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Postgres FD Implementation
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updates for new startup sequence, some reformatting

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<Chapter Id="protocol">
<DocInfo>
<Author>
<FirstName>Phil</FirstName>
<Surname>Thompson</Surname>
</Author>
<Date>1998-08-08</Date>
</DocInfo>
<Title>Frontend/Backend Protocol</Title>
<Para>
<Note>
<Para>
Written by Phil Thompson (<email>phil@river-bank.demon.co.uk</email>).
Updates for protocol 2.0 by Tom Lane (<email>tgl@sss.pgh.pa.us</email>).
</Para>
</Note>
</para>
<Para>
<ProductName>Postgres</ProductName> uses a message-based protocol for communication between frontends
and backends. The protocol is implemented over <Acronym>TCP/IP</Acronym> and also on Unix sockets.
<ProductName>Postgres</ProductName> 6.3 introduced version numbers into the protocol.
This was done in such
a way as to still allow connections from earlier versions of frontends, but
this document does not cover the protocol used by those earlier versions.
</para>
<Para>
This document describes version 2.0 of the protocol, implemented in
<ProductName>Postgres</ProductName> 6.4 and later.
</para>
<Para>
Higher level features built on this protocol (for example, how <FileName>libpq</FileName> passes
certain environment variables after the connection is established)
are covered elsewhere.
</para>
<Sect1 id="protocol-overview">
<Title>Overview</Title>
<Para>
The three major components are the frontend (running on the client) and the
postmaster and backend (running on the server). The postmaster and backend
have different roles but may be implemented by the same executable.
</para>
<Para>
A frontend sends a start-up packet to the postmaster. This includes the names
of the user and the database the user wants to connect to. The postmaster then
uses this, and the information in the <filename>pg_hba.conf</filename>
file
to determine what
further authentication information it requires the frontend to send (if any)
and responds to the frontend accordingly.
</para>
<Para>
The frontend then sends any required authentication information. Once the
postmaster validates this it responds to the frontend that it is authenticated
and hands over the connection to a backend. The backend then sends a message
indicating successful start-up (normal case) or failure (for example, an
invalid database name).
</para>
<Para>
Subsequent communications are query and result packets exchanged between the
frontend and the backend. The postmaster takes no further part in ordinary
query/result communication. (However, the postmaster is involved when the
frontend wishes to cancel a query currently being executed by its backend.
Further details about that appear below.)
</para>
<Para>
When the frontend wishes to disconnect it sends an appropriate packet and
closes the connection without waiting for a response for the backend.
</para>
<Para>
Packets are sent as a data stream. The first byte determines what should be
expected in the rest of the packet. The exception is packets sent from a
frontend to the postmaster, which comprise a packet length then the packet
itself. The difference is historical.
</para>
</sect1>
<Sect1 id="protocol-protocol">
<Title>Protocol</Title>
<Para>
This section describes the message flow. There are four different types of
flows depending on the state of the connection:
start-up, query, function call, and termination.
There are also special provisions for notification responses and command
cancellation, which can occur at any time after the start-up phase.
</para>
<Sect2>
<Title>Start-up</Title>
<Para>
Start-up is divided into an authentication phase and a backend start-up phase.
</para>
<Para>
Initially, the frontend sends a StartupPacket. The postmaster uses this info
and the contents of the pg_hba.conf file to determine what authentication
method the frontend must use. The postmaster then responds with one of the
following messages:
</para>
<Para>
<VariableList>
<VarListEntry>
<Term>
ErrorResponse
</Term>
<ListItem>
<Para>
The postmaster then immediately closes the connection.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
AuthenticationOk
</Term>
<ListItem>
<Para>
The postmaster then hands over to the backend. The postmaster
takes no further part in the communication.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
AuthenticationKerberosV4
</Term>
<ListItem>
<Para>
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/protocol.sgml,v 1.18 2001/06/22 23:27:48 petere Exp $ -->
<chapter id="protocol">
<title>Frontend/Backend Protocol</title>
<note>
<para>
Written by Phil Thompson (<email>phil@river-bank.demon.co.uk</email>).
Updates for protocol 2.0 by Tom Lane (<email>tgl@sss.pgh.pa.us</email>).
</para>
</note>
<para>
<application>PostgreSQL</application> uses a message-based protocol
for communication between frontends and backends. The protocol is
implemented over <acronym>TCP/IP</acronym> and also on Unix domain
sockets. <productname>PostgreSQL</productname> 6.3 introduced
version numbers into the protocol. This was done in such a way as
to still allow connections from earlier versions of frontends, but
this document does not cover the protocol used by those earlier
versions.
</para>
<para>
This document describes version 2.0 of the protocol, implemented in
<application>PostgreSQL</application> 6.4 and later.
</para>
<para>
Higher level features built on this protocol (for example, how
<application>libpq</application> passes certain environment
variables after the connection is established) are covered
elsewhere.
</para>
<sect1 id="protocol-overview">
<title>Overview</title>
<para>
A frontend opens a connection to the server and sends a start-up
packet. This includes the names of the user and the database the
user wants to connect to. The server then uses this, and the
information in the <filename>pg_hba.conf</filename> file to
determine what further authentication information it requires the
frontend to send (if any) and responds to the frontend accordingly.
</para>
<para>
The frontend then sends any required authentication information.
Once the server validates this it responds to the frontend that it
is authenticated and sends a message indicating successful start-up
(normal case) or failure (for example, an invalid database name).
</para>
<para>
In order to serve multiple clients efficiently, the server would
normally create a new child process to handle each incoming
connection. However, this is not required. In the current
implementation, a new child process is created immediately after an
incoming connection is detected. In earlier versions of PostgreSQL
(7.1 and earlier), the child process was created after sending the
authentication confirmation message.
</para>
<para>
When the frontend wishes to disconnect it sends an appropriate packet and
closes the connection without waiting for a response for the backend.
</para>
<para>
Packets are sent as a data stream. The first byte determines what
should be expected in the rest of the packet. The exceptions are
packets sent as part of the startup and authentication exchange,
which comprise a packet length followed by the packet itself. The
difference is historical.
</para>
</sect1>
<sect1 id="protocol-protocol">
<title>Protocol</title>
<para>
This section describes the message flow. There are four different
types of flows depending on the state of the connection: start-up,
query, function call, and termination. There are also special
provisions for notification responses and command cancellation,
which can occur at any time after the start-up phase.
</para>
<sect2>
<title>Start-up</Title>
<para>
Initially, the frontend sends a StartupPacket. The server uses
this info and the contents of the <filename>pg_hba.conf</filename>
file to determine what authentication method the frontend must
use. The server then responds with one of the following messages:
<variablelist>
<varlistentry>
<term>ErrorResponse</term>
<listitem>
<para>
The server then immediately closes the connection.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>AuthenticationOk</term>
<listitem>
<para>
The authentication exchange is completed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>AuthenticationKerberosV4</Term>
<listitem>
<para>
The frontend must then take part in a Kerberos V4
authentication dialog (not described here) with the postmaster.
If this is successful, the postmaster responds with an
AuthenticationOk, otherwise it responds with an ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
AuthenticationKerberosV5
</Term>
<ListItem>
<Para>
authentication dialog (not described here, part of the
Kerberos specification) with the server. If this is
successful, the server responds with an AuthenticationOk,
otherwise it responds with an ErrorResponse.
</para>
</listitem>
</varlistentry>
<varlistentry>
<Term>AuthenticationKerberosV5</Term>
<ListItem>
<Para>
The frontend must then take part in a Kerberos V5
authentication dialog (not described here) with the postmaster.
If this is successful, the postmaster responds with an
authentication dialog (not described here, part of the
Kerberos specification) with the server. If this is
successful, the server responds with an AuthenticationOk,
otherwise it responds with an ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>AuthenticationUnencryptedPassword</Term>
<ListItem>
<Para>
The frontend must then send an UnencryptedPasswordPacket. If
this is the correct password, the server responds with an
AuthenticationOk, otherwise it responds with an ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
AuthenticationUnencryptedPassword
</Term>
<ListItem>
<Para>
The frontend must then send an UnencryptedPasswordPacket.
If this is the correct password, the postmaster responds with
an AuthenticationOk, otherwise it responds with an
ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
AuthenticationEncryptedPassword
</Term>
<ListItem>
<Para>
The frontend must then send an EncryptedPasswordPacket.
If this is the correct password, the postmaster responds with
an AuthenticationOk, otherwise it responds with an
ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<Para>
If the frontend does not support the authentication method requested by the
postmaster, then it should immediately close the connection.
</para>
<Para>
After sending AuthenticationOk, the postmaster attempts to launch a backend
process. Since this might fail, or the backend might encounter a failure
during start-up, the frontend must wait for the backend to acknowledge
successful start-up. The frontend should send no messages at this point.
The possible messages from the backend during this phase are:
<VariableList>
<VarListEntry>
<Term>
BackendKeyData
</Term>
<ListItem>
<Para>
This message is issued after successful backend start-up.
It provides secret-key data that the frontend must save
if it wants to be able to issue cancel requests later.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>AuthenticationEncryptedPassword</Term>
<ListItem>
<Para>
The frontend must then send an EncryptedPasswordPacket. If
this is the correct password, the server responds with an
AuthenticationOk, otherwise it responds with an ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<para>
If the frontend does not support the authentication method
requested by the server, then it should immediately close the
connection.
</para>
<para>
After having received AuthenticationOk, the frontend should wait
for further messages from the server. The possible messages from
the backend in this phase are:
<VariableList>
<VarListEntry>
<Term>BackendKeyData</Term>
<ListItem>
<Para>
This message provides secret-key data that the frontend must
save if it wants to be able to issue cancel requests later.
The frontend should not respond to this message, but should
continue listening for a ReadyForQuery message.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
ReadyForQuery
</Term>
<ListItem>
<Para>
Backend start-up is successful. The frontend may now issue
query or function call messages.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
ErrorResponse
</Term>
<ListItem>
<Para>
Backend start-up failed. The connection is closed after
sending this message.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
NoticeResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>ReadyForQuery</Term>
<ListItem>
<Para>
Start-up is completed. The frontend may now issue query or
function call messages.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>ErrorResponse</Term>
<ListItem>
<Para>
Start-up failed. The connection is closed after sending this
message.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>NoticeResponse</Term>
<ListItem>
<Para>
A warning message has been issued. The frontend should
display the message but continue listening for ReadyForQuery
or ErrorResponse.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<Para>
The ReadyForQuery message is the same one that the backend will issue after
each query cycle. Depending on the coding needs of the frontend, it is
reasonable to consider ReadyForQuery as starting a query cycle (and then
BackendKeyData indicates successful conclusion of the start-up phase),
or to consider ReadyForQuery as ending the start-up phase and each subsequent
query cycle.
</para>
</sect2>
<Sect2>
<Title>Query</Title>
<Para>
A Query cycle is initiated by the frontend sending a Query message to the
backend. The backend then sends one or more response messages depending
on the contents of the query command string, and finally a ReadyForQuery
response message. ReadyForQuery informs the frontend that it may safely
send a new query or function call.
</para>
<Para>
The possible response messages from the backend are:
<VariableList>
<VarListEntry>
<Term>
CompletedResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<para>
The ReadyForQuery message is the same one that the backend will
issue after each query cycle. Depending on the coding needs of
the frontend, it is reasonable to consider ReadyForQuery as
starting a query cycle (and then BackendKeyData indicates
successful conclusion of the start-up phase), or to consider
ReadyForQuery as ending the start-up phase and each subsequent
query cycle.
</para>
</sect2>
<Sect2>
<Title>Query</Title>
<Para>
A Query cycle is initiated by the frontend sending a Query message
to the backend. The backend then sends one or more response
messages depending on the contents of the query command string,
and finally a ReadyForQuery response message. ReadyForQuery
informs the frontend that it may safely send a new query or
function call.
</para>
<Para>
The possible response messages from the backend are:
<VariableList>
<VarListEntry>
<Term>CompletedResponse</Term>
<ListItem>
<Para>
An SQL command completed normally.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CopyInResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>CopyInResponse</Term>
<ListItem>
<Para>
The backend is ready to copy data from the frontend to a
relation. The frontend should then send a CopyDataRows
message. The backend will then respond with a
CompletedResponse message with a tag of "COPY".
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CopyOutResponse
</Term>
<ListItem>
<Para>
The backend is ready to copy data from a relation to the
table. The frontend should then send a CopyDataRows message.
The backend will then respond with a CompletedResponse message
with a tag of <literal>COPY</literal>.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>CopyOutResponse</Term>
<ListItem>
<Para>
The backend is ready to copy data from a table to the
frontend. It then sends a CopyDataRows message, and then a
CompletedResponse message with a tag of "COPY".
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
CursorResponse
</Term>
<ListItem>
<Para>
The query was either an insert(l), delete(l), update(l),
fetch(l) or a select(l) command.
If the transaction has been
aborted then the backend sends a CompletedResponse message with
a tag of "*ABORT STATE*". Otherwise the following responses
are sent.
</Para>
<Para>
For an insert(l) command, the backend then sends a
CompletedResponse message with a tag of "INSERT <Replaceable>oid</Replaceable> <Replaceable>rows</Replaceable>"
where <Replaceable>rows</Replaceable> is the number of rows inserted, and <Replaceable>oid</Replaceable> is the
object ID of the inserted row if <Replaceable>rows</Replaceable> is 1, otherwise <Replaceable>oid</Replaceable>
is 0.
</Para>
<Para>
For a delete(l) command, the backend then sends a
CompletedResponse message with a tag of "DELETE <Replaceable>rows</Replaceable>" where
CompletedResponse message with a tag of <literal>COPY</literal>.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>CursorResponse</Term>
<ListItem>
<Para>
The query was either an <literal>INSERT</literal>,
<literal>UPDATE</literal>, <literal>DELETE</literal>,
<literal>FETCH</literal>, or a <literal>SELECT</literal>
command. If the transaction has been aborted then the backend
sends a CompletedResponse message with a tag of <literal>*ABORT
STATE*</literal>. Otherwise the following responses are sent.
</Para>
<Para>
For an <literal>INSERT</literal> command, the backend then
sends a CompletedResponse message with a tag of
<literal>INSERT <replaceable>oid</replaceable>
<replaceable>rows</replaceable></literal>, where
<replaceable>rows</replaceable> is the number of rows
inserted, and <replaceable>oid</replaceable> is the object ID
of the inserted row if <Replaceable>rows</Replaceable> is 1,
otherwise <Replaceable>oid</Replaceable> is 0.
</Para>
<Para>
For a <literal>DELETE</literal> command, the backend then
sends a CompletedResponse message with a tag of <literal>DELETE
<Replaceable>rows</Replaceable></literal> where
<Replaceable>rows</Replaceable> is the number of rows deleted.
</Para>
<Para>
For an update(l) command, the backend then sends a
CompletedResponse message with a tag of "UPDATE <Replaceable>rows</Replaceable>" where
<Replaceable>rows</Replaceable> is the number of rows deleted.
</Para>
<Para>
For a fetch(l) or select(l) command, the backend sends a
RowDescription message. This is then followed by an AsciiRow
or BinaryRow message (depending on whether a binary cursor was
specified) for each row being returned to the frontend.
Finally, the backend sends a CompletedResponse message with a
tag of "SELECT".
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
EmptyQueryResponse
</Term>
<ListItem>
<Para>
</Para>
<Para>
For an <literal>UPDATE</literal> command, the backend then
sends a CompletedResponse message with a tag of <literal>UPDATE
<Replaceable>rows</Replaceable></literal> where
<Replaceable>rows</Replaceable> is the number of rows affected
by the update.
</Para>
<Para>
For a <literal>FETCH</literal> or <literal>SELECT</literal>
command, the backend sends a RowDescription message. This is
then followed by an AsciiRow or BinaryRow message (depending
on whether a binary cursor was specified) for each row being
returned to the frontend. Finally, the backend sends a
CompletedResponse message with a tag of <literal>SELECT</literal>.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>EmptyQueryResponse</Term>
<ListItem>
<Para>
An empty query string was recognized. (The need to specially
distinguish this case is historical.)
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
ErrorResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>ErrorResponse</Term>
<ListItem>
<Para>
An error has occurred.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
ReadyForQuery
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>ReadyForQuery</Term>
<ListItem>
<Para>
Processing of the query string is complete. A separate
message is sent to indicate this because the query string
may contain multiple SQL commands. (CompletedResponse marks
the end of processing one SQL command, not the whole string.)
message is sent to indicate this because the query string may
contain multiple SQL commands. (CompletedResponse marks the
end of processing one SQL command, not the whole string.)
ReadyForQuery will always be sent, whether processing
terminates successfully or with an error.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
NoticeResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>NoticeResponse</Term>
<ListItem>
<Para>
A warning message has been issued in relation to the query.
Notices are in addition to other responses, i.e., the backend
will continue processing the command.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<Para>
A frontend must be prepared to accept ErrorResponse and NoticeResponse
messages whenever it is expecting any other type of message.
</para>
<Para>
Actually, it is possible for NoticeResponse to arrive even when the frontend
is not expecting any kind of message, that is, the backend is nominally idle.
(In particular, the backend can be commanded to terminate by its postmaster.
In that case it will send a NoticeResponse before closing the connection.)
It is recommended that the frontend check for such asynchronous notices just
before issuing any new command.
</para>
<Para>
Also, if the frontend issues any listen(l) commands then it must be prepared
to accept NotificationResponse messages at any time; see below.
</para>
</sect2>
<Sect2>
<Title>Function Call</Title>
<Para>
A Function Call cycle is initiated by the frontend sending a FunctionCall
message to the backend. The backend then sends one or more response messages
depending on the results of the function call, and finally a ReadyForQuery
response message. ReadyForQuery informs the frontend that it may safely send
a new query or function call.
</para>
<Para>
The possible response messages from the backend are:
<VariableList>
<VarListEntry>
<Term>
ErrorResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<para>
A frontend must be prepared to accept ErrorResponse and
NoticeResponse messages whenever it is expecting any other type of
message.
</para>
<Para>
Actually, it is possible for NoticeResponse to arrive even when
the frontend is not expecting any kind of message, that is, the
backend is nominally idle. (In particular, the backend can be
commanded to terminate by its parent process. In that case it will
send a NoticeResponse before closing the connection.) It is
recommended that the frontend check for such asynchronous notices
just before issuing any new command.
</para>
<Para>
Also, if the frontend issues any <literal>LISTEN</literal>
commands then it must be prepared to accept NotificationResponse
messages at any time; see below.
</para>
</sect2>
<Sect2>
<Title>Function Call</Title>
<Para>
A Function Call cycle is initiated by the frontend sending a
FunctionCall message to the backend. The backend then sends one
or more response messages depending on the results of the function
call, and finally a ReadyForQuery response message. ReadyForQuery
informs the frontend that it may safely send a new query or
function call.
</para>
<Para>
The possible response messages from the backend are:
<VariableList>
<VarListEntry>
<Term>ErrorResponse</Term>
<ListItem>
<Para>
An error has occurred.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
FunctionResultResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>FunctionResultResponse</Term>
<ListItem>
<Para>
The function call was executed and returned a result.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
FunctionVoidResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>FunctionVoidResponse</Term>
<ListItem>
<Para>
The function call was executed and returned no result.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
ReadyForQuery
</Term>
<ListItem>
<Para>
Processing of the function call is complete.
ReadyForQuery will always be sent, whether processing
terminates successfully or with an error.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
NoticeResponse
</Term>
<ListItem>
<Para>
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>ReadyForQuery</Term>
<ListItem>
<Para>
Processing of the function call is complete. ReadyForQuery
will always be sent, whether processing terminates
successfully or with an error.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>NoticeResponse</Term>
<ListItem>
<Para>
A warning message has been issued in relation to the function
call.
Notices are in addition to other responses, i.e., the backend
will continue processing the command.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<Para>
A frontend must be prepared to accept ErrorResponse and NoticeResponse
messages whenever it is expecting any other type of message. Also,
if it issues any listen(l) commands then it must be prepared to accept
NotificationResponse messages at any time; see below.
</para>
</sect2>
<Sect2>
<Title>Notification Responses</Title>
<Para>
If a frontend issues a listen(l) command, then the backend will send a
NotificationResponse message (not to be confused with NoticeResponse!)
whenever a notify(l) command is executed for the same notification name.
</para>
<Para>
Notification responses are permitted at any point in the protocol (after
start-up), except within another backend message. Thus, the frontend
must be prepared to recognize a NotificationResponse message whenever it is
expecting any message. Indeed, it should be able to handle
NotificationResponse messages even when it is not engaged in a query.
<VariableList>
<VarListEntry>
<Term>
NotificationResponse
</Term>
<ListItem>
<Para>
A notify(l) command has been executed for a name for which
a previous listen(l) command was executed. Notifications
may be sent at any time.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<Para>
It may be worth pointing out that the names used in listen and notify
commands need not have anything to do with names of relations (tables)
in the SQL database. Notification names are simply arbitrarily chosen
condition names.
</para>
</sect2>
<Sect2>
<Title>Cancelling Requests in Progress</Title>
<Para>
During the processing of a query, the frontend may request cancellation of the
query by sending an appropriate request to the postmaster. The cancel request
is not sent directly to the backend for reasons of implementation efficiency:
we don't want to have the backend constantly checking for new input from
the frontend during query processing. Cancel requests should be relatively
infrequent, so we make them slightly cumbersome in order to avoid a penalty
in the normal case.
</para>
<Para>
To issue a cancel request, the frontend opens a new connection to the
postmaster and sends a CancelRequest message, rather than the StartupPacket
message that would ordinarily be sent across a new connection. The postmaster
will process this request and then close the connection. For security
reasons, no direct reply is made to the cancel request message.
</para>
<Para>
A CancelRequest message will be ignored unless it contains the same key data
(PID and secret key) passed to the frontend during connection start-up. If the
request matches the PID and secret key for a currently executing backend, the
postmaster signals the backend to abort processing of the current query.
</para>
<Para>
The cancellation signal may or may not have any effect --- for example, if it
arrives after the backend has finished processing the query, then it will have
no effect. If the cancellation is effective, it results in the current
command being terminated early with an error message.
</para>
<Para>
The upshot of all this is that for reasons of both security and efficiency,
the frontend has no direct way to tell whether a cancel request has succeeded.
It must continue to wait for the backend to respond to the query. Issuing a
cancel simply improves the odds that the current query will finish soon,
and improves the odds that it will fail with an error message instead of
succeeding.
</para>
<Para>
Since the cancel request is sent to the postmaster and not across the
regular frontend/backend communication link, it is possible for the cancel
request to be issued by any process, not just the frontend whose query is
to be canceled. This may have some benefits of flexibility in building
multiple-process applications. It also introduces a security risk, in that
unauthorized persons might try to cancel queries. The security risk is
addressed by requiring a dynamically generated secret key to be supplied
in cancel requests.
</para>
</sect2>
<Sect2>
<Title>Termination</Title>
<Para>
The normal, graceful termination procedure is that the frontend sends a
Terminate message and immediately closes the connection. On receipt of the
message, the backend immediately closes the connection and terminates.
</para>
<Para>
An ungraceful termination may occur due to software failure (i.e., core dump)
at either end. If either frontend or backend sees an unexpected closure of
the connection, it should clean up and terminate. The frontend has the option
of launching a new backend by recontacting the postmaster, if it doesn't want
to terminate itself.
</para>
</sect2>
</sect1>
call. Notices are in addition to other responses, i.e., the
backend will continue processing the command.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<para>
A frontend must be prepared to accept ErrorResponse and
NoticeResponse messages whenever it is expecting any other type of
message. Also, if it issues any <literal>LISTEN</literal>
commands then it must be prepared to accept NotificationResponse
messages at any time; see below.
</para>
</sect2>
<sect2>
<title>Notification Responses</title>
<Para>
If a frontend issues a <literal>LISTEN</literal> command, then the
backend will send a NotificationResponse message (not to be
confused with NoticeResponse!) whenever a
<literal>NOTIFY</literal> command is executed for the same
notification name.
</para>
<para>
Notification responses are permitted at any point in the protocol
(after start-up), except within another backend message. Thus,
the frontend must be prepared to recognize a NotificationResponse
message whenever it is expecting any message. Indeed, it should
be able to handle NotificationResponse messages even when it is
not engaged in a query.
<VariableList>
<VarListEntry>
<Term>NotificationResponse</Term>
<ListItem>
<Para>
A <literal>NOTIFY</literal> command has been executed for a
name for which a previous <literal>LISTEN</literal> command
was executed. Notifications may be sent at any time.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
<para>
It may be worth pointing out that the names used in listen and
notify commands need not have anything to do with names of
relations (tables) in the SQL database. Notification names are
simply arbitrarily chosen condition names.
</para>
</sect2>
<Sect2>
<Title>Cancelling Requests in Progress</Title>
<Para>
During the processing of a query, the frontend may request
cancellation of the query. The cancel request is not sent
directly on the open connection to the backend for reasons of
implementation efficiency: we don't want to have the backend
constantly checking for new input from the frontend during query
processing. Cancel requests should be relatively infrequent, so
we make them slightly cumbersome in order to avoid a penalty in
the normal case.
</para>
<Para>
To issue a cancel request, the frontend opens a new connection to
the server and sends a CancelRequest message, rather than the
StartupPacket message that would ordinarily be sent across a new
connection. The server will process this request and then close
the connection. For security reasons, no direct reply is made to
the cancel request message.
</para>
<Para>
A CancelRequest message will be ignored unless it contains the
same key data (PID and secret key) passed to the frontend during
connection start-up. If the request matches the PID and secret
key for a currently executing backend, the processing of the
current query is aborted. (In the existing implemenation, this is
done by sending a special signal to the backend process that is
processing the query.)
</para>
<Para>
The cancellation signal may or may not have any effect --- for
example, if it arrives after the backend has finished processing
the query, then it will have no effect. If the cancellation is
effective, it results in the current command being terminated
early with an error message.
</para>
<Para>
The upshot of all this is that for reasons of both security and
efficiency, the frontend has no direct way to tell whether a
cancel request has succeeded. It must continue to wait for the
backend to respond to the query. Issuing a cancel simply improves
the odds that the current query will finish soon, and improves the
odds that it will fail with an error message instead of
succeeding.
</para>
<Para>
Since the cancel request is sent across a new connection to the
server and not across the regular frontend/backend communication
link, it is possible for the cancel request to be issued by any
process, not just the frontend whose query is to be canceled.
This may have some benefits of flexibility in building
multiple-process applications. It also introduces a security
risk, in that unauthorized persons might try to cancel queries.
The security risk is addressed by requiring a dynamically
generated secret key to be supplied in cancel requests.
</para>
</sect2>
<Sect2>
<Title>Termination</Title>
<Para>
The normal, graceful termination procedure is that the frontend
sends a Terminate message and immediately closes the connection.
On receipt of the message, the backend immediately closes the
connection and terminates.
</para>
<Para>
An ungraceful termination may occur due to software failure (i.e.,
core dump) at either end. If either frontend or backend sees an
unexpected closure of the connection, it should clean up and
terminate. The frontend has the option of launching a new backend
by recontacting the server if it doesn't want to terminate
itself.
</para>
</sect2>
</sect1>
<Sect1 id="protocol-message-types">
<Title>Message Data Types</Title>
......@@ -695,7 +678,7 @@ characters that don't fit into your fixed-size buffer.
<Para>
This section describes the detailed format of each message. Each can be sent
by either a frontend (F), a postmaster/backend (B), or both (F & B).
by either a frontend (F), a backend (B), or both (F & B).
</para>
<VariableList>
......@@ -1815,7 +1798,7 @@ StartupPacket (F)
<ListItem>
<Para>
Any additional command line arguments to be passed to the
backend by the postmaster.
backend child process by the server.
</Para>
</ListItem>
</VarListEntry>
......
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