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

 <note>
  <para>
   Written by Phil Thompson (<email>phil@river-bank.demon.co.uk</email>).
   Updates for protocol 2.0 by Tom Lane (<email>tgl@sss.pgh.pa.us</email>).
  </para>
 </note>

 <para>
  <application>PostgreSQL</application> uses a message-based protocol
  for communication between frontends and backends.  The protocol is
  implemented over <acronym>TCP/IP</acronym> and also on Unix domain
  sockets.  <productname>PostgreSQL</productname> 6.3 introduced
  version numbers into the protocol.  This was done in such a way as
  to still allow connections from earlier versions of frontends, but
  this document does not cover the protocol used by those earlier
  versions.
 </para>

 <para>
  This document describes version 2.0 of the protocol, implemented in
  <application>PostgreSQL</application> 6.4 and later.
 </para>

 <para>
  Higher level features built on this protocol (for example, how
  <application>libpq</application> passes certain environment
  variables after the connection is established) are covered
  elsewhere.
 </para>

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

  <para>
   A frontend opens a connection to the server and sends a start-up
   packet.  This includes the names of the user and of the database the
   user wants to connect to.  The server then uses this, and the
   information in the <filename>pg_hba.conf</filename> file to
   determine what further authentication information it requires the
   frontend to send (if any) and responds to the frontend accordingly.
  </para>

  <para>
   The frontend then sends any required authentication information.
   Once the server validates this it responds to the frontend that it
   is authenticated and sends a message indicating successful start-up
   (normal case) or failure (for example, an invalid database name).
  </para>

  <para>
   In order to serve multiple clients efficiently, the server launches
   a new <quote>backend</> process for each client.  This is transparent
   to the protocol, however.  In the current implementation, a new child
   process is created immediately after an incoming connection is detected.
  </para>

  <para>
   When the frontend wishes to disconnect it sends an appropriate packet and
   closes the connection without waiting for a response from the backend.
  </para>

  <para>
   Packets are sent as a data stream.  The first byte determines what
   should be expected in the rest of the packet.  The exceptions are
   packets sent as part of the startup and authentication exchange,
   which comprise a packet length followed by the packet itself.  The
   difference is historical.
  </para>
 </sect1>

 <sect1 id="protocol-protocol">
  <title>Protocol</title>

  <para>
   This section describes the message flow.  There are four different
   types of flows depending on the state of the connection: start-up,
   query, function call, and termination.  There are also special
   provisions for notification responses and command cancellation,
   which can occur at any time after the start-up phase.
  </para>

  <sect2>
   <title>Start-up</Title>

   <para>
    Initially, the frontend sends a StartupPacket.  The server uses
    this info and the contents of the <filename>pg_hba.conf</filename>
    file to determine what authentication method the frontend must
    use.  The server then responds with one of the following messages:

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

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

     <varlistentry>
      <term>AuthenticationKerberosV4</Term>
      <listitem>
       <para>
        The frontend must then 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 then 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 then send a PasswordPacket 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 then send a PasswordPacket containing the
	password encrypted via crypt(3), using the 2-character salt
	specified in the AuthenticationCryptPassword packet.  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 then send a PasswordPacket containing the
	password encrypted via MD5, using the 4-character salt
	specified in the AuthenticationMD5Password packet.  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 method 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 should wait
    for further messages from the server.  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>ReadyForQuery</Term>
      <ListItem>
       <Para>
        Start-up is completed.  The frontend may now issue query or
        function call messages.
       </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 query cycle.  Depending on the coding needs of
    the frontend, it is reasonable to consider ReadyForQuery as
    starting a query cycle (and then BackendKeyData indicates
    successful conclusion of the start-up phase), or to consider
    ReadyForQuery as ending the start-up phase and each subsequent
    query cycle.
   </para>
  </sect2>

  <Sect2>
   <Title>Query</Title>

   <Para>
    A 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 query or
    function call.
   </para>

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

    <VariableList>
     <VarListEntry>
      <Term>CompletedResponse</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.  The frontend should then send a CopyDataRows message.
        The backend will then respond with a CompletedResponse message
        with a tag of <literal>COPY</literal>.
       </Para>
      </ListItem>
     </VarListEntry>

     <VarListEntry>
      <Term>CopyOutResponse</Term>
      <ListItem>
       <Para>
        The backend is ready to copy data from a table to the
        frontend.  It then sends a CopyDataRows message, and then a
        CompletedResponse message with a tag of <literal>COPY</literal>.
       </Para>
      </ListItem>
     </VarListEntry>

     <VarListEntry>
      <Term>CursorResponse</Term>
      <ListItem>
       <Para>
        Beginning of the response to a <command>SELECT</command>,
        <command>FETCH</command>, <command>INSERT</command>,
        <command>UPDATE</command>, or <command>DELETE</command>
        query.  In the <command>FETCH</command> case the name of the
	cursor being fetched from is included in the message.  Otherwise
	the message always mentions the <quote>blank</> cursor.
       </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 an AsciiRow 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.  (CompletedResponse 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</> or <command>FETCH</> query
    normally consists of CursorResponse, RowDescription, zero or more
    AsciiRow or BinaryRow messages, and finally CompletedResponse.
    <command>INSERT</command>, <command>UPDATE</command>, and
    <command>DELETE</command> queries produce CursorResponse followed by
    CompletedResponse.
    <command>COPY</> to or from the frontend invokes special protocol
    as mentioned above.
    All other query types normally produce only
    a CompletedResponse 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.
    (The need to specially distinguish this case is historical.)
   </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.
   </para>

   <Para>
    Actually, it is possible for NoticeResponse to arrive even when
    the frontend is not expecting any kind of message, that is, the
    backend is nominally idle.  (In particular, the backend can be
    commanded to terminate by its parent process.  In that case it will
    send a NoticeResponse before closing the connection.)  It is
    recommended that the frontend check for such asynchronous notices
    just before issuing any new command.
   </para>

   <Para>
    Also, if the frontend issues any <command>LISTEN</command>
    commands then it must be prepared to accept NotificationResponse
    messages at any time; see below.
   </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>Function Call</Title>

   <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 result.
       </Para>
      </ListItem>
     </VarListEntry>

     <VarListEntry>
      <Term>FunctionVoidResponse</Term>
      <ListItem>
       <Para>
        The function call was executed and returned no result.
       </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>

   <para>
    A frontend must be prepared to accept ErrorResponse and
    NoticeResponse messages whenever it is expecting any other type of
    message.  Also, if it issues any <command>LISTEN</command>
    commands then it must be prepared to accept NotificationResponse
    messages at any time; see below.
   </para>
  </sect2>

  <sect2>
   <title>Notification Responses</title>

   <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>

   <para>
    Notification responses are permitted at any point in the protocol
    (after start-up), except within another backend message.  Thus,
    the frontend must be prepared to recognize a NotificationResponse
    message whenever it is expecting any message.  Indeed, it should
    be able to handle NotificationResponse messages even when it is
    not engaged in a query.

    <VariableList>
     <VarListEntry>
      <Term>NotificationResponse</Term>
      <ListItem>
       <Para>
        A <command>NOTIFY</command> command has been executed for a
        name for which a previous <command>LISTEN</command> command
        was executed.  Notifications may be sent at any time.
       </Para>
      </ListItem>
     </VarListEntry>
    </VariableList>
   </Para>

   <para>
    It may be worth pointing out that the names used in listen and
    notify commands need not have anything to do with names of
    relations (tables) in the SQL database.  Notification names are
    simply arbitrarily chosen condition names.
   </para>
  </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
    StartupPacket 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 the message, the backend immediately closes the
    connection and terminates.
   </para>

   <Para>
    An ungraceful termination may occur due to software failure (i.e.,
    core dump) at either end.  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.
   </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 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>
    Recent releases of <productname>PostgreSQL</> allow frontend/backend
    communication to 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 StartupPacket.  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 StartupPacket.  In this case the StartupPacket and
    all subsequent data will be SSL-encrypted.  To continue after
    <literal>N</>, send the usual StartupPacket 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 literal value.  Eg. Int16, Int32(42).
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString<Replaceable>n</Replaceable>(<Replaceable>s</Replaceable>)
</Term>
<ListItem>
<Para>
                A character array of exactly <Replaceable>n</Replaceable> bytes interpreted as a
                null-terminated string.  The zero-byte is omitted if there is
                insufficient room.  If <Replaceable>s</Replaceable> is specified it is the literal value.
                Eg. LimString32, LimString64("user").
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String(<Replaceable>s</Replaceable>)
</Term>
<ListItem>
<Para>
                A conventional C null-terminated string with no length
                limitation.
                If <Replaceable>s</Replaceable> is specified it is the literal value.
                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 literal
                value.  Eg. Byte, 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 can be sent
by either a frontend (F), a backend (B), or both (F & B).
</para>

<VariableList>
<VarListEntry>
<Term>
AsciiRow (B)
</Term>
<ListItem>
<Para>
<VariableList>
<VarListEntry>
<Term>
        Byte1('D')
</Term>
<ListItem>
<Para>
                Identifies the message as an <Acronym>ASCII</Acronym> data row.
                (A prior RowDescription message defines the number of
                fields in the row and their data types.)
</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>
<Para>
        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, including
                        this size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
                Byte<Replaceable>n</Replaceable>
</Term>
<ListItem>
<Para>
                        Specifies the value of the field itself in <Acronym>ASCII</Acronym>
                        characters.  <Replaceable>n</Replaceable> is the above
                        size minus 4.
                        There is no trailing zero-byte in the field data; the front
                        end must add one if it wants one.
</Para>
</ListItem>
</VarListEntry>
</VariableList>

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

</Para>
</ListItem>
</VarListEntry>
<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(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(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(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(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(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(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(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
</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.
                (A prior RowDescription message defines the number of
                fields in the row and their data types.)
</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>
<Para>
        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>
</VariableList>

</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CancelRequest (F)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Int32(16)
</Term>
<ListItem>
<Para>
                The size of the packet in bytes.
</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>
CompletedResponse (B)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Byte1('C')
</Term>
<ListItem>
<Para>
                Identifies the message as a completed response.
</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>
</ListItem>
</VarListEntry>
</VariableList>


</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CopyDataRows (B & F)
</Term>
<ListItem>
<Para>
        This is a stream of rows where each row is terminated by a Byte1('\n').
        This is then followed by the sequence Byte1('\\'), Byte1('.'),
        Byte1('\n').
</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 a CopyDataRows message.
</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 a CopyDataRows message.
</Para>
</ListItem>
</VarListEntry>
</VariableList>


</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CursorResponse (B)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Byte1('P')
</Term>
<ListItem>
<Para>
                Identifies the message as a cursor response.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String
</Term>
<ListItem>
<Para>
                The name of the cursor.  This will be <quote>blank</> if the cursor is
                implicit.
</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.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String("")
</Term>
<ListItem>
<Para>
                Unused.
</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>
        String
</Term>
<ListItem>
<Para>
                The error message itself.
</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>
        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>
<Para>
        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>
</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>
        Byte1('G')
</Term>
<ListItem>
<Para>
                Specifies that a nonempty 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>
        Byte1('0')
</Term>
<ListItem>
<Para>
                Specifies that an empty result was returned.
</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>
        String
</Term>
<ListItem>
<Para>
                The notice message itself.
</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>
                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>
</VariableList>


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

<VarListEntry>
<Term>
PasswordPacket (F)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                The size of the packet in bytes.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        String
</Term>
<ListItem>
<Para>
                The password (encrypted, if requested).
</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 query.
</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>
</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>
        Int16
</Term>
<ListItem>
<Para>
                Specifies the number of fields in a row (may be zero).
</Para>
<Para>
        Then, for each field, there is the following:
<VariableList>
<VarListEntry>
<Term>
                String
</Term>
<ListItem>
<Para>
                        Specifies the field name.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
                Int32
</Term>
<ListItem>
<Para>
                        Specifies the object ID of the field type.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
                Int16
</Term>
<ListItem>
<Para>
                        Specifies the type size.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
                Int32
</Term>
<ListItem>
<Para>
                        Specifies the type modifier.
</Para>
</ListItem>
</VarListEntry>
</VariableList>

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


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

<VarListEntry>
<Term>
SSLRequest (F)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Int32(8)
</Term>
<ListItem>
<Para>
                The size of the packet in bytes.
</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>
StartupPacket (F)
</Term>
<ListItem>
<Para>

<VariableList>
<VarListEntry>
<Term>
        Int32(296)
</Term>
<ListItem>
<Para>
                The size of the packet in bytes.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        Int32
</Term>
<ListItem>
<Para>
                The protocol version number.  The most significant 16 bits are
                the major version number.  The least 16 significant bits are
                the minor version number.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString64
</Term>
<ListItem>
<Para>
                The database name, defaults to the user name if empty.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString32
</Term>
<ListItem>
<Para>
                The user name.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString64
</Term>
<ListItem>
<Para>
                Any additional command line arguments to be passed to the
                backend child process by the server.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString64
</Term>
<ListItem>
<Para>
                Unused.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
        LimString64
</Term>
<ListItem>
<Para>
                The optional tty the backend should use for debugging messages.
		(Currently, this field is unsupported and ignored.)
</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>
</VariableList>


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

</VariableList>

</sect1>
</Chapter>
