@ -1,5 +1,5 @@
<!-- <!--
$PostgreSQL: pgsql/doc/src/sgml/ref/notify.sgml,v 1.31 2008/11/14 10:22:47 petere Exp $ $PostgreSQL: pgsql/doc/src/sgml/ref/notify.sgml,v 1.32 2010/02/16 22:34:43 tgl Exp $
PostgreSQL documentation PostgreSQL documentation
--> -->
@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv> <refsynopsisdiv>
<synopsis> <synopsis>
NOTIFY <replaceable class="PARAMETER">name</replaceable> NOTIFY <replaceable class="PARAMETER">channel</replaceable> [ , <replaceable class="PARAMETER">payload</replaceable> ]
</synopsis> </synopsis>
</refsynopsisdiv> </refsynopsisdiv>
@ -29,35 +29,39 @@ NOTIFY <replaceable class="PARAMETER">name</replaceable>
<title>Description</title> <title>Description</title>
<para> <para>
The <command>NOTIFY</command> command sends a notification event to each The <command>NOTIFY</command> command sends a notification event together
client application that has previously executed with an optional <quote>payload</> string to each client application that
<command>LISTEN <replaceable class="parameter">name</replaceable></command> has previously executed
for the specified notification name in the current database. <command>LISTEN <replaceable class="parameter">channel</></command>
for the specified channel name in the current database.
</para> </para>
<para> <para>
<command>NOTIFY</command> provides a simple form of signal or <command>NOTIFY</command> provides a simple
interprocess communication mechanism for a collection of processes interprocess communication mechanism for a collection of processes
accessing the same <productname>PostgreSQL</productname> database. accessing the same <productname>PostgreSQL</productname> database.
Higher-level mechanisms can be built by using tables in the database to A payload string can be sent along with the notification, and
pass additional data (beyond a mere notification name) from notifier to higher-level mechanisms for passing structured data can be built by using
listener(s). tables in the database to pass additional data from notifier to listener(s).
</para> </para>
<para> <para>
The information passed to the client for a notification event includes the notification The information passed to the client for a notification event includes the
name and the notifying session's server process <acronym>PID</>. It is up to the notification channel
database designer to define the notification names that will be used in a given name, the notifying session's server process <acronym>PID</>, and the
database and what each one means . payload string, which is an empty string if it has not been specified .
</para> </para>
<para> <para>
Commonly, the notification name is the same as the name of some table in It is up to the database designer to define the channel names that will
be used in a given database and what each one means.
Commonly, the channel name is the same as the name of some table in
the database, and the notify event essentially means, <quote>I changed this table, the database, and the notify event essentially means, <quote>I changed this table,
take a look at it to see what's new</quote>. But no such association is enforced by take a look at it to see what's new</quote>. But no such association is enforced by
the <command>NOTIFY</command> and <command>LISTEN</command> commands. For the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
example, a database designer could use several different notification names example, a database designer could use several different channel names
to signal different sorts of changes to a single table. to signal different sorts of changes to a single table. Alternatively,
the payload string could be used to differentiate various cases.
</para> </para>
<para> <para>
@ -89,19 +93,22 @@ NOTIFY <replaceable class="PARAMETER">name</replaceable>
</para> </para>
<para> <para>
<command>NOTIFY</command> behaves like Unix signals in one important If the same channel name is signaled multiple times from the same
respect: if the same notification name is signaled multiple times in quick transaction with identical payload strings, the
succession, recipients might get only one notification event for several executions database server can decide to deliver a single notification only.
of <command>NOTIFY</command>. So it is a bad idea to depend on the number On the other hand, notifications with distinct payload strings will
of notifications received. Instead, use <command>NOTIFY</command> to wake up always be delivered as distinct notifications. Similarly, notifications from
applications that need to pay attention to something, and use a database different transactions will never get folded into one notification.
object (such as a sequence) to keep track of what happened or how many times Except for dropping later instances of duplicate notifications,
it happened. <command>NOTIFY</command> guarantees that notifications from the same
transaction get delivered in the order they were sent. It is also
guaranteed that messages from different transactions are delivered in
the order in which the transactions committed.
</para> </para>
<para> <para>
It is common for a client that executes <command>NOTIFY</command> It is common for a client that executes <command>NOTIFY</command>
to be listening on the same notification nam e itself. In that case to be listening on the same notification chan nel itself. In that case
it will get back a notification event, just like all the other it will get back a notification event, just like all the other
listening sessions. Depending on the application logic, this could listening sessions. Depending on the application logic, this could
result in useless work, for example, reading a database table to result in useless work, for example, reading a database table to
@ -111,12 +118,7 @@ NOTIFY <replaceable class="PARAMETER">name</replaceable>
notification event message) is the same as one's own session's notification event message) is the same as one's own session's
<acronym>PID</> (available from <application>libpq</>). When they <acronym>PID</> (available from <application>libpq</>). When they
are the same, the notification event is one's own work bouncing are the same, the notification event is one's own work bouncing
back, and can be ignored. (Despite what was said in the preceding back, and can be ignored.
paragraph, this is a safe technique.
<productname>PostgreSQL</productname> keeps self-notifications
separate from notifications arriving from other sessions, so you
cannot miss an outside notification by ignoring your own
notifications.)
</para> </para>
</refsect1> </refsect1>
@ -125,16 +127,61 @@ NOTIFY <replaceable class="PARAMETER">name</replaceable>
<variablelist> <variablelist>
<varlistentry> <varlistentry>
<term><replaceable class="PARAMETER">nam e</replaceable></term> <term><replaceable class="PARAMETER">chan nel </replaceable></term>
<listitem> <listitem>
<para> <para>
Name of the notification to be signaled (any identifier). Name of the notification channel to be signaled (any identifier).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">payload</replaceable></term>
<listitem>
<para>
The <quote>payload</> string to be communicated along with the
notification. This string must be shorter than 8000 bytes, and
is treated as text.
(If binary data or large amounts of information need to be communicated,
it's best to put it in a database table and send the key of the record.)
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
</variablelist> </variablelist>
</refsect1> </refsect1>
<refsect1>
<title>Notes</title>
<indexterm>
<primary>pg_notify</primary>
</indexterm>
<para>
To send a notification you can also use the function
<literal><function>pg_notify</function>(<type>text</type>,
<type>text</type>)</literal>. The function takes the channel name as the
first argument and the payload as the second. The function is much easier
to use than the <command>NOTIFY</command> command if you need to work with
non-constant channel names and payloads.
</para>
<para>
There is a queue that holds notifications that have been sent but not
yet processed by all listening sessions. If this queue becomes full,
transactions calling <command>NOTIFY</command> will fail at commit.
The queue is quite large (8GB in a standard installation) and should be
sufficiently sized for almost every use case. However, no cleanup can take
place if a session executes <command>LISTEN</command> and then enters a
transaction for a very long time. Once the queue is half full you will see
warnings in the log file pointing you to the session that is preventing
cleanup. In this case you should make sure that this session ends its
current transaction so that cleanup can proceed.
</para>
<para>
A transaction that has executed <command>NOTIFY</command> cannot be
prepared for two-phase commit.
</para>
</refsect1>
<refsect1> <refsect1>
<title>Examples</title> <title>Examples</title>
@ -146,6 +193,12 @@ NOTIFY <replaceable class="PARAMETER">name</replaceable>
LISTEN virtual; LISTEN virtual;
NOTIFY virtual; NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448. Asynchronous notification "virtual" received from server process with PID 8448.
NOTIFY virtual, 'This is the payload';
Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.
LISTEN foo;
SELECT pg_notify('fo' || 'o', 'pay' || 'load');
Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.
</programlisting> </programlisting>
</para> </para>
</refsect1> </refsect1>
Tom Lane
16 years ago