<!-- $Header: /cvsroot/pgsql/doc/src/sgml/protocol.sgml,v 1.30 2003/04/24 21:16:42 tgl Exp $ -->

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

   <caution>
    <para>
     This is currently a <emphasis>DRAFT</> description of FE/BE protocol
     version 3.0.  Details are still subject to change.
     In particular, the representation of binary data is still under debate.
    </para>
   </caution>

 <para>
  <application>PostgreSQL</application> 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 may be used.
 </para>

 <para>
  This document describes version 3.0 of the protocol, implemented in
  <application>PostgreSQL</application> 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>
  Higher level features built on this protocol (for example, how
  <application>libpq</application> passes certain environment
  variables when the connection is established) are covered elsewhere.
 </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 may 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 planning of a textual query string.  A
    prepared statement is not necessarily ready to execute, because it may
    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 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 <command>SELECT</> query, the execute step can be told to fetch only
    a limited number of rows, so that multiple execute steps may 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, for use with
    queries that are to be executed and forgotten.  This is slightly
    more efficient than using named objects, since the backend knows that
    it need not save the object's state for re-use.
   </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.  There are several different sub-protocols
   depending on the state of the connection: start-up,
   query, function call, COPY, 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.  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).
    In principle the authentication request/response cycle could require
    multiple iterations, but none of the present authentication methods
    use more than one request and response.  In some methods, no response
    at all is needed from the frontend, and so no authentication request
    occurs.
   </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>AuthenticationKerberosV4</Term>
      <listitem>
       <para>
        The frontend must now take part in a Kerberos V4
        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>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>AuthenticationCryptPassword</Term>
      <ListItem>
       <Para>
        The frontend must now send a PasswordMessage containing the
	password encrypted via crypt(3), using the 2-character salt
	specified in the AuthenticationCryptPassword message.  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>

    </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
    BackendKeyData, some ParameterStatus messages, and finally ReadyForQuery.
   </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 <varname>client_encoding</>
	or <varname>DateStyle</>.  The frontend may ignore this message,
	or record the settings for its future use; see
	<xref linkend="protocol-async"> for more detail.
        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 may 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 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 may 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> or <command>FETCH</command> query.
	The message contents describe the layout of the rows.  This
	will be followed by a DataRow or BinaryRow message (depending on
	whether a binary cursor was specified) for each row being returned
        to the frontend.
       </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 may
        contain multiple SQL commands.  (CommandComplete marks the
        end of processing one SQL command, not the whole string.)
        ReadyForQuery will always be sent, whether processing
        terminates successfully or with an error.
       </Para>
      </ListItem>
     </VarListEntry>

     <VarListEntry>
      <Term>NoticeResponse</Term>
      <ListItem>
       <Para>
        A warning message has been issued in relation to the query.
        Notices are in addition to other responses, i.e., the backend
        will continue processing the command.
       </Para>
      </ListItem>
     </VarListEntry>

    </VariableList>
   </Para>

   <Para>
    The response to a <command>SELECT</>, <command>FETCH</>, or
    <command>SHOW</> query
    normally consists of RowDescription, zero or more
    DataRow or BinaryRow 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
    may occur partway through the sequence of messages generated by an
    individual query.
   </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 may 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>
   <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 datatypes 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 datatypes may be
    specified by OID; if not given, the parser attempts to infer the
    datatypes in the same way as it would do for untyped literal string
    constants.
   </para>

   <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 message is issued.
    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 response is either BindComplete or ErrorResponse.  The
    supplied parameter set must match those needed by the prepared statement.
   </para>

   <para>
    If successfully created, a named portal object lasts till
    the end of the current transaction, unless explicitly destroyed.  An
    unnamed portal is destroyed at the end of the transaction, or as soon
    as the next Parse or Bind message is executed.
    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), the desired output format (text or binary), and
    a maximum result-row count (zero meaning <quote>fetch all rows</>).
    The output format and result-row count are only meaningful for portals
    containing SELECT commands; they are ignored for other types of commands.
    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 to be issued.  Also, the choice between text and binary
    output (DataRow or BinaryRow messages) is determined by Execute's
    format field, regardless of the command; the <literal>BINARY</> attribute
    in cursor declarations is irrelevant when using this protocol.
   </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 resychronization 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 --- 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
    SELECT-type query; or ErrorResponse if there is no such portal.  In most
    situations the frontend will want to issue this message before issuing
    Execute, to obtain a description of the results it will get back.
   </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 (if any), followed by a RowDescription
    message describing the rows that will be returned when the statement is
    eventually executed (or NoData if there is no SELECT-type query in the
    prepared statement).  ErrorResponse is issued if there is no such prepared
    statement.  This message may be useful if the client library is
    uncertain about the parameters needed by a prepared statement.
   </para>

   <para>
    The Close message closes an existing prepared statement or portal
    and releases resources.
   </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, returning data 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, 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 is that it will not return ParseComplete, BindComplete, or
     NoData messages.
    </para>
   </note>
  </sect2>

  <Sect2>
   <Title>Function Call</Title>

   <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 may 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>FunctionResultResponse</Term>
      <ListItem>
       <Para>
        The function call was executed and returned a non-null result.
	(Note that the Function Call protocol can only handle a single
	scalar result, not a rowtype or set of results.)
       </Para>
      </ListItem>
     </VarListEntry>

     <VarListEntry>
      <Term>FunctionVoidResponse</Term>
      <ListItem>
       <Para>
        The function call was executed and returned a NULL value.
       </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, or indeed any frontend message other than
    CopyData or CopyDone), 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>
    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 (short of closing the connection),
    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 (or
    indeed any message type other than CopyData or CopyDone) as terminating
    the copy-out mode.
   </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 --- but it is also possible for parameter
    status changes to occur because the administrator changed a configuration
    file and then SIGHUP'd the postmaster.  Also, if a SET 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>version</> (backend version,
    a pseudo-parameter that cannot change after startup); 
    <literal>database_encoding</> (also not presently changeable after start);
    <literal>client_encoding</>, and
    <literal>DateStyle</>.
    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
    notification 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 may 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 may 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 may or may not have any effect --- 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 may have some benefits of flexibility in 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 may 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-SELECT 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 may be committed
    before the disconnection is recognized.
   </para>
  </sect2>

  <Sect2>
   <Title>SSL Session Encryption</Title>

   <Para>
    If <productname>PostgreSQL</> was built with SSL support, frontend/backend
    communications can be encrypted using SSL.  This provides communication
    security in environments where attackers might be able to capture the
    session traffic.
   </para>

   <para>
    To initiate an SSL-encrypted connection, the frontend initially sends
    an SSLRequest message rather than a StartupMessage.  The server then
    responds with a single byte containing <literal>Y</> or <literal>N</>,
    indicating that it is willing or unwilling to perform SSL, respectively.
    The frontend may close the connection at this point if it is dissatisfied
    with the response.  To continue after <literal>Y</>, perform an SSL
    startup handshake (not described here, part of the SSL 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 SSL-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 SSL support to <productname>PostgreSQL</>.
    In this case the connection must be closed, but the frontend may choose
    to open a fresh connection and proceed without requesting SSL.
   </para>

   <para>
    An initial SSLRequest may 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 SSL encryption, the administrator may 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.
                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>
        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
                <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 may 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
may not 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>
AuthenticationKerberosV4 (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(1)
</Term>
<ListItem>
<Para>
                Specifies that Kerberos V4 authentication is required.
</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 cleartext password is required.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
</ListItem>
</VarListEntry>


<VarListEntry>
<Term>
AuthenticationCryptPassword (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(10)
</Term>
<ListItem>
<Para>
                Length of message contents in bytes, including self.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32(4)
</Term>
<ListItem>
<Para>
                Specifies that a crypt()-encrypted password is required.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte2
</Term>
<ListItem>
<Para>
                The salt to use when encrypting the password.
</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>
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>
BinaryRow (B)
</Term>
<ListItem>
<Para>
<VariableList>
<VarListEntry>
<Term>
        Byte1('B')
</Term>
<ListItem>
<Para>
                Identifies the message as a binary data row.
                (Normally, a prior RowDescription message defines the number
		of fields in the row and their data types.  Note that the
		receiver <emphasis>must</> know the number of fields to be
		able to decode the message contents.)
</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>
                A bit map with one bit for each field in the row.  The 1st
                field corresponds to bit 7 (MSB) of the 1st byte, the 2nd
                field corresponds to bit 6 of the 1st byte, the 8th field
                corresponds to bit 0 (LSB) of the 1st byte, the 9th field
                corresponds to bit 7 of the 2nd byte, and so on.  Each bit
                is set if the value of the corresponding field is not NULL.
                If the number of fields is not a multiple of 8, the remainder
                of the last byte in the bit map is wasted.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        Then, for each field with a non-NULL value, there is the following:
<VariableList>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the size of the value of the field, excluding
                this size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                Specifies the value of the field itself in binary
                format.  <Replaceable>n</Replaceable> is the above size.
</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>
        Int8
</Term>
<ListItem>
<Para>
		0 if parameter values are specified in textual form.
                1 if parameter values are specified in binary form.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                The number of parameter values specified
		(may be zero).  This must match the number of parameters
		needed by the query.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        If parameter values are specified in textual form, the following
	appears for each parameter:
<VariableList>
<VarListEntry>
<Term>
        Int8
</Term>
<ListItem>
<Para>
                1 if the parameter is non-null.  0 if it is null.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String
</Term>
<ListItem>
<Para>
                The parameter value in textual form (that is, suitable
		input for the parameter's datatype's input converter).
		If the preceding byte specified a null parameter, then
		the string is omitted.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        If parameter values are specified in binary form, the following
	appears for each parameter:
<VariableList>
<VarListEntry>
<Term>
        Int16
</Term>
<ListItem>
<Para>
                Zero if the field is null, otherwise the <varname>typlen</>
		for the field datatype.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                The value of the field itself in binary format.
		Omitted if the field is null.
		<Replaceable>n</Replaceable> is the <varname>typlen</>
		value if <varname>typlen</> is positive.  If
		<varname>typlen</> is -1 then the field value begins with
		its own length as an Int32 (the length includes itself).
</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>
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, and <replaceable>oid</replaceable> is the object ID
        of the inserted row if <Replaceable>rows</Replaceable> is 1,
        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>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>
</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 COPY 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 COPY datastream.  Messages sent
		from the backend will always correspond to single data rows,
		but messages sent by frontends may divide the datastream
		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 COPY-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 COPY-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(5)
</Term>
<ListItem>
<Para>
                Length of message contents in bytes, including self.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int8
</Term>
<ListItem>
<Para>
                0 for textual copy, 1 for binary copy.
</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(5)
</Term>
<ListItem>
<Para>
                Length of message contents in bytes, including self.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int8
</Term>
<ListItem>
<Para>
                0 for textual copy, 1 for binary copy.
</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 text-format data row.
                (Normally, a prior RowDescription message defines the number
		of fields in the row and their data types.  Note that the
		receiver <emphasis>must</> know the number of fields to be
		able to decode the message contents.)
</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>
                A bit map with one bit for each field in the row.  The 1st
                field corresponds to bit 7 (MSB) of the 1st byte, the 2nd
                field corresponds to bit 6 of the 1st byte, the 8th field
                corresponds to bit 0 (LSB) of the 1st byte, the 9th field
                corresponds to bit 7 of the 2nd byte, and so on.  Each bit
                is set if the value of the corresponding field is not NULL.
                If the number of fields is not a multiple of 8, the remainder
                of the last byte in the bit map is wasted.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        Then, for each field with a non-NULL value, there is the following:
<VariableList>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the size of the value of the field, in
		bytes; the count includes itself.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                Specifies the value of the field itself in textual
		form (that is, the result of the output-conversion
		routine for the field's datatype).
		<Replaceable>n</Replaceable> is the above size minus 4.
                There is no trailing zero-byte in the field data; the
		frontend must add one if it wants one.
</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 terminator.  Fields may 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 may 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>
        Int8
</Term>
<ListItem>
<Para>
		0 to return results in textual form (DataRow messages).
                1 to return results in binary form (BinaryRow messages).
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Maximum number of rows to return, if portal contains
		a SELECT or FETCH query (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>
        String("")
</Term>
<ListItem>
<Para>
                Unused.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the object ID of the function to call.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the number of arguments being supplied to the
                function.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        Then, for each argument, there is the following:
<VariableList>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the size of the value of the argument,
                excluding this size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                Specifies the value of the field itself in binary
                format.  <Replaceable>n</Replaceable> is the above size.
</Para>
</ListItem>
</VarListEntry>
</VariableList>

</Para>
</ListItem>
</VarListEntry>


<VarListEntry>
<Term>
FunctionResultResponse (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>
        Byte1('G')
</Term>
<ListItem>
<Para>
                Specifies that a non-null result was returned.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                Specifies the size of the value of the result, excluding this
                size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                Specifies the value of the result itself in binary format.
                <Replaceable>n</Replaceable> is the above size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte1('0')
</Term>
<ListItem>
<Para>
                Unused.  (Strictly speaking, FunctionResultResponse and
                FunctionVoidResponse are the same thing but with some optional
                parts to the message.)
</Para>
</ListItem>
</VarListEntry>
</VariableList>

</Para>
</ListItem>
</VarListEntry>


<VarListEntry>
<Term>
FunctionVoidResponse (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(5)
</Term>
<ListItem>
<Para>
                Length of message contents in bytes, including self.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Byte1('0')
</Term>
<ListItem>
<Para>
                Specifies that a null result was returned.
</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 terminator.  Fields may 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 may 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 condition that the notify has been raised on.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String
</Term>
<ListItem>
<Para>
                Additional information passed from the notifying process.
		(Currently, this feature is unimplemented so the field
		is always an empty string.)
</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>
        Int32
</Term>
<ListItem>
<Para>
                The number of parameters used by the statement
		(may 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 datatype.
</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>
        Int32
</Term>
<ListItem>
<Para>
                The number of parameter datatypes specified
		(may 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 datatype.
		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.
</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 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 (may 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 datatype.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int16
</Term>
<ListItem>
<Para>
                The datatype 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>
</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 SSL 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
</Term>
<ListItem>
<Para>
                The protocol version number.  The most significant 16 bits are
                the major version number (3 or more for the format described
		here).
		The least significant 16 bits are the minor version number.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
        The protocol version number is followed by one or more pairs of
	parameter name and value strings.  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 may 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 may 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 (a 5-character
        string following SQL spec conventions).  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.  May 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.
	May 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>W</>
</Term>
<ListItem>
<Para>
        Where: an indication of the context in which the error occurred.
	Presently this includes a call stack traceback of active PL functions.
	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-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 may
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>
COPY data is now encapsulated into CopyData and CopyDone messages.  There
is a well-defined way to recover from errors during COPY.  The special
<quote><literal>\.</></quote> last line is not needed anymore, and is not sent
during COPY OUT.
(It is still recognized as a terminator during COPY IN, but its use is
deprecated and will eventually be removed.)  Binary COPY is supported.
The CopyInResponse and CopyOutResponse messages carry a field indicating
whether the COPY operation is text or binary.
</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 is presently empty but may someday carry additional data passed
from the NOTIFY event sender.
</para>

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

<note>
<para>
Additional changes will be documented as they are implemented.
</para>
</note>

</sect1>


</Chapter>
