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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>55.2. Message Flow</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="protocol-overview.html" title="55.1. Overview" /><link rel="next" href="sasl-authentication.html" title="55.3. SASL Authentication" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">55.2. Message Flow</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="protocol-overview.html" title="55.1. Overview">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="protocol.html" title="Chapter 55. Frontend/Backend Protocol">Up</a></td><th width="60%" align="center">Chapter 55. Frontend/Backend Protocol</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sasl-authentication.html" title="55.3. SASL Authentication">Next</a></td></tr></table><hr /></div><div class="sect1" id="PROTOCOL-FLOW"><div class="titlepage"><div><div><h2 class="title" style="clear: both">55.2. Message Flow</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.3">55.2.1. Start-up</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.4">55.2.2. Simple Query</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY">55.2.3. Extended Query</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-PIPELINING">55.2.4. Pipelining</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.7">55.2.5. Function Call</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-COPY">55.2.6. COPY Operations</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-ASYNC">55.2.7. Asynchronous Operations</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.10">55.2.8. Canceling Requests in Progress</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.11">55.2.9. Termination</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.12">55.2.10. <acronym class="acronym">SSL</acronym> Session Encryption</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.13">55.2.11. <acronym class="acronym">GSSAPI</acronym> Session Encryption</a></span></dt></dl></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 class="xref" href="protocol-message-formats.html" title="55.7. Message Formats">Section 55.7</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" id="id-1.10.6.7.3"><div class="titlepage"><div><div><h3 class="title">55.2.1. 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).
For all authentication methods except GSSAPI, SSPI and SASL, 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, SSPI and SASL, 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><div class="variablelist"><dl class="variablelist"><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. This is no
longer supported.
</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">AuthenticationMD5Password</span></dt><dd><p>
The frontend must now send a PasswordMessage containing the
password (with user name) 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 <code class="literal">concat('md5',
md5(concat(md5(concat(password, username)), random-salt)))</code>.
(Keep in mind the <code class="function">md5()</code> function returns its
result as a hex string.)
</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.
(This message type is only issued by pre-9.1 servers. It may
eventually be removed from the protocol specification.)
</p></dd><dt><span class="term">AuthenticationGSS</span></dt><dd><p>
The frontend must now initiate a GSSAPI negotiation. The frontend
will send a GSSResponse message 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><span class="term">AuthenticationSSPI</span></dt><dd><p>
The frontend must now initiate an SSPI negotiation. The frontend
will send a GSSResponse 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><span class="term">AuthenticationGSSContinue</span></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 GSSResponse message. 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><dt><span class="term">AuthenticationSASL</span></dt><dd><p>
The frontend must now initiate a SASL negotiation, using one of the
SASL mechanisms listed in the message. The frontend will send a
SASLInitialResponse with the name of the selected mechanism, and the
first part of the SASL data stream in response to this. If further
messages are needed, the server will respond with
AuthenticationSASLContinue. See <a class="xref" href="sasl-authentication.html" title="55.3. SASL Authentication">Section 55.3</a>
for details.
</p></dd><dt><span class="term">AuthenticationSASLContinue</span></dt><dd><p>
This message contains challenge data from the previous step of SASL
negotiation (AuthenticationSASL, or a previous
AuthenticationSASLContinue). The frontend must respond with a
SASLResponse message.
</p></dd><dt><span class="term">AuthenticationSASLFinal</span></dt><dd><p>
SASL authentication has completed with additional mechanism-specific
data for the client. The server will next send AuthenticationOk to
indicate successful authentication, or an ErrorResponse to indicate
failure. This message is sent only if the SASL mechanism specifies
additional data to be sent from server to client at completion.
</p></dd><dt><span class="term">NegotiateProtocolVersion</span></dt><dd><p>
The server does not support the minor protocol version requested
by the client, but does support an earlier version of the protocol;
this message indicates the highest supported minor version. This
message will also be sent if the client requested unsupported protocol
options (i.e., beginning with <code class="literal">_pq_.</code>) in the
startup packet. This message will be followed by an ErrorResponse or
a message indicating the success or failure of authentication.
</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) or the server to decline support for the requested
minor protocol version (NegotiateProtocolVersion), 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 class="variablelist"><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 class="xref" href="runtime-config-client.html#GUC-CLIENT-ENCODING">client_encoding</a> or <a class="xref" 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 class="xref" href="protocol-flow.html#PROTOCOL-ASYNC" title="55.2.7. Asynchronous Operations">Section 55.2.7</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 can 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" id="id-1.10.6.7.4"><div class="titlepage"><div><div><h3 class="title">55.2.2. 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 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><div class="variablelist"><dl class="variablelist"><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 class="xref" href="protocol-flow.html#PROTOCOL-COPY" title="55.2.6. COPY Operations">Section 55.2.6</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 class="xref" href="protocol-flow.html#PROTOCOL-COPY" title="55.2.6. COPY Operations">Section 55.2.6</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 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><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 class="xref" href="protocol-flow.html#PROTOCOL-COPY" title="55.2.6. COPY Operations">Section 55.2.6</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 <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 class="xref" href="protocol-flow.html#PROTOCOL-ASYNC" title="55.2.7. Asynchronous Operations">Section 55.2.7</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 class="sect3" id="PROTOCOL-FLOW-MULTI-STATEMENT"><div class="titlepage"><div><div><h4 class="title">55.2.2.1. Multiple Statements in a Simple Query</h4></div></div></div><p>
When a simple Query message contains more than one SQL statement
(separated by semicolons), those statements are executed as a single
transaction, unless explicit transaction control commands are included
to force a different behavior. For example, if the message contains
</p><pre class="programlisting">
INSERT INTO mytable VALUES(1);
SELECT 1/0;
INSERT INTO mytable VALUES(2);
</pre><p>
then the divide-by-zero failure in the <code class="command">SELECT</code> will force
rollback of the first <code class="command">INSERT</code>. Furthermore, because
execution of the message is abandoned at the first error, the second
<code class="command">INSERT</code> is never attempted at all.
</p><p>
If instead the message contains
</p><pre class="programlisting">
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELECT 1/0;
</pre><p>
then the first <code class="command">INSERT</code> is committed by the
explicit <code class="command">COMMIT</code> command. The second <code class="command">INSERT</code>
and the <code class="command">SELECT</code> are still treated as a single transaction,
so that the divide-by-zero failure will roll back the
second <code class="command">INSERT</code>, but not the first one.
</p><p>
This behavior is implemented by running the statements in a
multi-statement Query message in an <em class="firstterm">implicit transaction
block</em> unless there is some explicit transaction block for them to
run in. The main difference between an implicit transaction block and
a regular one is that an implicit block is closed automatically at the
end of the Query message, either by an implicit commit if there was no
error, or an implicit rollback if there was an error. This is similar
to the implicit commit or rollback that happens for a statement
executed by itself (when not in a transaction block).
</p><p>
If the session is already in a transaction block, as a result of
a <code class="command">BEGIN</code> in some previous message, then the Query message
simply continues that transaction block, whether the message contains
one statement or several. However, if the Query message contains
a <code class="command">COMMIT</code> or <code class="command">ROLLBACK</code> closing the existing
transaction block, then any following statements are executed in an
implicit transaction block.
Conversely, if a <code class="command">BEGIN</code> appears in a multi-statement Query
message, then it starts a regular transaction block that will only be
terminated by an explicit <code class="command">COMMIT</code> or <code class="command">ROLLBACK</code>,
whether that appears in this Query message or a later one.
If the <code class="command">BEGIN</code> follows some statements that were executed as
an implicit transaction block, those statements are not immediately
committed; in effect, they are retroactively included into the new
regular transaction block.
</p><p>
A <code class="command">COMMIT</code> or <code class="command">ROLLBACK</code> appearing in an implicit
transaction block is executed as normal, closing the implicit block;
however, a warning will be issued since a <code class="command">COMMIT</code>
or <code class="command">ROLLBACK</code> without a previous <code class="command">BEGIN</code> might
represent a mistake. If more statements follow, a new implicit
transaction block will be started for them.
</p><p>
Savepoints are not allowed in an implicit transaction block, since
they would conflict with the behavior of automatically closing the
block upon any error.
</p><p>
Remember that, regardless of any transaction control commands that may
be present, execution of the Query message stops at the first error.
Thus for example given
</p><pre class="programlisting">
BEGIN;
SELECT 1/0;
ROLLBACK;
</pre><p>
in a single Query message, the session will be left inside a failed
regular transaction block, since the <code class="command">ROLLBACK</code> is not
reached after the divide-by-zero error. Another <code class="command">ROLLBACK</code>
will be needed to restore the session to a usable state.
</p><p>
Another behavior of note is that initial lexical and syntactic
analysis is done on the entire query string before any of it is
executed. Thus simple errors (such as a misspelled keyword) in later
statements can prevent execution of any of the statements. This
is normally invisible to users since the statements would all roll
back anyway when done as an implicit transaction block. However,
it can be visible when attempting to do multiple transactions within a
multi-statement Query. For instance, if a typo turned our previous
example into
</p><pre class="programlisting">
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELCT 1/0;
</pre><p>
then none of the statements would get run, resulting in the visible
difference that the first <code class="command">INSERT</code> is not committed.
Errors detected at semantic analysis or later, such as a misspelled
table or column name, do not have this effect.
</p></div></div><div class="sect2" id="PROTOCOL-FLOW-EXT-QUERY"><div class="titlepage"><div><div><h3 class="title">55.2.3. 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 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"><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> pseudo-type). 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"><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
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 <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"><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 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 <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 <span class="quote">“<span class="quote">fetch all rows</span>”</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
<code class="command">BEGIN</code>/<code class="command">COMMIT</code> transaction block (<span class="quote">“<span class="quote">close</span>”</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"><em>while</em></span> processing Sync — this ensures that there is one
and only one ReadyForQuery sent for each Sync.)
</p><div class="note"><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"><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"><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" id="PROTOCOL-FLOW-PIPELINING"><div class="titlepage"><div><div><h3 class="title">55.2.4. Pipelining</h3></div></div></div><a id="id-1.10.6.7.6.2" class="indexterm"></a><p>
Use of the extended query protocol
allows <em class="firstterm">pipelining</em>, which means sending a series
of queries without waiting for earlier ones to complete. This reduces
the number of network round trips needed to complete a given series of
operations. However, the user must carefully consider the required
behavior if one of the steps fails, since later queries will already
be in flight to the server.
</p><p>
One way to deal with that is to make the whole query series be a
single transaction, that is wrap it in <code class="command">BEGIN</code> ...
<code class="command">COMMIT</code>. However, this does not help if one wishes
for some of the commands to commit independently of others.
</p><p>
The extended query protocol provides another way to manage this
concern, which is to omit sending Sync messages between steps that
are dependent. Since, after an error, the backend will skip command
messages until it finds Sync, this allows later commands in a pipeline
to be skipped automatically when an earlier one fails, without the
client having to manage that explicitly with <code class="command">BEGIN</code>
and <code class="command">COMMIT</code>. Independently-committable segments
of the pipeline can be separated by Sync messages.
</p><p>
If the client has not issued an explicit <code class="command">BEGIN</code>,
then each Sync ordinarily causes an implicit <code class="command">COMMIT</code>
if the preceding step(s) succeeded, or an
implicit <code class="command">ROLLBACK</code> if they failed. However, there
are a few DDL commands (such as <code class="command">CREATE DATABASE</code>)
that cannot be executed inside a transaction block. If one of
these is executed in a pipeline, it will fail unless it is the first
command in the pipeline. Furthermore, upon success it will force an
immediate commit to preserve database consistency. Thus a Sync
immediately following one of these commands has no effect except to
respond with ReadyForQuery.
</p><p>
When using this method, completion of the pipeline must be determined
by counting ReadyForQuery messages and waiting for that to reach the
number of Syncs sent. Counting command completion responses is
unreliable, since some of the commands may be skipped and thus not
produce a completion message.
</p></div><div class="sect2" id="id-1.10.6.7.7"><div class="titlepage"><div><div><h3 class="title">55.2.5. 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"><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 can safely send a new query or
function call.
</p><p>
The possible response messages from the backend are:
</p><div class="variablelist"><dl class="variablelist"><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" id="PROTOCOL-COPY"><div class="titlepage"><div><div><h3 class="title">55.2.6. 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 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 class="xref" href="protocol-flow.html#PROTOCOL-ASYNC" title="55.2.7. Asynchronous Operations">Section 55.2.7</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"><em>and</em></span> from the server.
Copy-both mode is initiated when a backend in walsender mode
executes a <code class="command">START_REPLICATION</code> statement. The
backend sends a CopyBothResponse message to the frontend. Both
the backend and the frontend may then send CopyData messages
until either end sends a CopyDone message. After the client
sends a CopyDone message, the connection goes from copy-both mode to
copy-out mode, and the client may not send any more CopyData messages.
Similarly, when the server sends a CopyDone message, the connection
goes into copy-in mode, and the server may not send any more CopyData
messages. After both sides have sent a CopyDone message, the copy mode
is terminated, and the backend reverts to the command-processing mode.
In the event of a backend-detected error during copy-both mode,
the backend will issue an ErrorResponse message, discard frontend messages
until a Sync message is received, and then issue ReadyForQuery and return
to normal processing. The frontend should treat receipt of ErrorResponse
as terminating the copy in both directions; no CopyDone should be sent
in this case. See <a class="xref" href="protocol-replication.html" title="55.4. Streaming Replication Protocol">Section 55.4</a> for more
information on the subprotocol transmitted over copy-both mode.
</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 <code class="command">COPY</code>
operation will use the same format, but the message design does not
assume this.)
</p></div><div class="sect2" id="PROTOCOL-ASYNC"><div class="titlepage"><div><div><h3 class="title">55.2.7. 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 <span class="quote">“<span class="quote">fast</span>”</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 <code class="command">SET</code> 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 <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="varname">server_version</code>,
<code class="varname">server_encoding</code>,
<code class="varname">client_encoding</code>,
<code class="varname">application_name</code>,
<code class="varname">default_transaction_read_only</code>,
<code class="varname">in_hot_standby</code>,
<code class="varname">is_superuser</code>,
<code class="varname">session_authorization</code>,
<code class="varname">DateStyle</code>,
<code class="varname">IntervalStyle</code>,
<code class="varname">TimeZone</code>,
<code class="varname">integer_datetimes</code>, and
<code class="varname">standard_conforming_strings</code>.
(<code class="varname">server_encoding</code>, <code class="varname">TimeZone</code>, and
<code class="varname">integer_datetimes</code> were not reported by releases before 8.0;
<code class="varname">standard_conforming_strings</code> was not reported by releases
before 8.1;
<code class="varname">IntervalStyle</code> was not reported by releases before 8.4;
<code class="varname">application_name</code> was not reported by releases before
9.0;
<code class="varname">default_transaction_read_only</code> and
<code class="varname">in_hot_standby</code> were not reported by releases before
14.)
Note that
<code class="varname">server_version</code>,
<code class="varname">server_encoding</code> and
<code class="varname">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
channel name.
</p><div class="note"><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 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></div></div><div class="sect2" id="id-1.10.6.7.10"><div class="titlepage"><div><div><h3 class="title">55.2.8. Canceling Requests in Progress</h3></div></div></div><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" id="id-1.10.6.7.11"><div class="titlepage"><div><div><h3 class="title">55.2.9. 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 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-<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 might be committed before the
disconnection is recognized.
</p></div><div class="sect2" id="id-1.10.6.7.12"><div class="titlepage"><div><div><h3 class="title">55.2.10. <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 class="xref" href="ssl-tcp.html" title="19.9. Secure TCP/IP Connections with SSL">Section 19.9</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 <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 might 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.
(Alternatively, it is permissible to issue a GSSENCRequest message
after an <code class="literal">N</code> response to try to
use <acronym class="acronym">GSSAPI</acronym> encryption instead
of <acronym class="acronym">SSL</acronym>.)
</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>
When <acronym class="acronym">SSL</acronym> encryption can be performed, the server
is expected to send only the single <code class="literal">S</code> byte and then
wait for the frontend to initiate an <acronym class="acronym">SSL</acronym> handshake.
If additional bytes are available to read at this point, it likely
means that a man-in-the-middle is attempting to perform a
buffer-stuffing attack
(<a class="ulink" href="https://www.postgresql.org/support/security/CVE-2021-23222/" target="_top">CVE-2021-23222</a>).
Frontends should be coded either to read exactly one byte from the
socket before turning the socket over to their SSL library, or to
treat it as a protocol violation if they find they have read additional
bytes.
</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 class="sect2" id="id-1.10.6.7.13"><div class="titlepage"><div><div><h3 class="title">55.2.11. <acronym class="acronym">GSSAPI</acronym> Session Encryption</h3></div></div></div><p>
If <span class="productname">PostgreSQL</span> was built with
<acronym class="acronym">GSSAPI</acronym> support, frontend/backend communications
can be encrypted using <acronym class="acronym">GSSAPI</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">GSSAPI</acronym>, see <a class="xref" href="gssapi-enc.html" title="19.10. Secure TCP/IP Connections with GSSAPI Encryption">Section 19.10</a>.
</p><p>
To initiate a <acronym class="acronym">GSSAPI</acronym>-encrypted connection, the
frontend initially sends a GSSENCRequest message rather than a
StartupMessage. The server then responds with a single byte
containing <code class="literal">G</code> or <code class="literal">N</code>, indicating that it
is willing or unwilling to perform <acronym class="acronym">GSSAPI</acronym> encryption,
respectively. The frontend might close the connection at this point
if it is dissatisfied with the response. To continue after
<code class="literal">G</code>, using the GSSAPI C bindings as discussed in
<a class="ulink" href="https://datatracker.ietf.org/doc/html/rfc2744" target="_top">RFC 2744</a>
or equivalent, perform a <acronym class="acronym">GSSAPI</acronym> initialization by
calling <code class="function">gss_init_sec_context()</code> in a loop and sending
the result to the server, starting with an empty input and then with each
result from the server, until it returns no output. When sending the
results of <code class="function">gss_init_sec_context()</code> to the server,
prepend the length of the message as a four byte integer in network byte
order.
To continue after
<code class="literal">N</code>, send the usual StartupMessage and proceed without
encryption.
(Alternatively, it is permissible to issue an SSLRequest message
after an <code class="literal">N</code> response to try to
use <acronym class="acronym">SSL</acronym> encryption instead
of <acronym class="acronym">GSSAPI</acronym>.)
</p><p>
The frontend should also be prepared to handle an ErrorMessage
response to GSSENCRequest from the server. This would only occur if
the server predates the addition of <acronym class="acronym">GSSAPI</acronym> encryption
support to <span class="productname">PostgreSQL</span>. 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">GSSAPI</acronym>
encryption.
</p><p>
When <acronym class="acronym">GSSAPI</acronym> encryption can be performed, the server
is expected to send only the single <code class="literal">G</code> byte and then
wait for the frontend to initiate a <acronym class="acronym">GSSAPI</acronym> handshake.
If additional bytes are available to read at this point, it likely
means that a man-in-the-middle is attempting to perform a
buffer-stuffing attack
(<a class="ulink" href="https://www.postgresql.org/support/security/CVE-2021-23222/" target="_top">CVE-2021-23222</a>).
Frontends should be coded either to read exactly one byte from the
socket before turning the socket over to their GSSAPI library, or to
treat it as a protocol violation if they find they have read additional
bytes.
</p><p>
An initial GSSENCRequest can also be used in a connection that is being
opened to send a CancelRequest message.
</p><p>
Once <acronym class="acronym">GSSAPI</acronym> encryption has been successfully
established, use <code class="function">gss_wrap()</code> to
encrypt the usual StartupMessage and all subsequent data, prepending the
length of the result from <code class="function">gss_wrap()</code> as a four byte
integer in network byte order to the actual encrypted payload. Note that
the server will only accept encrypted packets from the client which are less
than 16kB; <code class="function">gss_wrap_size_limit()</code> should be used by the
client to determine the size of the unencrypted message which will fit
within this limit and larger messages should be broken up into multiple
<code class="function">gss_wrap()</code> calls. Typical segments are 8kB of
unencrypted data, resulting in encrypted packets of slightly larger than 8kB
but well within the 16kB maximum. The server can be expected to not send
encrypted packets of larger than 16kB to the client.
</p><p>
While the protocol itself does not provide a way for the server to
force <acronym class="acronym">GSSAPI</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 /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="protocol-overview.html" title="55.1. Overview">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="protocol.html" title="Chapter 55. Frontend/Backend Protocol">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sasl-authentication.html" title="55.3. SASL Authentication">Next</a></td></tr><tr><td width="40%" align="left" valign="top">55.1. Overview </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 55.3. SASL Authentication</td></tr></table></div></body></html>
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