doc/src/sgml/ref/notify.sgml

<!--
doc/src/sgml/ref/notify.sgml
PostgreSQL documentation
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<refentry id="sql-notify">
 <indexterm zone="sql-notify">
  <primary>NOTIFY</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>NOTIFY</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>NOTIFY</refname>
  <refpurpose>generate a notification</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
NOTIFY <replaceable class="parameter">channel</replaceable> [ , <replaceable class="parameter">payload</replaceable> ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   The <command>NOTIFY</command> command sends a notification event together
   with an optional <quote>payload</quote> string to each client application that
   has previously executed
   <command>LISTEN <replaceable class="parameter">channel</replaceable></command>
   for the specified channel name in the current database.
   Notifications are visible to all users.
  </para>

  <para>
   <command>NOTIFY</command> provides a simple
   interprocess communication mechanism for a collection of processes
   accessing the same <productname>PostgreSQL</productname> database.
   A payload string can be sent along with the notification, and
   higher-level mechanisms for passing structured data can be built by using
   tables in the database to pass additional data from notifier to listener(s).
  </para>

  <para>
   The information passed to the client for a notification event includes the
   notification channel
   name, the notifying session's server process <acronym>PID</acronym>, and the
   payload string, which is an empty string if it has not been specified.
  </para>

  <para>
   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,
   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
   example, a database designer could use several different channel names
   to signal different sorts of changes to a single table.  Alternatively,
   the payload string could be used to differentiate various cases.
  </para>

  <para>
   When <command>NOTIFY</command> is used to signal the occurrence of changes
   to a particular table, a useful programming technique is to put the
   <command>NOTIFY</command> in a statement trigger that is triggered by table updates.
   In this way, notification happens automatically when the table is changed,
   and the application programmer cannot accidentally forget to do it.
  </para>

  <para>
   <command>NOTIFY</command> interacts with SQL transactions in some important
   ways.  Firstly, if a <command>NOTIFY</command> is executed inside a
   transaction, the notify events are not delivered until and unless the
   transaction is committed.  This is appropriate, since if the transaction
   is aborted, all the commands within it have had no
   effect, including <command>NOTIFY</command>.  But it can be disconcerting if one
   is expecting the notification events to be delivered immediately.  Secondly, if
   a listening session receives a notification signal while it is within a transaction,
   the notification event will not be delivered to its connected client until just
   after the transaction is completed (either committed or aborted).  Again, the
   reasoning is that if a notification were delivered within a transaction that was
   later aborted, one would want the notification to be undone somehow &mdash;
   but
   the server cannot <quote>take back</quote> a notification once it has sent it to the client.
   So notification events are only delivered between transactions.  The upshot of this
   is that applications using <command>NOTIFY</command> for real-time signaling
   should try to keep their transactions short.
  </para>

  <para>
   If the same channel name is signaled multiple times with identical
   payload strings within the same transaction, only one instance of the
   notification event is delivered to listeners.
   On the other hand, notifications with distinct payload strings will
   always be delivered as distinct notifications. Similarly, notifications from
   different transactions will never get folded into one notification.
   Except for dropping later instances of duplicate notifications,
   <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>
   It is common for a client that executes <command>NOTIFY</command>
   to be listening on the same notification channel itself.  In that case
   it will get back a notification event, just like all the other
   listening sessions.  Depending on the application logic, this could
   result in useless work, for example, reading a database table to
   find the same updates that that session just wrote out.  It is
   possible to avoid such extra work by noticing whether the notifying
   session's server process <acronym>PID</acronym> (supplied in the
   notification event message) is the same as one's own session's
   <acronym>PID</acronym> (available from <application>libpq</application>).  When they
   are the same, the notification event is one's own work bouncing
   back, and can be ignored.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">channel</replaceable></term>
    <listitem>
     <para>
      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</quote> string to be communicated along with the
      notification.  This must be specified as a simple string literal.
      In the default configuration it must be shorter than 8000 bytes.
      (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>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <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>
   The function <function>pg_notification_queue_usage</function> returns the
   fraction of the queue that is currently occupied by pending notifications.
   See <xref linkend="functions-info"/> for more information.
  </para>
  <para>
   A transaction that has executed <command>NOTIFY</command> cannot be
   prepared for two-phase commit.
  </para>

  <refsect2>
   <title>pg_notify</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>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Configure and execute a listen/notify sequence from
   <application>psql</application>:

<programlisting>
LISTEN virtual;
NOTIFY virtual;
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></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>NOTIFY</command> statement in the SQL
   standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-listen"/></member>
   <member><xref linkend="sql-unlisten"/></member>
  </simplelist>
 </refsect1>
</refentry>