| Current File : //usr/share/doc/postgresql-9.2.24/html/protocol-flow.html |
<!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 — 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 — 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 — 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
>