1#@!#!123s

: / usr / share / doc / postgresql-9.2.24 / html /
Filename : protocol-flow.html
back
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Message Flow</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.2.24 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Frontend/Backend Protocol"
HREF="protocol.html"><LINK
REL="PREVIOUS"
TITLE="Overview"
HREF="protocol-overview.html"><LINK
REL="NEXT"
TITLE="Streaming Replication Protocol"
HREF="protocol-replication.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2017-11-06T22:43:11"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.2.24 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Overview"
HREF="protocol-overview.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="protocol.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 46. Frontend/Backend Protocol</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Streaming Replication Protocol"
HREF="protocol-replication.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PROTOCOL-FLOW"
>46.2. Message Flow</A
></H1
><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"
>Section 46.5</A
>.)  There are
   several different sub-protocols depending on the state of the
   connection: start-up, query, function call,
   <TT
CLASS="COMMAND"
>COPY</TT
>, 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"
><H2
CLASS="SECT2"
><A
NAME="AEN95777"
>46.2.1. Start-up</A
></H2
><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
    <TT
CLASS="FILENAME"
>pg_hba.conf</TT
>) 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).
    For all authentication methods except GSSAPI and SSPI, there is at most
    one request and one response. In some methods, no response
    at all is needed from the frontend, and so no authentication request
    occurs. For GSSAPI and SSPI, multiple exchanges of packets may be needed
    to complete the authentication.
   </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
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>ErrorResponse</DT
><DD
><P
>        The connection attempt has been rejected.
        The server then immediately closes the connection.
       </P
></DD
><DT
>AuthenticationOk</DT
><DD
><P
>        The authentication exchange is successfully completed.
       </P
></DD
><DT
>AuthenticationKerberosV5</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
>AuthenticationCleartextPassword</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
>AuthenticationMD5Password</DT
><DD
><P
>        The frontend must now send a PasswordMessage containing the
        password (with username) encrypted via MD5, then encrypted
        again using the 4-byte random salt specified in the
        AuthenticationMD5Password message.  If this is the correct
        password, the server responds with an AuthenticationOk,
        otherwise it responds with an ErrorResponse.  The actual
        PasswordMessage can be computed in SQL as <TT
CLASS="LITERAL"
>concat('md5',
        md5(concat(md5(concat(password, username)), random-salt)))</TT
>.
        (Keep in mind the <CODE
CLASS="FUNCTION"
>md5()</CODE
> function returns its
        result as a hex string.)
       </P
></DD
><DT
>AuthenticationSCMCredential</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.
        (This message type is only issued by pre-9.1 servers.  It may
        eventually be removed from the protocol specification.)
       </P
></DD
><DT
>AuthenticationGSS</DT
><DD
><P
>        The frontend must now initiate a GSSAPI negotiation. The frontend
        will send a PasswordMessage with the first part of the GSSAPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </P
></DD
><DT
>AuthenticationSSPI</DT
><DD
><P
>        The frontend must now initiate a SSPI negotiation. The frontend
        will send a PasswordMessage with the first part of the SSPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </P
></DD
><DT
>AuthenticationGSSContinue</DT
><DD
><P
>        This message contains the response data from the previous step
        of GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI
        or a previous AuthenticationGSSContinue). If the GSSAPI
        or SSPI data in this message
        indicates more data is needed to complete the authentication,
        the frontend must send that data as another PasswordMessage. If
        GSSAPI or SSPI authentication is completed by this message, the server
        will next send AuthenticationOk to indicate successful authentication
        or ErrorResponse to indicate failure.
       </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
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>BackendKeyData</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
>ParameterStatus</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 can ignore this message, or record the settings
         for its future use; see <A
HREF="protocol-flow.html#PROTOCOL-ASYNC"
>Section 46.2.6</A
> for
         more details.  The frontend should not respond to this
         message, but should continue listening for a ReadyForQuery
         message.
       </P
></DD
><DT
>ReadyForQuery</DT
><DD
><P
>        Start-up is completed.  The frontend can now issue commands.
       </P
></DD
><DT
>ErrorResponse</DT
><DD
><P
>        Start-up failed.  The connection is closed after sending this
        message.
       </P
></DD
><DT
>NoticeResponse</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"
><H2
CLASS="SECT2"
><A
NAME="AEN95852"
>46.2.2. Simple Query</A
></H2
><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 can safely send a new command.
    (It is not actually necessary for the frontend to wait for
    ReadyForQuery before issuing another command, but the frontend must
    then take responsibility for figuring out what happens if the earlier
    command fails and already-issued later commands succeed.)
   </P
><P
>    The possible response messages from the backend are:

    <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>CommandComplete</DT
><DD
><P
>        An SQL command completed normally.
       </P
></DD
><DT
>CopyInResponse</DT
><DD
><P
>        The backend is ready to copy data from the frontend to a
        table; see <A
HREF="protocol-flow.html#PROTOCOL-COPY"
>Section 46.2.5</A
>.
       </P
></DD
><DT
>CopyOutResponse</DT
><DD
><P
>        The backend is ready to copy data from a table to the
        frontend; see <A
HREF="protocol-flow.html#PROTOCOL-COPY"
>Section 46.2.5</A
>.
       </P
></DD
><DT
>RowDescription</DT
><DD
><P
>        Indicates that rows are about to be returned in response to
        a <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>FETCH</TT
>, 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
>DataRow</DT
><DD
><P
>        One of the set of rows returned by
        a <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>FETCH</TT
>, etc query.
       </P
></DD
><DT
>EmptyQueryResponse</DT
><DD
><P
>        An empty query string was recognized.
       </P
></DD
><DT
>ErrorResponse</DT
><DD
><P
>        An error has occurred.
       </P
></DD
><DT
>ReadyForQuery</DT
><DD
><P
>        Processing of the query string is complete.  A separate
        message is sent to indicate this because the query string might
        contain multiple SQL commands.  (CommandComplete marks the
        end of processing one SQL command, not the whole string.)
        ReadyForQuery will always be sent, whether processing
        terminates successfully or with an error.
       </P
></DD
><DT
>NoticeResponse</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 <TT
CLASS="COMMAND"
>SELECT</TT
> query (or other queries that
    return row sets, such as <TT
CLASS="COMMAND"
>EXPLAIN</TT
> or <TT
CLASS="COMMAND"
>SHOW</TT
>)
    normally consists of RowDescription, zero or more
    DataRow messages, and then CommandComplete.
    <TT
CLASS="COMMAND"
>COPY</TT
> to or from the frontend invokes special protocol
    as described in <A
HREF="protocol-flow.html#PROTOCOL-COPY"
>Section 46.2.5</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
    might 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 <TT
CLASS="COMMAND"
>FETCH</TT
> from a cursor
    declared with the <TT
CLASS="LITERAL"
>BINARY</TT
> 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"
>Section 46.2.6</A
> concerning messages
    that the backend might 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"
><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-FLOW-EXT-QUERY"
>46.2.3. Extended Query</A
></H2
><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 can be
    specified by OID; if not given, the parser attempts to infer the
    data types in the same way as it would do for untyped literal string
    constants.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     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 (<TT
CLASS="LITERAL"
>$</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)
     used in the query string.  Another special case is that a parameter's
     type can be specified as <TT
CLASS="TYPE"
>void</TT
> (that is, the OID of the
     <TT
CLASS="TYPE"
>void</TT
> 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 <TT
CLASS="TYPE"
>void</TT
> 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 <TT
CLASS="LITERAL"
>foo($1,$2,$3,$4)</TT
> could match a function with
     two IN and two OUT arguments, if <TT
CLASS="LITERAL"
>$3</TT
> and <TT
CLASS="LITERAL"
>$4</TT
>
     are specified as having type <TT
CLASS="TYPE"
>void</TT
>.
    </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     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
></BLOCKQUOTE
></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
    another Parse message, but this is not required for the unnamed statement.
    Named prepared statements can also be created and accessed at the SQL
    command level, using <TT
CLASS="COMMAND"
>PREPARE</TT
> and <TT
CLASS="COMMAND"
>EXECUTE</TT
>.
   </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 <TT
CLASS="TYPE"
>void</TT
> 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"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     The choice between text and binary output is determined by the format
     codes given in Bind, regardless of the SQL command involved.  The
     <TT
CLASS="LITERAL"
>BINARY</TT
> attribute in cursor declarations is irrelevant when
     using extended query protocol.
    </P
></BLOCKQUOTE
></DIV
><P
>    Query planning typically occurs when the Bind message is processed.
    If the prepared statement has no parameters, or is executed repeatedly,
    the server might save the created plan and re-use it during subsequent
    Bind messages for the same prepared statement.  However, it will do so
    only if it finds that a generic plan can be created that is not much
    less efficient than a plan that depends on the specific parameter values
    supplied.  This happens transparently so far as the protocol is concerned.
   </P
><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 another
    Bind message, but this is not required for the unnamed portal.
    Named portals can also be created and accessed at the SQL
    command level, using <TT
CLASS="COMMAND"
>DECLARE CURSOR</TT
> and <TT
CLASS="COMMAND"
>FETCH</TT
>.
   </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 <SPAN
CLASS="QUOTE"
>"fetch all rows"</SPAN
>).
    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
    <TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>COMMIT</TT
> transaction block (<SPAN
CLASS="QUOTE"
>"close"</SPAN
>
    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"
><I
CLASS="EMPHASIS"
>while</I
></SPAN
> processing Sync &mdash; this ensures that there is one
    and only one ReadyForQuery sent for each Sync.)
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     Sync does not cause a transaction block opened with <TT
CLASS="COMMAND"
>BEGIN</TT
>
     to be closed.  It is possible to detect this situation since the
     ReadyForQuery message includes transaction status information.
    </P
></BLOCKQUOTE
></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"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>     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
></BLOCKQUOTE
></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"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     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
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN95963"
>46.2.4. Function Call</A
></H2
><P
>    The Function Call sub-protocol allows the client to request a direct
    call of any function that exists in the database's
    <TT
CLASS="STRUCTNAME"
>pg_proc</TT
> system catalog.  The client must have
    execute permission for the function.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     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 <TT
CLASS="LITERAL"
>SELECT function($1, ...)</TT
>.
     The Function Call cycle can then be replaced with Bind/Execute.
    </P
></BLOCKQUOTE
></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 can safely send a new query or
    function call.
   </P
><P
>    The possible response messages from the backend are:

    <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>ErrorResponse</DT
><DD
><P
>        An error has occurred.
       </P
></DD
><DT
>FunctionCallResponse</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
>ReadyForQuery</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
>NoticeResponse</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"
><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-COPY"
>46.2.5. COPY Operations</A
></H2
><P
>    The <TT
CLASS="COMMAND"
>COPY</TT
> 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 <TT
CLASS="COMMAND"
>COPY FROM STDIN</TT
> 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 <TT
CLASS="COMMAND"
>COPY</TT
> SQL statement to fail with an
    error).  The backend then reverts to the command-processing mode it was
    in before the <TT
CLASS="COMMAND"
>COPY</TT
> 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 <TT
CLASS="COMMAND"
>COPY</TT
> 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 <TT
CLASS="COMMAND"
>COPY</TT
> 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 <TT
CLASS="COMMAND"
>COPY FROM STDIN</TT
>.)
   </P
><P
>    Copy-out mode (data transfer from the server) is initiated when the
    backend executes a <TT
CLASS="COMMAND"
>COPY TO STDOUT</TT
> 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 <TT
CLASS="COMMAND"
>COPY</TT
> 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 as
    terminating the copy-out mode.
   </P
><P
>    It is possible for NoticeResponse and ParameterStatus messages to be
    interspersed between CopyData messages; frontends must handle these cases,
    and should be prepared for other asynchronous message types as well (see
    <A
HREF="protocol-flow.html#PROTOCOL-ASYNC"
>Section 46.2.6</A
>).  Otherwise, any message type other than
    CopyData or CopyDone may be treated as terminating copy-out mode.
   </P
><P
>    There is another Copy-related mode called Copy-both, which allows
    high-speed bulk data transfer to <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>and</I
></SPAN
> from the server.
    Copy-both mode is initiated when a backend in walsender mode
    executes a <TT
CLASS="COMMAND"
>START_REPLICATION</TT
> statement.  The
    backend sends a CopyBothResponse message to the frontend.  Both
    the backend and the frontend may then send CopyData messages
    until the connection is terminated.  See <A
HREF="protocol-replication.html"
>Section 46.3</A
>.
   </P
><P
>    The CopyInResponse, CopyOutResponse and CopyBothResponse messages
    include fields that inform the frontend of the number of columns
    per row and the format codes being used for each column.  (As of
    the present implementation, all columns in a given <TT
CLASS="COMMAND"
>COPY</TT
>
    operation will use the same format, but the message design does not
    assume this.)
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-ASYNC"
>46.2.6. Asynchronous Operations</A
></H2
><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 <SPAN
CLASS="QUOTE"
>"fast"</SPAN
> 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 <TT
CLASS="COMMAND"
>SET</TT
> 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 server.  Also,
    if a <TT
CLASS="COMMAND"
>SET</TT
> 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
    <TT
CLASS="VARNAME"
>server_version</TT
>,
    <TT
CLASS="VARNAME"
>server_encoding</TT
>,
    <TT
CLASS="VARNAME"
>client_encoding</TT
>,
    <TT
CLASS="VARNAME"
>application_name</TT
>,
    <TT
CLASS="VARNAME"
>is_superuser</TT
>,
    <TT
CLASS="VARNAME"
>session_authorization</TT
>,
    <TT
CLASS="VARNAME"
>DateStyle</TT
>,
    <TT
CLASS="VARNAME"
>IntervalStyle</TT
>,
    <TT
CLASS="VARNAME"
>TimeZone</TT
>,
    <TT
CLASS="VARNAME"
>integer_datetimes</TT
>, and
    <TT
CLASS="VARNAME"
>standard_conforming_strings</TT
>.
    (<TT
CLASS="VARNAME"
>server_encoding</TT
>, <TT
CLASS="VARNAME"
>TimeZone</TT
>, and
    <TT
CLASS="VARNAME"
>integer_datetimes</TT
> were not reported by releases before 8.0;
    <TT
CLASS="VARNAME"
>standard_conforming_strings</TT
> was not reported by releases
    before 8.1;
    <TT
CLASS="VARNAME"
>IntervalStyle</TT
> was not reported by releases before 8.4;
    <TT
CLASS="VARNAME"
>application_name</TT
> was not reported by releases before 9.0.)
    Note that
    <TT
CLASS="VARNAME"
>server_version</TT
>,
    <TT
CLASS="VARNAME"
>server_encoding</TT
> and
    <TT
CLASS="VARNAME"
>integer_datetimes</TT
>
    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 <TT
CLASS="COMMAND"
>LISTEN</TT
> command, then the
    backend will send a NotificationResponse message (not to be
    confused with NoticeResponse!)  whenever a
    <TT
CLASS="COMMAND"
>NOTIFY</TT
> command is executed for the same
    channel name.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     At present, NotificationResponse can only be sent outside a
     transaction, and thus it will not occur in the middle of a
     command-response series, though it might occur just before ReadyForQuery.
     It is unwise to design frontend logic that assumes that, however.
     Good practice is to be able to accept NotificationResponse at any
     point in the protocol.
    </P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN96049"
>46.2.7. Canceling Requests in Progress</A
></H2
><P
>    During the processing of a query, the frontend might request
    cancellation of the query.  The cancel request is not sent
    directly on the open connection to the backend for reasons of
    implementation efficiency: we don't want to have the backend
    constantly checking for new input from the frontend during query
    processing.  Cancel requests should be relatively infrequent, so
    we make them slightly cumbersome in order to avoid a penalty in
    the normal case.
   </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 might or might not have any effect &mdash; for
    example, if it arrives after the backend has finished processing
    the query, then it will have no effect.  If the cancellation is
    effective, it results in the current command being terminated
    early with an error message.
   </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 might provide additional flexibility when building
    multiple-process applications.  It also introduces a security
    risk, in that unauthorized persons might try to cancel queries.
    The security risk is addressed by requiring a dynamically
    generated secret key to be supplied in cancel requests.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN96057"
>46.2.8. Termination</A
></H2
><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 might disconnect without any frontend request to do so.
    In such cases the backend will attempt to send an error or notice message
    giving the reason for the disconnection before it closes the connection.
   </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-<TT
CLASS="COMMAND"
>SELECT</TT
> query
    is being processed, the backend will probably finish the query
    before noticing the disconnection.  If the query is outside any
    transaction block (<TT
CLASS="COMMAND"
>BEGIN</TT
> ... <TT
CLASS="COMMAND"
>COMMIT</TT
>
    sequence) then its results might be committed before the
    disconnection is recognized.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN96066"
>46.2.9. <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> Session Encryption</A
></H2
><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"
>Section 17.9</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 <TT
CLASS="LITERAL"
>S</TT
> or <TT
CLASS="LITERAL"
>N</TT
>, indicating that it is
    willing or unwilling to perform <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
>,
    respectively.  The frontend might close the connection at this point
    if it is dissatisfied with the response.  To continue after
    <TT
CLASS="LITERAL"
>S</TT
>, 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
    <TT
CLASS="LITERAL"
>N</TT
>, 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
>.  (Such servers are now very ancient,
    and likely do not exist in the wild anymore.)
    In this case the connection must
    be closed, but the frontend might choose to open a fresh connection
    and proceed without requesting <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
>.
   </P
><P
>    An initial SSLRequest can 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 can
    configure the server to reject unencrypted sessions as a byproduct
    of authentication checking.
   </P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="protocol-overview.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="protocol-replication.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Overview</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="protocol.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Streaming Replication Protocol</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>