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updates for new startup sequence, some reformatting

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Peter Eisentraut 2001-06-22 23:27:48 +00:00
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@ -1,247 +1,217 @@
<Chapter Id="protocol"> <!-- $Header: /cvsroot/pgsql/doc/src/sgml/protocol.sgml,v 1.18 2001/06/22 23:27:48 petere Exp $ -->
<DocInfo>
<Author>
<FirstName>Phil</FirstName>
<Surname>Thompson</Surname>
</Author>
<Date>1998-08-08</Date>
</DocInfo>
<Title>Frontend/Backend Protocol</Title>
<Para> <chapter id="protocol">
<Note> <title>Frontend/Backend Protocol</title>
<Para>
<note>
<para>
Written by Phil Thompson (<email>phil@river-bank.demon.co.uk</email>). 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>). Updates for protocol 2.0 by Tom Lane (<email>tgl@sss.pgh.pa.us</email>).
</Para> </para>
</Note> </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>
<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 This document describes version 2.0 of the protocol, implemented in
<ProductName>Postgres</ProductName> 6.4 and later. <application>PostgreSQL</application> 6.4 and later.
</para> </para>
<Para> <para>
Higher level features built on this protocol (for example, how <FileName>libpq</FileName> passes Higher level features built on this protocol (for example, how
certain environment variables after the connection is established) <application>libpq</application> passes certain environment
are covered elsewhere. variables after the connection is established) are covered
elsewhere.
</para> </para>
<Sect1 id="protocol-overview"> <sect1 id="protocol-overview">
<Title>Overview</Title> <title>Overview</title>
<Para> <para>
The three major components are the frontend (running on the client) and the A frontend opens a connection to the server and sends a start-up
postmaster and backend (running on the server). The postmaster and backend packet. This includes the names of the user and the database the
have different roles but may be implemented by the same executable. 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>
<Para> <para>
A frontend sends a start-up packet to the postmaster. This includes the names The frontend then sends any required authentication information.
of the user and the database the user wants to connect to. The postmaster then Once the server validates this it responds to the frontend that it
uses this, and the information in the <filename>pg_hba.conf</filename> is authenticated and sends a message indicating successful start-up
file (normal case) or failure (for example, an invalid database name).
to determine what
further authentication information it requires the frontend to send (if any)
and responds to the frontend accordingly.
</para> </para>
<Para> <para>
The frontend then sends any required authentication information. Once the In order to serve multiple clients efficiently, the server would
postmaster validates this it responds to the frontend that it is authenticated normally create a new child process to handle each incoming
and hands over the connection to a backend. The backend then sends a message connection. However, this is not required. In the current
indicating successful start-up (normal case) or failure (for example, an implementation, a new child process is created immediately after an
invalid database name). 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>
<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 When the frontend wishes to disconnect it sends an appropriate packet and
closes the connection without waiting for a response for the backend. closes the connection without waiting for a response for the backend.
</para> </para>
<Para> <para>
Packets are sent as a data stream. The first byte determines what should be Packets are sent as a data stream. The first byte determines what
expected in the rest of the packet. The exception is packets sent from a should be expected in the rest of the packet. The exceptions are
frontend to the postmaster, which comprise a packet length then the packet packets sent as part of the startup and authentication exchange,
itself. The difference is historical. which comprise a packet length followed by the packet itself. The
difference is historical.
</para> </para>
</sect1> </sect1>
<Sect1 id="protocol-protocol"> <sect1 id="protocol-protocol">
<Title>Protocol</Title> <title>Protocol</title>
<Para> <para>
This section describes the message flow. There are four different types of This section describes the message flow. There are four different
flows depending on the state of the connection: types of flows depending on the state of the connection: start-up,
start-up, query, function call, and termination. query, function call, and termination. There are also special
There are also special provisions for notification responses and command provisions for notification responses and command cancellation,
cancellation, which can occur at any time after the start-up phase. which can occur at any time after the start-up phase.
</para> </para>
<sect2>
<title>Start-up</Title>
<Sect2> <para>
<Title>Start-up</Title> 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:
<Para> <variablelist>
Start-up is divided into an authentication phase and a backend start-up phase. <varlistentry>
<term>ErrorResponse</term>
<listitem>
<para>
The server then immediately closes the connection.
</para> </para>
</listitem>
</varlistentry>
<Para> <varlistentry>
Initially, the frontend sends a StartupPacket. The postmaster uses this info <term>AuthenticationOk</term>
and the contents of the pg_hba.conf file to determine what authentication <listitem>
method the frontend must use. The postmaster then responds with one of the <para>
following messages: The authentication exchange is completed.
</para> </para>
</listitem>
</varlistentry>
<Para> <varlistentry>
<VariableList> <term>AuthenticationKerberosV4</Term>
<VarListEntry> <listitem>
<Term> <para>
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>
The frontend must then take part in a Kerberos V4 The frontend must then take part in a Kerberos V4
authentication dialog (not described here) with the postmaster. authentication dialog (not described here, part of the
If this is successful, the postmaster responds with an Kerberos specification) with the server. If this is
AuthenticationOk, otherwise it responds with an ErrorResponse. successful, the server responds with an AuthenticationOk,
</Para> otherwise it responds with an ErrorResponse.
</ListItem> </para>
</VarListEntry> </listitem>
<VarListEntry> </varlistentry>
<Term>
AuthenticationKerberosV5 <varlistentry>
</Term> <Term>AuthenticationKerberosV5</Term>
<ListItem> <ListItem>
<Para> <Para>
The frontend must then take part in a Kerberos V5 The frontend must then take part in a Kerberos V5
authentication dialog (not described here) with the postmaster. authentication dialog (not described here, part of the
If this is successful, the postmaster responds with an 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. AuthenticationOk, otherwise it responds with an ErrorResponse.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>AuthenticationEncryptedPassword</Term>
AuthenticationUnencryptedPassword
</Term>
<ListItem> <ListItem>
<Para> <Para>
The frontend must then send an UnencryptedPasswordPacket. The frontend must then send an EncryptedPasswordPacket. If
If this is the correct password, the postmaster responds with this is the correct password, the server responds with an
an AuthenticationOk, otherwise it responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.
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> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
</VariableList> </VariableList>
</Para> </Para>
<Para> <para>
If the frontend does not support the authentication method requested by the If the frontend does not support the authentication method
postmaster, then it should immediately close the connection. requested by the server, then it should immediately close the
connection.
</para> </para>
<Para> <para>
After sending AuthenticationOk, the postmaster attempts to launch a backend After having received AuthenticationOk, the frontend should wait
process. Since this might fail, or the backend might encounter a failure for further messages from the server. The possible messages from
during start-up, the frontend must wait for the backend to acknowledge the backend in this phase are:
successful start-up. The frontend should send no messages at this point.
The possible messages from the backend during this phase are:
<VariableList> <VariableList>
<VarListEntry> <VarListEntry>
<Term> <Term>BackendKeyData</Term>
BackendKeyData
</Term>
<ListItem> <ListItem>
<Para> <Para>
This message is issued after successful backend start-up. This message provides secret-key data that the frontend must
It provides secret-key data that the frontend must save save if it wants to be able to issue cancel requests later.
if it wants to be able to issue cancel requests later.
The frontend should not respond to this message, but should The frontend should not respond to this message, but should
continue listening for a ReadyForQuery message. continue listening for a ReadyForQuery message.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>ReadyForQuery</Term>
ReadyForQuery
</Term>
<ListItem> <ListItem>
<Para> <Para>
Backend start-up is successful. The frontend may now issue Start-up is completed. The frontend may now issue query or
query or function call messages. function call messages.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>ErrorResponse</Term>
ErrorResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
Backend start-up failed. The connection is closed after Start-up failed. The connection is closed after sending this
sending this message. message.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>NoticeResponse</Term>
NoticeResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
A warning message has been issued. The frontend should A warning message has been issued. The frontend should
@ -253,12 +223,13 @@ The possible messages from the backend during this phase are:
</VariableList> </VariableList>
</Para> </Para>
<Para> <para>
The ReadyForQuery message is the same one that the backend will issue after The ReadyForQuery message is the same one that the backend will
each query cycle. Depending on the coding needs of the frontend, it is issue after each query cycle. Depending on the coding needs of
reasonable to consider ReadyForQuery as starting a query cycle (and then the frontend, it is reasonable to consider ReadyForQuery as
BackendKeyData indicates successful conclusion of the start-up phase), starting a query cycle (and then BackendKeyData indicates
or to consider ReadyForQuery as ending the start-up phase and each subsequent successful conclusion of the start-up phase), or to consider
ReadyForQuery as ending the start-up phase and each subsequent
query cycle. query cycle.
</para> </para>
</sect2> </sect2>
@ -267,11 +238,12 @@ query cycle.
<Title>Query</Title> <Title>Query</Title>
<Para> <Para>
A Query cycle is initiated by the frontend sending a Query message to the A Query cycle is initiated by the frontend sending a Query message
backend. The backend then sends one or more response messages depending to the backend. The backend then sends one or more response
on the contents of the query command string, and finally a ReadyForQuery messages depending on the contents of the query command string,
response message. ReadyForQuery informs the frontend that it may safely and finally a ReadyForQuery response message. ReadyForQuery
send a new query or function call. informs the frontend that it may safely send a new query or
function call.
</para> </para>
<Para> <Para>
@ -279,84 +251,88 @@ The possible response messages from the backend are:
<VariableList> <VariableList>
<VarListEntry> <VarListEntry>
<Term> <Term>CompletedResponse</Term>
CompletedResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
An SQL command completed normally. An SQL command completed normally.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>CopyInResponse</Term>
CopyInResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
The backend is ready to copy data from the frontend to a The backend is ready to copy data from the frontend to a
relation. The frontend should then send a CopyDataRows table. The frontend should then send a CopyDataRows message.
message. The backend will then respond with a The backend will then respond with a CompletedResponse message
CompletedResponse message with a tag of "COPY". with a tag of <literal>COPY</literal>.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>CopyOutResponse</Term>
CopyOutResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
The backend is ready to copy data from a relation to the The backend is ready to copy data from a table to the
frontend. It then sends a CopyDataRows message, and then a frontend. It then sends a CopyDataRows message, and then a
CompletedResponse message with a tag of "COPY". CompletedResponse message with a tag of <literal>COPY</literal>.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>CursorResponse</Term>
CursorResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
The query was either an insert(l), delete(l), update(l), The query was either an <literal>INSERT</literal>,
fetch(l) or a select(l) command. <literal>UPDATE</literal>, <literal>DELETE</literal>,
If the transaction has been <literal>FETCH</literal>, or a <literal>SELECT</literal>
aborted then the backend sends a CompletedResponse message with command. If the transaction has been aborted then the backend
a tag of "*ABORT STATE*". Otherwise the following responses sends a CompletedResponse message with a tag of <literal>*ABORT
are sent. STATE*</literal>. Otherwise the following responses are sent.
</Para> </Para>
<Para> <Para>
For an insert(l) command, the backend then sends a For an <literal>INSERT</literal> command, the backend then
CompletedResponse message with a tag of "INSERT <Replaceable>oid</Replaceable> <Replaceable>rows</Replaceable>" sends a CompletedResponse message with a tag of
where <Replaceable>rows</Replaceable> is the number of rows inserted, and <Replaceable>oid</Replaceable> is the <literal>INSERT <replaceable>oid</replaceable>
object ID of the inserted row if <Replaceable>rows</Replaceable> is 1, otherwise <Replaceable>oid</Replaceable> <replaceable>rows</replaceable></literal>, where
is 0. <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>
<Para> <Para>
For a delete(l) command, the backend then sends a For a <literal>DELETE</literal> command, the backend then
CompletedResponse message with a tag of "DELETE <Replaceable>rows</Replaceable>" where sends a CompletedResponse message with a tag of <literal>DELETE
<Replaceable>rows</Replaceable></literal> where
<Replaceable>rows</Replaceable> is the number of rows deleted. <Replaceable>rows</Replaceable> is the number of rows deleted.
</Para> </Para>
<Para> <Para>
For an update(l) command, the backend then sends a For an <literal>UPDATE</literal> command, the backend then
CompletedResponse message with a tag of "UPDATE <Replaceable>rows</Replaceable>" where sends a CompletedResponse message with a tag of <literal>UPDATE
<Replaceable>rows</Replaceable> is the number of rows deleted. <Replaceable>rows</Replaceable></literal> where
<Replaceable>rows</Replaceable> is the number of rows affected
by the update.
</Para> </Para>
<Para> <Para>
For a fetch(l) or select(l) command, the backend sends a For a <literal>FETCH</literal> or <literal>SELECT</literal>
RowDescription message. This is then followed by an AsciiRow command, the backend sends a RowDescription message. This is
or BinaryRow message (depending on whether a binary cursor was then followed by an AsciiRow or BinaryRow message (depending
specified) for each row being returned to the frontend. on whether a binary cursor was specified) for each row being
Finally, the backend sends a CompletedResponse message with a returned to the frontend. Finally, the backend sends a
tag of "SELECT". CompletedResponse message with a tag of <literal>SELECT</literal>.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>EmptyQueryResponse</Term>
EmptyQueryResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
An empty query string was recognized. (The need to specially An empty query string was recognized. (The need to specially
@ -364,35 +340,32 @@ The possible response messages from the backend are:
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>ErrorResponse</Term>
ErrorResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
An error has occurred. An error has occurred.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>ReadyForQuery</Term>
ReadyForQuery
</Term>
<ListItem> <ListItem>
<Para> <Para>
Processing of the query string is complete. A separate Processing of the query string is complete. A separate
message is sent to indicate this because the query string message is sent to indicate this because the query string may
may contain multiple SQL commands. (CompletedResponse marks contain multiple SQL commands. (CompletedResponse marks the
the end of processing one SQL command, not the whole string.) end of processing one SQL command, not the whole string.)
ReadyForQuery will always be sent, whether processing ReadyForQuery will always be sent, whether processing
terminates successfully or with an error. terminates successfully or with an error.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>NoticeResponse</Term>
NoticeResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
A warning message has been issued in relation to the query. A warning message has been issued in relation to the query.
@ -401,26 +374,30 @@ The possible response messages from the backend are:
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
</VariableList> </VariableList>
</Para> </Para>
<Para> <para>
A frontend must be prepared to accept ErrorResponse and NoticeResponse A frontend must be prepared to accept ErrorResponse and
messages whenever it is expecting any other type of message. NoticeResponse messages whenever it is expecting any other type of
message.
</para> </para>
<Para> <Para>
Actually, it is possible for NoticeResponse to arrive even when the frontend Actually, it is possible for NoticeResponse to arrive even when
is not expecting any kind of message, that is, the backend is nominally idle. the frontend is not expecting any kind of message, that is, the
(In particular, the backend can be commanded to terminate by its postmaster. backend is nominally idle. (In particular, the backend can be
In that case it will send a NoticeResponse before closing the connection.) commanded to terminate by its parent process. In that case it will
It is recommended that the frontend check for such asynchronous notices just send a NoticeResponse before closing the connection.) It is
before issuing any new command. recommended that the frontend check for such asynchronous notices
just before issuing any new command.
</para> </para>
<Para> <Para>
Also, if the frontend issues any listen(l) commands then it must be prepared Also, if the frontend issues any <literal>LISTEN</literal>
to accept NotificationResponse messages at any time; see below. commands then it must be prepared to accept NotificationResponse
messages at any time; see below.
</para> </para>
</sect2> </sect2>
@ -428,11 +405,12 @@ to accept NotificationResponse messages at any time; see below.
<Title>Function Call</Title> <Title>Function Call</Title>
<Para> <Para>
A Function Call cycle is initiated by the frontend sending a FunctionCall A Function Call cycle is initiated by the frontend sending a
message to the backend. The backend then sends one or more response messages FunctionCall message to the backend. The backend then sends one
depending on the results of the function call, and finally a ReadyForQuery or more response messages depending on the results of the function
response message. ReadyForQuery informs the frontend that it may safely send call, and finally a ReadyForQuery response message. ReadyForQuery
a new query or function call. informs the frontend that it may safely send a new query or
function call.
</para> </para>
<Para> <Para>
@ -440,108 +418,103 @@ The possible response messages from the backend are:
<VariableList> <VariableList>
<VarListEntry> <VarListEntry>
<Term> <Term>ErrorResponse</Term>
ErrorResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
An error has occurred. An error has occurred.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>FunctionResultResponse</Term>
FunctionResultResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
The function call was executed and returned a result. The function call was executed and returned a result.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>FunctionVoidResponse</Term>
FunctionVoidResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
The function call was executed and returned no result. The function call was executed and returned no result.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>ReadyForQuery</Term>
ReadyForQuery
</Term>
<ListItem> <ListItem>
<Para> <Para>
Processing of the function call is complete. Processing of the function call is complete. ReadyForQuery
ReadyForQuery will always be sent, whether processing will always be sent, whether processing terminates
terminates successfully or with an error. successfully or with an error.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
<VarListEntry> <VarListEntry>
<Term> <Term>NoticeResponse</Term>
NoticeResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
A warning message has been issued in relation to the function A warning message has been issued in relation to the function
call. call. Notices are in addition to other responses, i.e., the
Notices are in addition to other responses, i.e., the backend backend will continue processing the command.
will continue processing the command.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
</VariableList> </VariableList>
</Para> </Para>
<Para> <para>
A frontend must be prepared to accept ErrorResponse and NoticeResponse A frontend must be prepared to accept ErrorResponse and
messages whenever it is expecting any other type of message. Also, NoticeResponse messages whenever it is expecting any other type of
if it issues any listen(l) commands then it must be prepared to accept message. Also, if it issues any <literal>LISTEN</literal>
NotificationResponse messages at any time; see below. commands then it must be prepared to accept NotificationResponse
messages at any time; see below.
</para> </para>
</sect2> </sect2>
<Sect2> <sect2>
<Title>Notification Responses</Title> <title>Notification Responses</title>
<Para> <Para>
If a frontend issues a listen(l) command, then the backend will send a If a frontend issues a <literal>LISTEN</literal> command, then the
NotificationResponse message (not to be confused with NoticeResponse!) backend will send a NotificationResponse message (not to be
whenever a notify(l) command is executed for the same notification name. confused with NoticeResponse!) whenever a
<literal>NOTIFY</literal> command is executed for the same
notification name.
</para> </para>
<Para> <para>
Notification responses are permitted at any point in the protocol (after Notification responses are permitted at any point in the protocol
start-up), except within another backend message. Thus, the frontend (after start-up), except within another backend message. Thus,
must be prepared to recognize a NotificationResponse message whenever it is the frontend must be prepared to recognize a NotificationResponse
expecting any message. Indeed, it should be able to handle message whenever it is expecting any message. Indeed, it should
NotificationResponse messages even when it is not engaged in a query. be able to handle NotificationResponse messages even when it is
not engaged in a query.
<VariableList> <VariableList>
<VarListEntry> <VarListEntry>
<Term> <Term>NotificationResponse</Term>
NotificationResponse
</Term>
<ListItem> <ListItem>
<Para> <Para>
A notify(l) command has been executed for a name for which A <literal>NOTIFY</literal> command has been executed for a
a previous listen(l) command was executed. Notifications name for which a previous <literal>LISTEN</literal> command
may be sent at any time. was executed. Notifications may be sent at any time.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>
</VariableList> </VariableList>
</Para> </Para>
<Para> <para>
It may be worth pointing out that the names used in listen and notify It may be worth pointing out that the names used in listen and
commands need not have anything to do with names of relations (tables) notify commands need not have anything to do with names of
in the SQL database. Notification names are simply arbitrarily chosen relations (tables) in the SQL database. Notification names are
condition names. simply arbitrarily chosen condition names.
</para> </para>
</sect2> </sect2>
@ -549,55 +522,63 @@ condition names.
<Title>Cancelling Requests in Progress</Title> <Title>Cancelling Requests in Progress</Title>
<Para> <Para>
During the processing of a query, the frontend may request cancellation of the During the processing of a query, the frontend may request
query by sending an appropriate request to the postmaster. The cancel request cancellation of the query. The cancel request is not sent
is not sent directly to the backend for reasons of implementation efficiency: directly on the open connection to the backend for reasons of
we don't want to have the backend constantly checking for new input from implementation efficiency: we don't want to have the backend
the frontend during query processing. Cancel requests should be relatively constantly checking for new input from the frontend during query
infrequent, so we make them slightly cumbersome in order to avoid a penalty processing. Cancel requests should be relatively infrequent, so
in the normal case. we make them slightly cumbersome in order to avoid a penalty in
the normal case.
</para> </para>
<Para> <Para>
To issue a cancel request, the frontend opens a new connection to the To issue a cancel request, the frontend opens a new connection to
postmaster and sends a CancelRequest message, rather than the StartupPacket the server and sends a CancelRequest message, rather than the
message that would ordinarily be sent across a new connection. The postmaster StartupPacket message that would ordinarily be sent across a new
will process this request and then close the connection. For security connection. The server will process this request and then close
reasons, no direct reply is made to the cancel request message. the connection. For security reasons, no direct reply is made to
the cancel request message.
</para> </para>
<Para> <Para>
A CancelRequest message will be ignored unless it contains the same key data A CancelRequest message will be ignored unless it contains the
(PID and secret key) passed to the frontend during connection start-up. If the same key data (PID and secret key) passed to the frontend during
request matches the PID and secret key for a currently executing backend, the connection start-up. If the request matches the PID and secret
postmaster signals the backend to abort processing of the current query. 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>
<Para> <Para>
The cancellation signal may or may not have any effect --- for example, if it The cancellation signal may or may not have any effect --- for
arrives after the backend has finished processing the query, then it will have example, if it arrives after the backend has finished processing
no effect. If the cancellation is effective, it results in the current the query, then it will have no effect. If the cancellation is
command being terminated early with an error message. effective, it results in the current command being terminated
early with an error message.
</para> </para>
<Para> <Para>
The upshot of all this is that for reasons of both security and efficiency, The upshot of all this is that for reasons of both security and
the frontend has no direct way to tell whether a cancel request has succeeded. efficiency, the frontend has no direct way to tell whether a
It must continue to wait for the backend to respond to the query. Issuing a cancel request has succeeded. It must continue to wait for the
cancel simply improves the odds that the current query will finish soon, backend to respond to the query. Issuing a cancel simply improves
and improves the odds that it will fail with an error message instead of the odds that the current query will finish soon, and improves the
odds that it will fail with an error message instead of
succeeding. succeeding.
</para> </para>
<Para> <Para>
Since the cancel request is sent to the postmaster and not across the Since the cancel request is sent across a new connection to the
regular frontend/backend communication link, it is possible for the cancel server and not across the regular frontend/backend communication
request to be issued by any process, not just the frontend whose query is link, it is possible for the cancel request to be issued by any
to be canceled. This may have some benefits of flexibility in building process, not just the frontend whose query is to be canceled.
multiple-process applications. It also introduces a security risk, in that This may have some benefits of flexibility in building
unauthorized persons might try to cancel queries. The security risk is multiple-process applications. It also introduces a security
addressed by requiring a dynamically generated secret key to be supplied risk, in that unauthorized persons might try to cancel queries.
in cancel requests. The security risk is addressed by requiring a dynamically
generated secret key to be supplied in cancel requests.
</para> </para>
</sect2> </sect2>
@ -605,17 +586,19 @@ in cancel requests.
<Title>Termination</Title> <Title>Termination</Title>
<Para> <Para>
The normal, graceful termination procedure is that the frontend sends a The normal, graceful termination procedure is that the frontend
Terminate message and immediately closes the connection. On receipt of the sends a Terminate message and immediately closes the connection.
message, the backend immediately closes the connection and terminates. On receipt of the message, the backend immediately closes the
connection and terminates.
</para> </para>
<Para> <Para>
An ungraceful termination may occur due to software failure (i.e., core dump) An ungraceful termination may occur due to software failure (i.e.,
at either end. If either frontend or backend sees an unexpected closure of core dump) at either end. If either frontend or backend sees an
the connection, it should clean up and terminate. The frontend has the option unexpected closure of the connection, it should clean up and
of launching a new backend by recontacting the postmaster, if it doesn't want terminate. The frontend has the option of launching a new backend
to terminate itself. by recontacting the server if it doesn't want to terminate
itself.
</para> </para>
</sect2> </sect2>
</sect1> </sect1>
@ -695,7 +678,7 @@ characters that don't fit into your fixed-size buffer.
<Para> <Para>
This section describes the detailed format of each message. Each can be sent 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> </para>
<VariableList> <VariableList>
@ -1815,7 +1798,7 @@ StartupPacket (F)
<ListItem> <ListItem>
<Para> <Para>
Any additional command line arguments to be passed to the Any additional command line arguments to be passed to the
backend by the postmaster. backend child process by the server.
</Para> </Para>
</ListItem> </ListItem>
</VarListEntry> </VarListEntry>