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