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$Header: /cvsroot/pgsql/doc/src/sgml/ref/notify.sgml,v 1.20 2003/04/26 23:56:51 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-NOTIFY">
 <refmeta>
  <refentrytitle id="sql-notify-title">NOTIFY</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <refsynopsisdiv>
<synopsis>
NOTIFY <replaceable class="PARAMETER">name</replaceable>        
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   The <command>NOTIFY</command> command sends a notification event to each
   client application that has previously executed
   <command>LISTEN <replaceable class="parameter">name</replaceable></command>
   for the specified notification name in the current database.
  </para>

  <para>
   The information passed to the client for a notifiation event includes the notification
   name and the notifying session's server process <acronym>PID</>.  It is up to the
   database designer to define the notification names that will be used in a given
   database and what each one means.
  </para>

  <para>
   Commonly, the notification 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 notification names
   to signal different sorts of changes to a single table.
  </para>

  <para>
   <command>NOTIFY</command> provides a simple form of signal or
   interprocess communication mechanism for a collection of processes
   accessing the same <productname>PostgreSQL</productname> database.
   Higher-level mechanisms can be built by using tables in the database to
   pass additional data (beyond a mere notification name) from notifier to
   listener(s).
  </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 rule that is triggered by table updates.
   In this way, notification happens automatically when the table is changed,
   and the application programmer can't 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---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>
   <command>NOTIFY</command> behaves like Unix signals in one important
   respect: if the same notification name is signaled multiple times in quick
   succession, recipients may get only one notification event for several executions
   of <command>NOTIFY</command>.  So it is a bad idea to depend on the number
   of notifications received.  Instead, use <command>NOTIFY</command> to wake up
   applications that need to pay attention to something, and use a database
   object (such as a sequence) to keep track of what happened or how many times
   it happened.
  </para>

  <para>
   It is common for a client that executes <command>NOTIFY</command>
   to be listening on the same notification name 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</> (supplied in the
   notification event message) is the same as one's own session's
   <acronym>PID</> (available from <application>libpq</>).  When they
   are the same, the notification event is one's own work bouncing
   back, and can be ignored.  (Despite what was said in the preceding
   paragraph, this is a safe technique.
   <productname>PostgreSQL</productname> keeps self-notifiications
   separate from notifications arriving from other sessions, so you
   cannot miss an outside notification by ignoring your own
   notifications.)
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">name</replaceable></term>
    <listitem>
     <para>
      Name of the notification to be signaled (any identifier).
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Diagnostics</title>

  <variablelist>
   <varlistentry>
    <term><computeroutput>NOTIFY</computeroutput></term>
    <listitem>
     <para>
      Message returned when the commmand has executed.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Examples</title>

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

<programlisting>
LISTEN virtual;
NOTIFY virtual;
Asynchronous NOTIFY 'virtual' from backend with pid '8448' received. 
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

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

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