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LISTEN/NOTIFY doc improvements.
This commit is contained in:
Tom Lane
parent
30debec6e5
commit
55a5639e93
@ -10,20 +10,20 @@ LISTEN
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LISTEN
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</REFNAME>
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<REFPURPOSE>
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Listen for notification on a relation
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Listen for notification on a notify condition
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</REFPURPOSE>
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<REFSYNOPSISDIV>
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<REFSYNOPSISDIVINFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSYNOPSISDIVINFO>
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<SYNOPSIS>
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LISTEN <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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LISTEN <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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</SYNOPSIS>
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<REFSECT2 ID="R2-SQL-LISTEN-1">
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<REFSECT2INFO>
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<DATE>1998-09-01</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT2INFO>
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<TITLE>
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Inputs
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@ -33,11 +33,11 @@ Inputs
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<VARIABLELIST>
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<VARLISTENTRY>
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<TERM>
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<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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</TERM>
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<LISTITEM>
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<PARA>
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Table object used for notification.
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Name of notify condition.
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</VARIABLELIST>
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@ -62,63 +62,95 @@ Outputs
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<VARIABLELIST>
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<VARLISTENTRY>
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<TERM>
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LISTEN
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<returnvalue>LISTEN</returnvalue>
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</TERM>
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<LISTITEM>
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<PARA>
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Message returned upon successful completion of registration.
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</PARA>
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</LISTITEM>
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</VARLISTENTRY>
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<VARLISTENTRY>
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<TERM>
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<returnvalue>NOTICE Async_Listen: We are already listening on notifyname</returnvalue>
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</TERM>
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<LISTITEM>
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<PARA>
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If this backend is already registered for that notify condition.
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</PARA>
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</LISTITEM>
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</VARLISTENTRY>
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</variablelist>
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</LISTITEM>
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</VARLISTENTRY>
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</VARIABLELIST>
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</VARIABLELIST>
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</REFSECT2>
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</REFSYNOPSISDIV>
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<REFSECT1 ID="R1-SQL-LISTEN-1">
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<REFSECT1INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT1INFO>
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<TITLE>
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Description
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</TITLE>
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<PARA>
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LISTEN is used to register the current backend as a listener on the relation
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<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>.
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When the command
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<command>NOTIFY <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE></command>
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is called either from within a rule or at the query level, the
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frontend applications corresponding to the listening backends
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are notified. When the backend process exits, this registration
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is cleared.
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LISTEN registers the current <productname>Postgres</productname> backend as a
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listener on the notify condition
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>.
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<para>
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This event notification is performed through the libpq protocol
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and frontend application interface. The application program
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must call the routine
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<function>PQnotifies</function>
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in order to find out the name of the class to which a given
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notification corresponds. If this code is not included in
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the application, the event notification will be queued and
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never be processed.
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Whenever the command
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<command>NOTIFY <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE></command>
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is invoked, either by this backend or another one connected to
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the same database, all the backends currently listening on that notify
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condition are notified, and each will in turn notify its connected
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frontend application. See the discussion of <command>NOTIFY</command>
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for more information.
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<para>
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A backend can be deregistered for a given notify condition with the
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<command>UNLISTEN</command> command. Also, a backend's listen registrations
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are automatically cleared when the backend process exits.
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<para>
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The method a frontend application must use to detect notify events depends on
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which <productname>Postgres</productname> application programming interface it
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uses. With the basic libpq library, the application issues
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<command>LISTEN</command> as an ordinary SQL command, and then must
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periodically call the routine <function>PQnotifies</function> to find out
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whether any notify events have been received. Other interfaces such as
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libpgtcl provide higher-level methods for handling notify events; indeed,
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with libpgtcl the application programmer should not even issue
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<command>LISTEN</command> or <command>UNLISTEN</command> directly. See the
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documentation for the library you are using for more details.
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<para>
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The reference page for <command>NOTIFY</command> contains a more extensive
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discussion of the use of <command>LISTEN</command> and
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<command>NOTIFY</command>.
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<REFSECT2 ID="R2-SQL-LISTEN-3">
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<REFSECT2INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT2INFO>
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<TITLE>
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Notes
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</TITLE>
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<para>
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Note that <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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needs not to be a valid class name but can be any string valid
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as a name up to 32 characters long.
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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can be any string valid as a name;
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it need not correspond to the name of any actual table. If
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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is enclosed in double-quotes, it need not even be a syntactically
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valid name, but can be any string up to 31 characters long.
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<para>
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A restriction in some previous releases of
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<productname>Postgres</productname> that a
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<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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which does not correspond to an actual table must be enclosed in double-quotes
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is no longer present.
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In some previous releases of
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<productname>Postgres</productname>,
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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had to be enclosed in double-quotes when it did not correspond to any existing
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table name, even if syntactically valid as a name. That is no longer required.
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</REFSECT2>
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@ -128,6 +160,7 @@ Usage
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</TITLE>
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<PARA>
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<ProgramListing>
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-- Configure and execute a listen/notify sequence from psql
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postgres=> listen virtual;
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LISTEN
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postgres=> notify virtual;
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@ -10,22 +10,22 @@ NOTIFY
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NOTIFY
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</REFNAME>
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<REFPURPOSE>
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Signals all frontends and backends listening on a class
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Signals all frontends and backends listening on a notify condition
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</REFPURPOSE>
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<REFSYNOPSISDIV>
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<REFSYNOPSISDIVINFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSYNOPSISDIVINFO>
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<SYNOPSIS>
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<REPLACEABLE CLASS="PARAMETER">
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</REPLACEABLE>
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NOTIFY <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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NOTIFY <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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</SYNOPSIS>
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<REFSECT2 ID="R2-SQL-NOTIFY-1">
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<REFSECT2INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT2INFO>
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<TITLE>
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Inputs
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@ -35,11 +35,11 @@ Inputs
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<VARIABLELIST>
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<VARLISTENTRY>
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<TERM>
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<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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</TERM>
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<LISTITEM>
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<PARA>
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Table or arbitrary relation class used for notification.
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Notify condition to be signaled.
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</VARIABLELIST>
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@ -47,7 +47,7 @@ Table or arbitrary relation class used for notification.
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<REFSECT2 ID="R2-SQL-NOTIFY-2">
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<REFSECT2INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT2INFO>
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<TITLE>
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Outputs
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@ -69,66 +69,141 @@ NOTIFY
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</TERM>
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<LISTITEM>
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<PARA>
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Notification message from backend.
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Acknowledgement that notify command has executed.
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</PARA>
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</LISTITEM>
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</VARLISTENTRY>
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<VARLISTENTRY>
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<TERM>
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Notify events
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</TERM>
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<LISTITEM>
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<PARA>
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Events are delivered to listening frontends; whether and how each frontend
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application reacts depends on its programming.
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</PARA>
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</LISTITEM>
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</VARLISTENTRY>
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</variablelist>
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</LISTITEM>
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</VARLISTENTRY>
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</VARIABLELIST>
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</VARIABLELIST>
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</REFSECT2>
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</REFSYNOPSISDIV>
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<REFSECT1 ID="R1-SQL-NOTIFY-1">
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<REFSECT1INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT1INFO>
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<TITLE>
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Description
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</TITLE>
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<PARA>
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<command>NOTIFY</command> is used to awaken all sessions which have
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previously executed
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<command>LISTEN <replaceable class="parameter">classname</replaceable></command>.
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This can be used either within an instance-level rule
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as part of the action body or from a normal query.
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The <command>NOTIFY</command> command sends a notify event to each
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frontend application that has previously executed
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<command>LISTEN <replaceable class="parameter">notifyname</replaceable></command>
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for the specified notify condition in the current database.
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<para>
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When used from within a normal query,
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this can be thought of as interprocess communication (IPC). When
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used from within a rule, this can be thought of as an alert mechanism.
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The information passed to the frontend for a notify event includes the notify
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condition name and the notifying backend process's PID. It is up to the
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database designer to define the condition names that will be used in a given
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database and what each one means.
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<para>
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Note that the mere fact that a <command>NOTIFY</command> has been
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executed does not imply anything in particular about the state
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of the class (e.g., that it has been updated),
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nor does the notification protocol transmit any useful information
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other than the class name.
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Therefore, all <command>NOTIFY</command> does is indicate that some backend
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wishes its peers to
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examine <replaceable class="parameter">classname</replaceable>
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in some application-specific way.
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Commonly, the notify condition name is the same as the name of some table in
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the database, and the notify event essentially means "I changed this table,
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take a look at it to see what's new". But no such association is enforced by
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the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
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example, a database designer could use several different condition names
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to signal different sorts of changes to a single table.
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<para>
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In fact, <replaceable class="parameter">classname</replaceable>
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need not be the name of an SQL class at all.
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It is best thought of as a condition name
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that the application programmer selects.
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<command>NOTIFY</command> provides a simple form of signal or
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IPC (interprocess communication) mechanism for a collection of processes
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accessing the same <productname>Postgres</productname> database.
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Higher-level mechanisms can be built by using tables in the database to
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pass additional data (beyond a mere condition name) from notifier to
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listener(s).
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<para>
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This event notification is performed through the libpq protocol
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and frontend application interface. The application program
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must call the routine <function>PQnotifies</function>
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in order to find out the name of the class to which a given
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notification corresponds.
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If this code is not included in the application,
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the event notification will be
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queued and never be processed.
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When <command>NOTIFY</command> is used to signal the occurrence of changes
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to a particular table, a useful programming technique is to put the
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<command>NOTIFY</command> in a rule that is triggered by table updates.
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In this way, notification happens automatically when the table is changed,
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and the application programmer can't accidentally forget to do it.
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<para>
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<command>NOTIFY</command> interacts with SQL transactions in some important
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ways. Firstly, if a <command>NOTIFY</command> is executed inside a
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transaction, the notify events are not delivered until and unless the
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transaction is committed. This is appropriate, since if the transaction
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is aborted we would like all the commands within it to have had no effect
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--- including <command>NOTIFY</command>. But it can be disconcerting if one
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is expecting the notify events to be delivered immediately. Secondly, if
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a listening backend receives a notify signal while it is within a transaction,
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the notify event will not be delivered to its connected frontend until just
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after the transaction is completed (either committed or aborted). Again, the
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reasoning is that if a notify were delivered within a transaction that was
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later aborted, one would want the notification to be undone somehow --- but
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the backend cannot "take back" a notify once it has sent it to the frontend.
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So notify events are delivered only between transactions. The upshot of this
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is that applications using <command>NOTIFY</command> for real-time signaling
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should try to keep their transactions short.
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<para>
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<command>NOTIFY</command> behaves rather like Unix signals in one important
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respect: if the same notify name is signaled multiple times in quick
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succession, recipients may get only one notify event for several executions
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of <command>NOTIFY</command>. So it is a bad idea to depend on the number
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of notifies received; instead use <command>NOTIFY</command> to wake up
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applications that need to pay attention to something, and use a database
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object (such as a sequence) to keep track of what happened or how many times
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it happened.
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<para>
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It is common for a frontend that sends <command>NOTIFY</command> to be
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listening on the same notify name itself. In that case it will get back a
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notify event, just like all the other listening frontends. Depending on the
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application logic, this could result in useless work --- for example,
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re-reading a database table to find the same updates that that frontend just
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wrote out. In <productname>Postgres</productname> 6.4 and later, it is
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possible to avoid such extra work by noticing whether the notifying backend
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process's PID (supplied in the notify event message) is the same as one's own
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backend's PID (available from libpq). When they are the same, the notify
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event is one's own work bouncing back, and can be ignored. (Despite what was
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said in the preceding paragraph, this is a safe technique.
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<productname>Postgres</productname> keeps self-notifies separate from notifies
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arriving from other backends, so you cannot miss an outside notify by ignoring
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your own notifies.)
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<REFSECT2 ID="R2-SQL-NOTIFY-3">
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<REFSECT2INFO>
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<DATE>1998-09-24</DATE>
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<DATE>1998-10-07</DATE>
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</REFSECT2INFO>
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<TITLE>
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Notes
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</TITLE>
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<PARA>
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</PARA>
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<para>
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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can be any string valid as a name;
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it need not correspond to the name of any actual table. If
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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is enclosed in double-quotes, it need not even be a syntactically
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valid name, but can be any string up to 31 characters long.
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<para>
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In some previous releases of
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<productname>Postgres</productname>,
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<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
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had to be enclosed in double-quotes when it did not correspond to any existing
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table name, even if syntactically valid as a name. That is no longer required.
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<para>
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In <productname>Postgres</productname> releases prior to 6.4, the backend
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PID delivered in a notify message is always the PID of the frontend's own
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backend. So it is not possible to distinguish one's own notifies from other
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clients' notifies in those earlier releases.
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</REFSECT2>
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@ -137,19 +212,13 @@ Notes
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Usage
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</TITLE>
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<PARA>
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</PARA>
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<ProgramListing>
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-- Configure and execute a listen/notify sequence
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-- from psql
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CREATE TABLE t (i int4);
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<computeroutput>
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LISTEN t;
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</computeroutput>
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NOTIFY t;
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<computeroutput>
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-- Configure and execute a listen/notify sequence from psql
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postgres=> listen virtual;
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LISTEN
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postgres=> notify virtual;
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NOTIFY
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ASYNC NOTIFY of 't' from backend pid '10949' received
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</computeroutput>
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ASYNC NOTIFY of 'virtual' from backend pid '11239' received
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</ProgramListing>
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</REFSECT1>
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