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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="protocol-flow"></a>43.2.Message Flow</h2></div></div></div>
<p>   This section describes the message flow and the semantics of each
   message type.  (Details of the exact representation of each message
   appear in <a href="protocol-message-formats.html" title="43.4.Message Formats">Section43.4, &#8220;Message Formats&#8221;</a>.)  There are
   several different sub-protocols depending on the state of the
   connection: start-up, query, function call,
   <code class="command">COPY</code>, 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.
  </p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id834234"></a>43.2.1.Start-Up</h3></div></div></div>
<p>    To begin a session, a frontend opens a connection to the server and sends
    a startup message.  This message includes the names of the user and of the
    database the user wants to connect to; it also identifies the particular
    protocol version to be used.  (Optionally, the startup message can include
    additional settings for run-time parameters.)
    The server then uses this information and
    the contents of its configuration files (such as
    <code class="filename">pg_hba.conf</code>) to determine
    whether the connection is provisionally acceptable, and what additional
    authentication is required (if any).
   </p>
<p>    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.
   </p>
<p>    The authentication cycle ends with the server either rejecting the
    connection attempt (ErrorResponse), or sending AuthenticationOk.
   </p>
<p>    The possible messages from the server in this phase are:

    </p>
<div class="variablelist"><dl>
<dt><span class="term">ErrorResponse</span></dt>
<dd><p>        The connection attempt has been rejected.
        The server then immediately closes the connection.
       </p></dd>
<dt><span class="term">AuthenticationOk</span></dt>
<dd><p>        The authentication exchange is successfully completed.
       </p></dd>
<dt><span class="term">AuthenticationKerberosV5</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">AuthenticationCleartextPassword</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">AuthenticationCryptPassword</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">AuthenticationMD5Password</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">AuthenticationSCMCredential</span></dt>
<dd><p>        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.
       </p></dd>
</dl></div>
<p>
   </p>
<p>    If the frontend does not support the authentication method
    requested by the server, then it should immediately close the
    connection.
   </p>
<p>    After having received AuthenticationOk, the frontend must wait
    for further messages from the server.  In this phase a backend process
    is being started, and the frontend is just an interested bystander.
    It is still possible for the startup attempt
    to fail (ErrorResponse), but in the normal case the backend will send
    some ParameterStatus messages, BackendKeyData, and finally ReadyForQuery.
   </p>
<p>    During this phase the backend will attempt to apply any additional
    run-time parameter settings that were given in the startup message.
    If successful, these values become session defaults.  An error causes
    ErrorResponse and exit.
   </p>
<p>    The possible messages from the backend in this phase are:

    </p>
<div class="variablelist"><dl>
<dt><span class="term">BackendKeyData</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">ParameterStatus</span></dt>
<dd><p>        This message informs the frontend about the current (initial)
         setting of backend parameters, such as <a href="runtime-config-client.html#guc-client-encoding">client_encoding</a> or <a href="runtime-config-client.html#guc-datestyle">DateStyle</a>.
         The frontend may ignore this message, or record the settings
         for its future use; see <a href="protocol-flow.html#protocol-async" title="43.2.6.Asynchronous Operations">Section43.2.6, &#8220;Asynchronous Operations&#8221;</a> for
         more details.  The frontend should not respond to this
         message, but should continue listening for a ReadyForQuery
         message.
       </p></dd>
<dt><span class="term">ReadyForQuery</span></dt>
<dd><p>        Start-up is completed.  The frontend may now issue commands.
       </p></dd>
<dt><span class="term">ErrorResponse</span></dt>
<dd><p>        Start-up failed.  The connection is closed after sending this
        message.
       </p></dd>
<dt><span class="term">NoticeResponse</span></dt>
<dd><p>        A warning message has been issued.  The frontend should
        display the message but continue listening for ReadyForQuery
        or ErrorResponse.
       </p></dd>
</dl></div>
<p>
   </p>
<p>    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.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id834480"></a>43.2.2.Simple Query</h3></div></div></div>
<p>    A simple query cycle is initiated by the frontend sending a Query message
    to the backend.  The message includes an SQL command (or commands)
    expressed as a text string.
    The backend then sends one or more response
    messages depending on the contents of the query command string,
    and finally a ReadyForQuery response message.  ReadyForQuery
    informs the frontend that it 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.)
   </p>
<p>    The possible response messages from the backend are:

    </p>
<div class="variablelist"><dl>
<dt><span class="term">CommandComplete</span></dt>
<dd><p>        An SQL command completed normally.
       </p></dd>
<dt><span class="term">CopyInResponse</span></dt>
<dd><p>        The backend is ready to copy data from the frontend to a
        table; see <a href="protocol-flow.html#protocol-copy" title="43.2.5.COPY Operations">Section43.2.5, &#8220;COPY Operations&#8221;</a>.
       </p></dd>
<dt><span class="term">CopyOutResponse</span></dt>
<dd><p>        The backend is ready to copy data from a table to the
        frontend; see <a href="protocol-flow.html#protocol-copy" title="43.2.5.COPY Operations">Section43.2.5, &#8220;COPY Operations&#8221;</a>.
       </p></dd>
<dt><span class="term">RowDescription</span></dt>
<dd><p>        Indicates that rows are about to be returned in response to
        a <code class="command">SELECT</code>, <code class="command">FETCH</code>, etc query.
        The contents of this message describe the column layout of the rows.
        This will be followed by a DataRow message for each row being returned
        to the frontend.
       </p></dd>
<dt><span class="term">DataRow</span></dt>
<dd><p>        One of the set of rows returned by
        a <code class="command">SELECT</code>, <code class="command">FETCH</code>, etc query.
       </p></dd>
<dt><span class="term">EmptyQueryResponse</span></dt>
<dd><p>        An empty query string was recognized.
       </p></dd>
<dt><span class="term">ErrorResponse</span></dt>
<dd><p>        An error has occurred.
       </p></dd>
<dt><span class="term">ReadyForQuery</span></dt>
<dd><p>        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.
       </p></dd>
<dt><span class="term">NoticeResponse</span></dt>
<dd><p>        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.
       </p></dd>
</dl></div>
<p>
   </p>
<p>    The response to a <code class="command">SELECT</code> query (or other queries that
    return row sets, such as <code class="command">EXPLAIN</code> or <code class="command">SHOW</code>)
    normally consists of RowDescription, zero or more
    DataRow messages, and then CommandComplete.
    <code class="command">COPY</code> to or from the frontend invokes special protocol
    as described in <a href="protocol-flow.html#protocol-copy" title="43.2.5.COPY Operations">Section43.2.5, &#8220;COPY Operations&#8221;</a>.
    All other query types normally produce only
    a CommandComplete message.
   </p>
<p>    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.
   </p>
<p>    If a completely empty (no contents other than whitespace) query string
    is received, the response is EmptyQueryResponse followed by ReadyForQuery.
   </p>
<p>    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.
   </p>
<p>    In simple Query mode, the format of retrieved values is always text,
    except when the given command is a <code class="command">FETCH</code> from a cursor
    declared with the <code class="literal">BINARY</code> option.  In that case, the
    retrieved values are in binary format.  The format codes given in
    the RowDescription message tell which format is being used.
   </p>
<p>    A frontend must be prepared to accept ErrorResponse and
    NoticeResponse messages whenever it is expecting any other type of
    message.  See also <a href="protocol-flow.html#protocol-async" title="43.2.6.Asynchronous Operations">Section43.2.6, &#8220;Asynchronous Operations&#8221;</a> concerning messages
    that the backend may generate due to outside events.
   </p>
<p>    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.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id834738"></a>43.2.3.Extended Query</h3></div></div></div>
<p>    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.
   </p>
<p>    In the extended protocol, the frontend first sends a Parse message,
    which contains a textual query string, optionally some information
    about data types of parameter placeholders, and the
    name of a destination prepared-statement object (an empty string
    selects the unnamed prepared statement).  The response is
    either ParseComplete or ErrorResponse.  Parameter data types may be
    specified by OID; if not given, the parser attempts to infer the
    data types in the same way as it would do for untyped literal string
    constants.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     A parameter data type can be left unspecified by setting it to zero,
     or by making the array of parameter type OIDs shorter than the
     number of parameter symbols (<code class="literal">$</code><em class="replaceable"><code>n</code></em>)
     used in the query string.  Another special case is that a parameter's
     type can be specified as <code class="type">void</code> (that is, the OID of the
     <code class="type">void</code> pseudotype).  This is meant to allow parameter symbols
     to be used for function parameters that are actually OUT parameters.
     Ordinarily there is no context in which a <code class="type">void</code> parameter
     could be used, but if such a parameter symbol appears in a function's
     parameter list, it is effectively ignored.  For example, a function
     call such as <code class="literal">foo($1,$2,$3,$4)</code> could match a function with
     two IN and two OUT arguments, if <code class="literal">$3</code> and <code class="literal">$4</code>
     are specified as having type <code class="type">void</code>.
    </p>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     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.
    </p>
</div>
<p>    If successfully created, a named prepared-statement object lasts till
    the end of the current session, unless explicitly destroyed.  An unnamed
    prepared statement lasts only until the next Parse statement specifying
    the unnamed statement as destination is issued.  (Note that a simple
    Query message also destroys the unnamed statement.)  Named prepared
    statements must be explicitly closed before they can be redefined by
    a Parse message, but this is not required for the unnamed statement.
    Named prepared statements can also be created and accessed at the SQL
    command level, using <code class="command">PREPARE</code> and <code class="command">EXECUTE</code>.
   </p>
<p>    Once a prepared statement exists, it can be readied for execution using a
    Bind message.  The Bind message gives the name of the source prepared
    statement (empty string denotes the unnamed prepared statement), the name
    of the destination portal (empty string denotes the unnamed portal), and
    the values to use for any parameter placeholders present in the prepared
    statement.  The
    supplied parameter set must match those needed by the prepared statement.
    (If you declared any <code class="type">void</code> parameters in the Parse message,
    pass NULL values for them in the Bind message.)
    Bind also specifies the format to use for any data returned
    by the query; the format can be specified overall, or per-column.
    The response is either BindComplete or ErrorResponse.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     The choice between text and binary output is determined by the format
     codes given in Bind, regardless of the SQL command involved.  The
     <code class="literal">BINARY</code> attribute in cursor declarations is irrelevant when
     using extended query protocol.
    </p>
</div>
<p>    Query planning for named prepared-statement objects occurs when the Parse
    message is received. If a query will be repeatedly executed with
    different parameters, it may be beneficial to send a single Parse message
    containing a parameterized query, followed by multiple Bind
    and Execute messages. This will avoid replanning the query on each
    execution.
   </p>
<p>    The unnamed prepared statement is likewise planned during Parse processing
    if the Parse message defines no parameters.  But if there are parameters,
    query planning is delayed until the first Bind message for the statement
    is received. The planner will consider the actual values of the parameters
    provided in the Bind message when planning the query.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     Query plans generated from a parameterized query may be less
     efficient than query plans generated from an equivalent query with actual
     parameter values substituted. The query planner cannot make decisions
     based on actual parameter values (for example, index selectivity) when
     planning a parameterized query assigned to a named prepared-statement
     object.  This possible penalty is avoided when using the unnamed
     statement, since it is not planned until actual parameter values are
     available.
    </p>
<p>     If a second or subsequent Bind referencing the unnamed prepared-statement
     object is received without an intervening Parse, the query is
     not replanned. The parameter values used in the first Bind message may
     produce a query plan that is only efficient for a subset of possible
     parameter values. To force replanning of the query for a fresh set of
     parameters, send another Parse message to replace the unnamed
     prepared-statement object. 
    </p>
</div>
<p>    If successfully created, a named portal object lasts till the end of the
    current transaction, unless explicitly destroyed.  An unnamed portal is
    destroyed at the end of the transaction, or as soon as the next Bind
    statement specifying the unnamed portal as destination is issued.  (Note
    that a simple Query message also destroys the unnamed portal.)  Named
    portals must be explicitly closed before they can be redefined by a Bind
    message, but this is not required for the unnamed portal.
    Named portals can also be created and accessed at the SQL
    command level, using <code class="command">DECLARE CURSOR</code> and <code class="command">FETCH</code>.
   </p>
<p>    Once a portal exists, it can be executed using an Execute message.
    The Execute message specifies the portal name (empty string denotes the
    unnamed portal) and
    a maximum result-row count (zero meaning &#8220;<span class="quote">fetch all rows</span>&#8221;).
    The result-row count is only meaningful for portals
    containing commands that return row sets; in other cases the command is
    always executed to completion, and the row count is ignored.
    The possible
    responses to Execute are the same as those described above for queries
    issued via simple query protocol, except that Execute doesn't cause
    ReadyForQuery or RowDescription to be issued.
   </p>
<p>    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.
   </p>
<p>    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
    <code class="command">BEGIN</code>/<code class="command">COMMIT</code> transaction block (&#8220;<span class="quote">close</span>&#8221;
    meaning to commit if no error, or roll back if error).  Then a
    ReadyForQuery response is issued.  The purpose of Sync is to provide
    a resynchronization point for error recovery.  When an error is detected
    while processing any extended-query message, the backend issues
    ErrorResponse, then reads and discards messages until a Sync is reached,
    then issues ReadyForQuery and returns to normal message processing.
    (But note that no skipping occurs if an error is detected
    <span class="emphasis"><em>while</em></span> processing Sync [mdash ] this ensures that there is one
    and only one ReadyForQuery sent for each Sync.)
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     Sync does not cause a transaction block opened with <code class="command">BEGIN</code>
     to be closed.  It is possible to detect this situation since the
     ReadyForQuery message includes transaction status information.
    </p>
</div>
<p>    In addition to these fundamental, required operations, there are several
    optional operations that can be used with extended-query protocol.
   </p>
<p>    The Describe message (portal variant) specifies the name of an existing
    portal (or an empty string for the unnamed portal).  The response is a
    RowDescription message describing the rows that will be returned by
    executing the portal; or a NoData message if the portal does not contain a
    query that will return rows; or ErrorResponse if there is no such portal.
   </p>
<p>    The Describe message (statement variant) specifies the name of an existing
    prepared statement (or an empty string for the unnamed prepared
    statement).  The response is a ParameterDescription message describing the
    parameters needed by the statement, followed by a RowDescription message
    describing the rows that will be returned when the statement is eventually
    executed (or a NoData message if the statement will not return rows).
    ErrorResponse is issued if there is no such prepared statement.  Note that
    since Bind has not yet been issued, the formats to be used for returned
    columns are not yet known to the backend; the format code fields in the
    RowDescription message will be zeroes in this case.
   </p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>     In most scenarios the frontend should issue one or the other variant
     of Describe before issuing Execute, to ensure that it knows how to
     interpret the results it will get back.
    </p>
</div>
<p>    The Close message closes an existing prepared statement or portal
    and releases resources.  It is not an error to issue Close against
    a nonexistent statement or portal name.  The response is normally
    CloseComplete, but could be ErrorResponse if some difficulty is
    encountered while releasing resources.  Note that closing a prepared
    statement implicitly closes any open portals that were constructed
    from that statement.
   </p>
<p>    The Flush message does not cause any specific output to be generated,
    but forces the backend to deliver any data pending in its output
    buffers.  A Flush must be sent after any extended-query command except
    Sync, if the frontend wishes to examine the results of that command before
    issuing more commands.  Without Flush, messages returned by the backend
    will be combined into the minimum possible number of packets to minimize
    network overhead.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     The simple Query message is approximately equivalent to the series Parse,
     Bind, portal Describe, Execute, Close, Sync, using the unnamed prepared
     statement and portal objects and no parameters.  One difference is that
     it will accept multiple SQL statements in the query string, automatically
     performing the bind/describe/execute sequence for each one in succession.
     Another difference is that it will not return ParseComplete, BindComplete,
     CloseComplete, or NoData messages.
    </p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id835121"></a>43.2.4.Function Call</h3></div></div></div>
<p>    The Function Call sub-protocol allows the client to request a direct
    call of any function that exists in the database's
    <code class="structname">pg_proc</code> system catalog.  The client must have
    execute permission for the function.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     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 <code class="literal">SELECT function($1, ...)</code>.
     The Function Call cycle can then be replaced with Bind/Execute.
    </p>
</div>
<p>    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.
   </p>
<p>    The possible response messages from the backend are:

    </p>
<div class="variablelist"><dl>
<dt><span class="term">ErrorResponse</span></dt>
<dd><p>        An error has occurred.
       </p></dd>
<dt><span class="term">FunctionCallResponse</span></dt>
<dd><p>        The function call was completed and returned the result given
        in the message.
        (Note that the Function Call protocol can only handle a single
        scalar result, not a row type or set of results.)
       </p></dd>
<dt><span class="term">ReadyForQuery</span></dt>
<dd><p>        Processing of the function call is complete.  ReadyForQuery
        will always be sent, whether processing terminates
        successfully or with an error.
       </p></dd>
<dt><span class="term">NoticeResponse</span></dt>
<dd><p>        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.
       </p></dd>
</dl></div>
<p>
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="protocol-copy"></a>43.2.5.COPY Operations</h3></div></div></div>
<p>    The <code class="command">COPY</code> 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.
   </p>
<p>    Copy-in mode (data transfer to the server) is initiated when the
    backend executes a <code class="command">COPY FROM STDIN</code> 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 <code class="command">COPY</code> SQL statement to fail with an
    error).  The backend then reverts to the command-processing mode it was
    in before the <code class="command">COPY</code> started, which will be either simple or
    extended query protocol.  It will next send either CommandComplete
    (if successful) or ErrorResponse (if not).
   </p>
<p>    In the event of a backend-detected error during copy-in mode (including
    receipt of a CopyFail message), the backend will issue an ErrorResponse 
    message.  If the <code class="command">COPY</code> 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 <code class="command">COPY</code> 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.
   </p>
<p>    The backend will ignore Flush and Sync messages received during copy-in
    mode.  Receipt of any other non-copy message type constitutes an error
    that will abort the copy-in state as described above.  (The exception for
    Flush and Sync is for the convenience of client libraries that always
    send Flush or Sync after an Execute message, without checking whether
    the command to be executed is a <code class="command">COPY FROM STDIN</code>.)
   </p>
<p>    Copy-out mode (data transfer from the server) is initiated when the
    backend executes a <code class="command">COPY TO STDOUT</code> 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 <code class="command">COPY</code> started, and sends CommandComplete.
    The frontend cannot abort the transfer (except by closing the connection
    or issuing a Cancel request),
    but it can discard unwanted CopyData and CopyDone messages.
   </p>
<p>    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.
   </p>
<p>    The CopyInResponse and CopyOutResponse messages include fields that
    inform the frontend of the number of columns per row and the format
    codes being used for each column.  (As of the present implementation,
    all columns in a given <code class="command">COPY</code> operation will use the same
    format, but the message design does not assume this.)
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="protocol-async"></a>43.2.6.Asynchronous Operations</h3></div></div></div>
<p>    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.
   </p>
<p>    It is possible for NoticeResponse messages to be generated due to
    outside activity; for example, if the database administrator commands
    a &#8220;<span class="quote">fast</span>&#8221; 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.
   </p>
<p>    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 <code class="command">SET</code> SQL command executed by the frontend, and
    this case is effectively synchronous [mdash ] but it is also possible
    for parameter status changes to occur because the administrator
    changed a configuration file and then sent the
    <span class="systemitem">SIGHUP</span> signal to the postmaster.  Also,
    if a <code class="command">SET</code> command is rolled back, an appropriate
    ParameterStatus message will be generated to report the current
    effective value.
   </p>
<p>    At present there is a hard-wired set of parameters for which
    ParameterStatus will be generated: they are
    <code class="literal">server_version</code>,
    <code class="literal">server_encoding</code>,
    <code class="literal">client_encoding</code>,
    <code class="literal">is_superuser</code>,
    <code class="literal">session_authorization</code>,
    <code class="literal">DateStyle</code>,
    <code class="literal">TimeZone</code>,
    <code class="literal">integer_datetimes</code>, and
    <code class="literal">standard_conforming_strings</code>.
    (<code class="literal">server_encoding</code>, <code class="literal">TimeZone</code>, and
    <code class="literal">integer_datetimes</code> were not reported by releases before 8.0;
    <code class="literal">standard_conforming_strings</code> was not reported by releases
    before 8.1.)
    Note that
    <code class="literal">server_version</code>,
    <code class="literal">server_encoding</code> and
    <code class="literal">integer_datetimes</code>
    are pseudo-parameters that cannot change after startup.
    This set might change in the future, or even become configurable.
    Accordingly, a frontend should simply ignore ParameterStatus for
    parameters that it does not understand or care about.
   </p>
<p>    If a frontend issues a <code class="command">LISTEN</code> command, then the
    backend will send a NotificationResponse message (not to be
    confused with NoticeResponse!)  whenever a
    <code class="command">NOTIFY</code> command is executed for the same
    notification name.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     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.
    </p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id835564"></a>43.2.7.Cancelling Requests in Progress</h3></div></div></div>
<p>    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.
   </p>
<p>    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.
   </p>
<p>    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.)
   </p>
<p>    The cancellation signal may or may not have any effect [mdash ] for
    example, if it arrives after the backend has finished processing
    the query, then it will have no effect.  If the cancellation is
    effective, it results in the current command being terminated
    early with an error message.
   </p>
<p>    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.
   </p>
<p>    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.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id835628"></a>43.2.8.Termination</h3></div></div></div>
<p>    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.
   </p>
<p>    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.
   </p>
<p>    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.
   </p>
<p>    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-<code class="command">SELECT</code> query
    is being processed, the backend will probably finish the query
    before noticing the disconnection.  If the query is outside any
    transaction block (<code class="command">BEGIN</code> ... <code class="command">COMMIT</code>
    sequence) then its results may be committed before the
    disconnection is recognized.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id835688"></a>43.2.9.<acronym class="acronym">SSL</acronym> Session Encryption</h3></div></div></div>
<p>    If <span class="productname">PostgreSQL</span> was built with
    <acronym class="acronym">SSL</acronym> support, frontend/backend communications
    can be encrypted using <acronym class="acronym">SSL</acronym>.  This provides
    communication security in environments where attackers might be
    able to capture the session traffic. For more information on
    encrypting <span class="productname">PostgreSQL</span> sessions with
    <acronym class="acronym">SSL</acronym>, see <a href="ssl-tcp.html" title="16.7.Secure TCP/IP Connections with SSL">Section16.7, &#8220;Secure TCP/IP Connections with SSL&#8221;</a>.
   </p>
<p>    To initiate an <acronym class="acronym">SSL</acronym>-encrypted connection, the
    frontend initially sends an SSLRequest message rather than a
    StartupMessage.  The server then responds with a single byte
    containing <code class="literal">S</code> or <code class="literal">N</code>, indicating that it is
    willing or unwilling to perform <acronym class="acronym">SSL</acronym>,
    respectively.  The frontend may close the connection at this point
    if it is dissatisfied with the response.  To continue after
    <code class="literal">S</code>, perform an <acronym class="acronym">SSL</acronym> startup handshake
    (not described here, part of the <acronym class="acronym">SSL</acronym>
    specification) with the server.  If this is successful, continue
    with sending the usual StartupMessage.  In this case the
    StartupMessage and all subsequent data will be
    <acronym class="acronym">SSL</acronym>-encrypted.  To continue after
    <code class="literal">N</code>, send the usual StartupMessage and proceed without
    encryption.
   </p>
<p>    The frontend should also be prepared to handle an ErrorMessage
    response to SSLRequest from the server.  This would only occur if
    the server predates the addition of <acronym class="acronym">SSL</acronym> support
    to <span class="productname">PostgreSQL</span>.  In this case the connection must
    be closed, but the frontend may choose to open a fresh connection
    and proceed without requesting <acronym class="acronym">SSL</acronym>.
   </p>
<p>    An initial SSLRequest may also be used in a connection that is being
    opened to send a CancelRequest message.
   </p>
<p>    While the protocol itself does not provide a way for the server to
    force <acronym class="acronym">SSL</acronym> encryption, the administrator may
    configure the server to reject unencrypted sessions as a byproduct
    of authentication checking.
   </p>
</div>
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