Remove pre-7.4 documentaiton mentions, now that 8.0 is the oldest

[postgresql] / doc / src / sgml / protocol.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/protocol.sgml,v 1.84 2010/02/24 03:33:49 momjian Exp $ -->

<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 3.0 of the protocol, implemented in
  <productname>PostgreSQL</productname> 7.4 and later.  For descriptions
  of the earlier protocol versions, see previous releases of the
  <productname>PostgreSQL</productname> documentation.  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, and then the server follows that protocol
  if it is able.
 </para>

  <para>
   In order to serve multiple clients efficiently, the server launches
   a new <quote>backend</> 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</> and <quote>server</> are
   interchangeable; likewise <quote>frontend</> and <quote>client</>
   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</>)
   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</> protocol, the frontend
   just sends a textual query string, which is parsed and immediately
   executed by the backend.  In the <quote>extended query</> 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</>.
  </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</> and
    <firstterm>portals</>.  A prepared statement represents the result of
    parsing, semantic analysis, and (optionally) planning of a textual query
    string.
    A prepared statement is not necessarily ready to execute, because it might
    lack specific values for <firstterm>parameters</>.  A portal represents
    a ready-to-execute or already-partially-executed statement, with any
    missing parameter values filled in.  (For <command>SELECT</> 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</> statements.)
   </para>

   <para>
    The overall execution cycle consists of a <firstterm>parse</> step,
    which creates a prepared statement from a textual query string; a
    <firstterm>bind</> step, which creates a portal given a prepared
    statement and values for any needed parameters; and an
    <firstterm>execute</> step that runs a portal's query.  In the case of
    a query that returns rows (<command>SELECT</>, <command>SHOW</>, 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</> 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</>.
    The only supported formats are <quote>text</> and <quote>binary</>,
    but the protocol makes provision for future extensions.  The desired
    format for any value is specified by a <firstterm>format code</>.
    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>
 </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>
   <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 and SSPI, 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 and SSPI, 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.
       </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 encrypted via MD5, using the 4-character salt
        specified in the AuthenticationMD5Password message.  If
        this is the correct password, the server responds with an
        AuthenticationOk, otherwise it responds with an ErrorResponse.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSCMCredential</term>
      <listitem>
       <para>
        This response is only possible for local Unix-domain connections
        on platforms that support SCM credential messages.  The frontend
        must issue an SCM credential message and then send a single data
        byte.  (The contents of the data byte are uninteresting; it's
        only used to ensure that the server waits long enough to receive
        the credential message.)  If the credential is acceptable,
        the server responds with an
        AuthenticationOk, otherwise it responds with an ErrorResponse.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationGSS</term>
      <listitem>
       <para>
        The frontend must now initiate a GSSAPI negotiation. The frontend
        will send a PasswordMessage 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 a SSPI negotiation. The frontend
        will send a PasswordMessage 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 PasswordMessage. 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>

    </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), 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>
      </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>
   <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</> query (or other queries that
    return row sets, such as <command>EXPLAIN</> or <command>SHOW</>)
    normally consists of RowDescription, zero or more
    DataRow messages, and then CommandComplete.
    <command>COPY</> 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</> from a cursor
    declared with the <literal>BINARY</> 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>
  </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>$</><replaceable>n</>)
     used in the query string.  Another special case is that a parameter's
     type can be specified as <type>void</> (that is, the OID of the
     <type>void</> pseudotype).  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</> 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)</> could match a function with
     two IN and two OUT arguments, if <literal>$3</> and <literal>$4</>
     are specified as having type <type>void</>.
    </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
    a 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</> and <command>EXECUTE</>.
   </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</> 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</> attribute in cursor declarations is irrelevant when
     using extended query protocol.
    </para>
   </note>

   <para>
    Query planning for named prepared-statement objects occurs when the Parse
    message is processed. If a query will be repeatedly executed with
    different parameters, it might be beneficial to send a single Parse message
    containing a parameterized query, followed by multiple Bind
    and Execute messages. This will avoid replanning the query on each
    execution.
   </para>

   <para>
    The unnamed prepared statement is likewise planned during Parse processing
    if the Parse message defines no parameters.  But if there are parameters,
    query planning occurs every time Bind parameters are supplied.  This allows the
    planner to make use of the actual values of the parameters provided by
    each Bind message, rather than use generic estimates.
   </para>

   <note>
    <para>
     Query plans generated from a parameterized query might be less
     efficient than query plans generated from an equivalent query with actual
     parameter values substituted. The query planner cannot make decisions
     based on actual parameter values (for example, index selectivity) when
     planning a parameterized query assigned to a named prepared-statement
     object.  This possible penalty is avoided when using the unnamed
     statement, since it is not planned until actual parameter values are
     available.  The cost is that planning must occur afresh for each Bind,
     even if the query stays the same.
    </para>
   </note>

   <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 a 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</> and <command>FETCH</>.
   </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</>).
    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>COMMIT</> transaction block (<quote>close</>
    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</> 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</>
     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>
   <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, ...)</>.
     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 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</> 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</> SQL statement to fail with an
    error).  The backend then reverts to the command-processing mode it was
    in before the <command>COPY</> 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 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 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</>.)
   </para>

   <para>
    Copy-out mode (data transfer from the server) is initiated when the
    backend executes a <command>COPY TO STDOUT</> 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</> 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>
    The CopyInResponse and CopyOutResponse 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</> 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</> 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</> 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
    <literal>server_version</>,
    <literal>server_encoding</>,
    <literal>client_encoding</>,
    <literal>application_name</>,
    <literal>is_superuser</>,
    <literal>session_authorization</>,
    <literal>DateStyle</>,
    <literal>IntervalStyle</>,
    <literal>TimeZone</>,
    <literal>integer_datetimes</>, and
    <literal>standard_conforming_strings</>.
    (<literal>server_encoding</>, <literal>TimeZone</>, and
    <literal>integer_datetimes</> were not reported by releases before 8.0;
    <literal>standard_conforming_strings</> was not reported by releases
    before 8.1;
    <literal>IntervalStyle</> was not reported by releases before 8.4;
    <literal>application_name</> was not reported by releases before 9.0.)
    Note that
    <literal>server_version</>,
    <literal>server_encoding</> and
    <literal>integer_datetimes</>
    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>
   <title>Cancelling 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>
   <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>COMMIT</>
    sequence) then its results might be committed before the
    disconnection is recognized.
   </para>
  </sect2>

  <sect2>
   <title><acronym>SSL</acronym> Session Encryption</title>

   <para>
    If <productname>PostgreSQL</> 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</> or <literal>N</>, 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</>, 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</>, send the usual StartupMessage and proceed without
    encryption.
   </para>

   <para>
    The frontend should also be prepared to handle an ErrorMessage
    response to SSLRequest from the server.  This would only occur if
    the server predates the addition of <acronym>SSL</acronym> support
    to <productname>PostgreSQL</>.  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>
    An initial SSLRequest can also be used in a connection that is being
    opened to send a CancelRequest message.
   </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>
 </sect1>

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

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

<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>
</para>
</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,
the message format is defined so that the message end can be found without
reference to the byte count.  This aids validity checking.  (The CopyData
message is an exception, because it forms part of a data stream; the contents
of any individual CopyData message cannot be interpretable on their own.)
</para>

<variablelist>


<varlistentry>
<term>
AuthenticationOk (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationKerberosV5 (B)
</term>
<listitem>
<para>

<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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationCleartextPassword (B)
</term>
<listitem>
<para>

<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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationMD5Password (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationSCMCredential (B)
</term>
<listitem>
<para>

<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(6)
</term>
<listitem>
<para>
                Specifies that an SCM credentials message is required.
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationGSS (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
AuthenticationSSPI (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
AuthenticationGSSContinue (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
BackendKeyData (B)
</term>
<listitem>
<para>

<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(12)
</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>
        Int32
</term>
<listitem>
<para>
                The secret key of this backend.
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Bind (F)
</term>
<listitem>
<para>

<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</> 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</>]
</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>
        Next, the following pair of fields appear for each parameter:
<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>
        After the last parameter, the following fields appear:
<variablelist>
<varlistentry>
<term>
        Int16
</term>
<listitem>
<para>
                The number of result-column format codes that follow
                (denoted <replaceable>R</> 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</>]
</term>
<listitem>
<para>
                The result-column format codes.  Each must presently be
                zero (text) or one (binary).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
BindComplete (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CancelRequest (F)
</term>
<listitem>
<para>

<variablelist>
<varlistentry>
<term>
        Int32(16)
</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</> in the most significant 16 bits, and <literal>5678</> in the
                least 16 significant 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>
        Int32
</term>
<listitem>
<para>
                The secret key for the target backend.
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Close (F)
</term>
<listitem>
<para>

<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</>' to close a prepared statement; or
                '<literal>P</>' 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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CloseComplete (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CommandComplete (B)
</term>
<listitem>
<para>

<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> is the object ID
        of the inserted row if <replaceable>rows</replaceable> is 1
        and the target table has OIDs;
        otherwise <replaceable>oid</replaceable> is 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>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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CopyData (F &amp; B)
</term>
<listitem>
<para>
<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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CopyDone (F &amp; B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CopyFail (F)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CopyInResponse (B)
</term>
<listitem>
<para>

<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" endterm="sql-copy-title">
                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</> below).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
        Int16[<replaceable>N</>]
</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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
CopyOutResponse (B)
</term>
<listitem>
<para>

<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"
                endterm="sql-copy-title"> 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</> below).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
        Int16[<replaceable>N</>]
</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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
DataRow (B)
</term>
<listitem>
<para>
<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>
        Next, the following pair of fields appear for each column:
<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Describe (F)
</term>
<listitem>
<para>

<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</>' to describe a prepared statement; or
                '<literal>P</>' 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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
EmptyQueryResponse (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
ErrorResponse (B)
</term>
<listitem>
<para>

<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>
        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:
<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Execute (F)
</term>
<listitem>
<para>

<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</>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Flush (F)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
FunctionCall (F)
</term>
<listitem>
<para>

<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</> 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</>]
</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>
        Next, the following pair of fields appear for each argument:
<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>
        After the last argument, the following field appears:
<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
FunctionCallResponse (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
NoData (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
NoticeResponse (B)
</term>
<listitem>
<para>

<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>
        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:
<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
NotificationResponse (B)
</term>
<listitem>
<para>

<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</> string passed from the notifying process.
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
ParameterDescription (B)
</term>
<listitem>
<para>

<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>
        Then, for each parameter, there is the following:
<variablelist>
<varlistentry>
<term>
        Int32
</term>
<listitem>
<para>
                Specifies the object ID of the parameter data type.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
ParameterStatus (B)
</term>
<listitem>
<para>

<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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Parse (F)
</term>
<listitem>
<para>

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


<varlistentry>
<term>
ParseComplete (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
PasswordMessage (F)
</term>
<listitem>
<para>

<variablelist>
<varlistentry>
<term>
        Byte1('p')
</term>
<listitem>
<para>
                Identifies the message as a password response. Note that
                this is also used for GSSAPI and SSPI response messages
                (which is really a design error, since the contained data
                is not a null-terminated string in that case, but can be
                arbitrary binary data).
</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>
</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
PortalSuspended (B)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Query (F)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
ReadyForQuery (B)
</term>
<listitem>
<para>

<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</>' if idle (not in
                a transaction block); '<literal>T</>' if in a transaction
                block; or '<literal>E</>' if in a failed transaction
                block (queries will be rejected until block is ended).
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
RowDescription (B)
</term>
<listitem>
<para>

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

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
SSLRequest (F)
</term>
<listitem>
<para>

<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</> in the most significant 16 bits, and <literal>5679</> in the
                least 16 significant bits.  (To avoid confusion, this code
                must not be the same as any protocol version number.)
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
StartupMessage (F)
</term>
<listitem>
<para>

<variablelist>
<varlistentry>
<term>
        Int32
</term>
<listitem>
<para>
                Length of message contents in bytes, including self.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
        Int32(196608)
</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
                (0 for the protocol described here).
</para>
</listitem>
</varlistentry>
</variablelist>
        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</> is required, others are optional.
        Each parameter is specified as:
<variablelist>
<varlistentry>
<term>
        String
</term>
<listitem>
<para>
                The parameter name.  Currently recognized names are:

<variablelist>
<varlistentry>
<term>
                <literal>user</>
</term>
<listitem>
<para>
                        The database user name to connect as.  Required;
                        there is no default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
                <literal>database</>
</term>
<listitem>
<para>
                        The database to connect to.  Defaults to the user name.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
                <literal>options</>
</term>
<listitem>
<para>
                        Command-line arguments for the backend.  (This is
                        deprecated in favor of setting individual run-time
                        parameters.)
</para>
</listitem>
</varlistentry>
</variablelist>

                In addition to the above, any run-time parameter that can be
                set at backend start time might be listed.  Such settings
                will be applied during backend start (after parsing the
                command-line options if any).  The values will act as
                session defaults.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
        String
</term>
<listitem>
<para>
                The parameter value.
</para>
</listitem>
</varlistentry>
</variablelist>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Sync (F)
</term>
<listitem>
<para>

<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>

</para>
</listitem>
</varlistentry>


<varlistentry>
<term>
Terminate (F)
</term>
<listitem>
<para>

<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>

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

<varlistentry>
<term>
<literal>C</>
</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</>
</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</>
</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</>
</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</>
</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</>
</term>
<listitem>
<para>
        Internal position: this is defined the same as the <literal>P</>
        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</> field will always appear when this field appears.
</para>
</listitem>
</varlistentry>

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

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

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

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

</variablelist>

<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-replication">
<title>Streaming Replication Protocol</title>

<para>
To initiate streaming replication, the frontend sends the "replication"
parameter in the startup message. This tells the backend to go into
walsender mode, where a small set of replication commands can be issued
instead of SQL statements. Only the simple query protocol can be used in
walsender mode.

The commands accepted in walsender mode are:

<variablelist>
  <varlistentry>
    <term>IDENTIFY_SYSTEM</term>
    <listitem>
     <para>
      Requests the server to idenfity itself. Server replies with a result
      set of a single row, and two fields:

      systemid: The unique system identifier identifying the cluster. This
      can be used to check that the base backup used to initialize the
      slave came from the same cluster.

      timeline: Current TimelineID. Also used to check that the slave is
      consistent with the master.
     </para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term>START_REPLICATION XXX/XXX</term>
    <listitem>
     <para>
      Instructs backend to start streaming WAL, starting at point XXX/XXX.
      Server can reply with an error e.g if the requested piece of WAL has
      already been recycled. On success, server responds with a
      CopyOutResponse message, and backend starts to stream WAL as CopyData
      messages.
      The payload in CopyData message consists of the following format.
     </para>

     <para>
      <variablelist>
      <varlistentry>
      <term>
          XLogData (B)
      </term>
      <listitem>
      <para>
      <variablelist>
      <varlistentry>
      <term>
          Byte1('w')
      </term>
      <listitem>
      <para>
          Identifies the message as WAL data.
      </para>
      </listitem>
      </varlistentry>
      <varlistentry>
      <term>
          Int32
      </term>
      <listitem>
      <para>
          The log file number of the LSN, indicating the starting point of
          the WAL in the message.
      </para>
      </listitem>
      </varlistentry>
      <varlistentry>
      <term>
          Int32
      </term>
      <listitem>
      <para>
          The byte offset of the LSN, indicating the starting point of
          the WAL in the message.
      </para>
      </listitem>
      </varlistentry>
      <varlistentry>
      <term>
          Byte<replaceable>n</replaceable>
      </term>
      <listitem>
      <para>
          Data that forms part of WAL data stream.
      </para>
      </listitem>
      </varlistentry>
      </variablelist>
      </para>
      </listitem>
      </varlistentry>
      </variablelist>
     </para>
     <para>
       A single WAL record is never split across two CopyData messages. When
       a WAL record crosses a WAL page boundary, however, 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 split across different CopyData messages.
     </para>
    </listitem>
  </varlistentry>
</variablelist>

</para>

</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</> field, but given the
limited width of <literal>options</> 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</>' and '<literal>N</>')
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</>') 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</> 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>\.</></quote> last line is not needed anymore, and is not sent
during <command>COPY OUT</command>.
(It is still recognized as a terminator during <command>COPY IN</command>, but its use is
deprecated and will 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</>') 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</>') 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</>') message is no longer generated by
the backend.
</para>

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

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

</sect1>

</chapter>