with a specific table, the
      name of the table.  (Refer to the schema name field for the name of
      the table's schema.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>c</literal></term>
    <listitem>
     <para>
      Column name: if the error was associated with a specific table column,
      the name of the column.  (Refer to the schema and table name fields to
      identify the table.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>d</literal></term>
    <listitem>
     <para>
      Data type name: if the error was associated with a specific data type,
      the name of the data type.  (Refer to the schema name field for the
      name of the data type's schema.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>n</literal></term>
    <listitem>
     <para>
      Constraint name: if the error was associated with a specific
      constraint, the name of the constraint.  Refer to fields listed above
      for the associated table or domain.  (For this purpose, indexes are
      treated as constraints, even if they weren't created with constraint
      syntax.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>F</literal></term>
    <listitem>
     <para>
      File: the file name of the source-code location where the error
      was reported.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>L</literal></term>
    <listitem>
     <para>
      Line: the line number of the source-code location where the error
      was reported.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>R</literal></term>
    <listitem>
     <para>
      Routine: the name of the source-code routine reporting the error.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <note>
   <para>
    The fields for schema name, table name, column name, data type name, and
    constraint name are supplied only for a limited number of error types;
    see <xref linkend="errcodes-appendix"/>.  Frontends should not assume that
    the presence of any of these fields guarantees the presence of another
    field.  Core error sources observe the interrelationships noted above, but
    user-defined functions may use these fields in other ways.  In the same
    vein, clients should not assume that these fields denote contemporary
    objects in the current database.
   </para>
  </note>

  <para>
   The client is responsible for formatting displayed information to meet its
   needs; in particular it should break long lines as needed.  Newline characters
   appearing in the error message fields should be treated as paragraph breaks,
   not line breaks.
  </para>
 </sect1>

 <sect1 id="protocol-logicalrep-message-formats">
  <title>Logical Replication Message Formats</title>

  <para>
   This section describes the detailed format of each logical replication
   message.  These messages are either returned by the replication slot SQL
   interface or are sent by a walsender.  In the case of a walsender, they are
   encapsulated inside replication protocol WAL messages as described in
   <xref linkend="protocol-replication"/>, and generally obey the same message
   flow as physical replication.
  </para>

  <variablelist>
   <varlistentry id="protocol-logicalrep-message-formats-Begin">
    <term>Begin</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('B')</term>
       <listitem>
        <para>
         Identifies the message as a begin message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The final LSN of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Commit timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Message">
    <term>Message</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('M')</term>
       <listitem>
        <para>
         Identifies the message as a logical decoding message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         Flags; Either 0 for no flags or 1 if the logical decoding
         message is transactional.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the logical decoding message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The prefix of the logical decoding message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Length of the content.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte<replaceable>n</replaceable></term>
       <listitem>
        <para>
         The content of the logical decoding message.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Commit">
    <term>Commit</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('C')</term>
       <listitem>
        <para>
         Identifies the message as a commit message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the commit.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Commit timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Origin">
    <term>Origin</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('O')</term>
       <listitem>
        <para>
         Identifies the message as an origin message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the commit on the origin server.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Name of the origin.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Note that there can be multiple Origin messages inside a single transaction.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Relation">
    <term>Relation</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('R')</term>
       <listitem>
        <para>
         Identifies the message as a relation message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the relation.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Namespace (empty string for <literal>pg_catalog</literal>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Relation name.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         Replica identity setting for the relation (same as
         <structfield>relreplident</structfield> in <structname>pg_class</structname>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         Number of columns.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Next, the following message part appears for each column included in
      the publication:
     </para>

     <variablelist>
      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         Flags for the column. Currently can be either 0 for no flags
         or 1 which marks the column as part of the key.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Name of the column.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the column's data type.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Type modifier of the column (<structfield>atttypmod</structfield>).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Type">
    <term>Type</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('Y')</term>
       <listitem>
        <para>
         Identifies the message as a type message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the data type.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Namespace (empty string for <literal>pg_catalog</literal>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         Name of the data type.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Insert">
    <term>Insert</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('I')</term>
       <listitem>
        <para>
         Identifies the message as an insert message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the relation corresponding to the ID in the relation
         message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('N')</term>
       <listitem>
        <para>
         Identifies the following TupleData message as a new tuple.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>TupleData</term>
       <listitem>
        <para>
         TupleData message part representing the contents of new tuple.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Update">
    <term>Update</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('U')</term>
       <listitem>
        <para>
         Identifies the message as an update message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the relation corresponding to the ID in the relation
         message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('K')</term>
       <listitem>
        <para>
         Identifies the following TupleData submessage as a key.
         This field is optional and is only present if
         the update changed data in any of the column(s) that are
         part of the REPLICA IDENTITY index.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('O')</term>
       <listitem>
        <para>
         Identifies the following TupleData submessage as an old tuple.
         This field is optional and is only present if table in which
         the update happened has REPLICA IDENTITY set to FULL.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>TupleData</term>
       <listitem>
        <para>
         TupleData message part representing the contents of the old tuple
         or primary key. Only present if the previous 'O' or 'K' part
         is present.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('N')</term>
       <listitem>
        <para>
         Identifies the following TupleData message as a new tuple.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>TupleData</term>
       <listitem>
        <para>
         TupleData message part representing the contents of a new tuple.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The Update message may contain either a 'K' message part or an 'O' message part
      or neither of them, but never both of them.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Delete">
    <term>Delete</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('D')</term>
       <listitem>
        <para>
         Identifies the message as a delete message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the relation corresponding to the ID in the relation
         message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('K')</term>
       <listitem>
        <para>
         Identifies the following TupleData submessage as a key.
         This field is present if the table in which the delete has
         happened uses an index as REPLICA IDENTITY.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Byte1('O')</term>
       <listitem>
        <para>
         Identifies the following TupleData message as an old tuple.
         This field is present if the table in which the delete
         happened has REPLICA IDENTITY set to FULL.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>TupleData</term>
       <listitem>
        <para>
         TupleData message part representing the contents of the old tuple
         or primary key, depending on the previous field.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      The Delete message may contain either a 'K' message part or an 'O' message part,
      but never both of them.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Truncate">
    <term>Truncate</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('T')</term>
       <listitem>
        <para>
         Identifies the message as a truncate message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction (only present for streamed transactions).
         This field is available since protocol version 2.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32</term>
       <listitem>
        <para>
         Number of relations
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         Option bits for <command>TRUNCATE</command>:
         1 for <literal>CASCADE</literal>, 2 for <literal>RESTART IDENTITY</literal>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (Oid)</term>
       <listitem>
        <para>
         OID of the relation corresponding to the ID in the relation
         message.  This field is repeated for each relation.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The following messages (Stream Start, Stream Stop, Stream Commit, and
   Stream Abort) are available since protocol version 2.
  </para>

  <variablelist>
   <varlistentry id="protocol-logicalrep-message-formats-Stream-Start">
    <term>Stream Start</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('S')</term>
       <listitem>
        <para>
         Identifies the message as a stream start message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8</term>
       <listitem>
        <para>
         A value of 1 indicates this is the first stream segment for
         this XID, 0 for any other stream segment.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Stream-Stop">
    <term>Stream Stop</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('E')</term>
       <listitem>
        <para>
         Identifies the message as a stream stop message.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Stream-Commit">
    <term>Stream Commit</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('c')</term>
       <listitem>
        <para>
         Identifies the message as a stream commit message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the commit.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Commit timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Stream-Abort">
    <term>Stream Abort</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('A')</term>
       <listitem>
        <para>
         Identifies the message as a stream abort message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the subtransaction (will be same as xid of the transaction for top-level
         transactions).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the abort operation, present only when streaming is set to parallel.
         This field is available since protocol version 4.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Abort timestamp of the transaction, present only when streaming is set to
         parallel. The value is in number of microseconds since PostgreSQL epoch (2000-01-01).
         This field is available since protocol version 4.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The following messages (Begin Prepare, Prepare, Commit Prepared, Rollback Prepared, Stream Prepare)
   are available since protocol version 3.
  </para>

  <variablelist>
   <varlistentry id="protocol-logicalrep-message-formats-Begin-Prepare">
    <term>Begin Prepare</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('b')</term>
       <listitem>
        <para>
         Identifies the message as the beginning of a prepared transaction message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the prepare.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Prepare timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The user defined GID of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Prepare">
    <term>Prepare</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('P')</term>
       <listitem>
        <para>
         Identifies the message as a prepared transaction message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the prepare.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Prepare timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The user defined GID of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Commit-Prepared">
    <term>Commit Prepared</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('K')</term>
       <listitem>
        <para>
         Identifies the message as the commit of a prepared transaction message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the commit of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the commit of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Commit timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The user defined GID of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Rollback-Prepared">
    <term>Rollback Prepared</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('r')</term>
       <listitem>
        <para>
         Identifies the message as the rollback of a prepared transaction message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the rollback of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Prepare timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Rollback timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The user defined GID of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>

   <varlistentry id="protocol-logicalrep-message-formats-Stream-Prepare">
    <term>Stream Prepare</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Byte1('p')</term>
       <listitem>
        <para>
         Identifies the message as a stream prepared transaction message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int8(0)</term>
       <listitem>
        <para>
         Flags; currently unused.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The LSN of the prepare.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (XLogRecPtr)</term>
       <listitem>
        <para>
         The end LSN of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int64 (TimestampTz)</term>
       <listitem>
        <para>
         Prepare timestamp of the transaction. The value is in number
         of microseconds since PostgreSQL epoch (2000-01-01).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Int32 (TransactionId)</term>
       <listitem>
        <para>
         Xid of the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>String</term>
       <listitem>
        <para>
         The user defined GID of the prepared transaction.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The following message parts are shared by the above messages.
  </para>

  <variablelist>
   <varlistentry id="protocol-logicalrep-message-formats-TupleData">
    <term>TupleData</term>
    <listitem>
     <variablelist>
      <varlistentry>
       <term>Int16</term>
       <listitem>
        <para>
         Number of columns.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Next, one of the following submessages appears for each published column:

      <variablelist>
       <varlistentry>
        <term>Byte1('n')</term>
        <listitem>
         <para>
          Identifies the data as NULL value.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
      Or
      <variablelist>
       <varlistentry>
        <term>Byte1('u')</term>
        <listitem>
         <para>
          Identifies unchanged TOASTed value (the actual value is not
          sent).
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
      Or
      <variablelist>
       <varlistentry>
        <term>Byte1('t')</term>
        <listitem>
         <para>
          Identifies the data as text formatted value.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
      Or
      <variablelist>
       <varlistentry>
        <term>Byte1('b')</term>
        <listitem>
         <para>
          Identifies the data as binary formatted value.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term>Int32</term>
        <listitem>
         <para>
          Length of the column value.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term>Byte<replaceable>n</replaceable></term>
        <listitem>
         <para>
          The value of the column, either in binary or in text format.
          (As specified in the preceding format byte).
          <replaceable>n</replaceable> is the above length.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect1>

 <sect1 id="protocol-changes">
  <title>Summary of Changes since Protocol 2.0</title>

  <para>
   This section provides a quick checklist of changes, for the benefit of
   developers trying to update existing client libraries to protocol 3.0.
  </para>

  <para>
   The initial startup packet uses a flexible list-of-strings format
   instead of a fixed format.  Notice that session default values for run-time
   parameters can now be specified directly in the startup packet.  (Actually,
   you could do that before using the <literal>options</literal> field, but given the
   limited width of <literal>options</literal> and the lack of any way to quote
   whitespace in the values, it wasn't a very safe technique.)
  </para>

  <para>
   All messages now have a length count immediately following the message type
   byte (except for startup packets, which have no type byte).  Also note that
   PasswordMessage now has a type byte.
  </para>

  <para>
   ErrorResponse and NoticeResponse ('<literal>E</literal>' and '<literal>N</literal>')
   messages now contain multiple fields, from which the client code can
   assemble an error message of the desired level of verbosity.  Note that
   individual fields will typically not end with a newline, whereas the single
   string sent in the older protocol always did.
  </para>

  <para>
   The ReadyForQuery ('<literal>Z</literal>') message includes a transaction status
   indicator.
  </para>

  <para>
   The distinction between BinaryRow and DataRow message types is gone; the
   single DataRow message type serves for returning data in all formats.
   Note that the layout of DataRow has changed to make it easier to parse.
   Also, the representation of binary values has changed: it is no longer
   directly tied to the server's internal representation.
  </para>

  <para>
   There is a new <quote>extended query</quote> sub-protocol, which adds the frontend
   message types Parse, Bind, Execute, Describe, Close, Flush, and Sync, and the
   backend message types ParseComplete, BindComplete, PortalSuspended,
   ParameterDescription, NoData, and CloseComplete.  Existing clients do not
   have to concern themselves with this sub-protocol, but making use of it
   might allow improvements in performance or functionality.
  </para>

  <para>
   <command>COPY</command> data is now encapsulated into CopyData and CopyDone messages.  There
   is a well-defined way to recover from errors during <command>COPY</command>.  The special
   <quote><literal>\.</literal></quote> last line is not needed anymore, and is not sent
   during <command>COPY OUT</command>.
   (It is still recognized as a terminator during text-mode <command>COPY
   IN</command>, but not in CSV mode.  The text-mode behavior is
   deprecated and may eventually be removed.)  Binary <command>COPY</command> is supported.
   The CopyInResponse and CopyOutResponse messages include fields indicating
   the number of columns and the format of each column.
  </para>

  <para>
   The layout of FunctionCall and FunctionCallResponse messages has changed.
   FunctionCall can now support passing NULL arguments to functions.  It also
   can handle passing parameters and retrieving results in either text or
   binary format.  There is no longer any reason to consider FunctionCall a
   potential security hole, since it does not offer direct access to internal
   server data representations.
  </para>

  <para>
   The backend sends ParameterStatus ('<literal>S</literal>') messages during connection
   startup for all parameters it considers interesting to the client library.
   Subsequently, a ParameterStatus message is sent whenever the active value
   changes for any of these parameters.
  </para>

  <para>
   The RowDescription ('<literal>T</literal>') message carries new table OID and column
   number fields for each column of the described row.  It also shows the format
   code for each column.
  </para>

  <para>
   The CursorResponse ('<literal>P</literal>') message is no longer generated by
   the backend.
  </para>

  <para>
   The NotificationResponse ('<literal>A</literal>') message has an additional string
   field, which can carry a <quote>payload</quote> string passed
   from the <command>NOTIFY</command> event sender.
  </para>

  <para>
   The EmptyQueryResponse ('<literal>I</literal>') message used to include an empty
   string parameter; this has been removed.
  </para>
 </sect1>
</chapter>
