postgresql/doc/src/sgml/ref/notify.sgml

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Postgres 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>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
NOTIFY <replaceable class="PARAMETER">name</replaceable>        
  </synopsis>

  <refsect2 id="R2-SQL-NOTIFY-1">
   <refsect2info>
    <date>1998-10-07</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">notifyname</replaceable></term>
      <listitem>
       <para>
	Notify condition to be signaled.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-NOTIFY-2">
   <refsect2info>
    <date>1998-10-07</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
NOTIFY
       </computeroutput></term>
      <listitem>
       <para>
	Acknowledgement that notify command has executed.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable>Notify events</replaceable></term>
      <listitem>
       <para>
	Events are delivered to listening frontends; whether and how each frontend
	application reacts depends on its programming.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-NOTIFY-1">
  <refsect1info>
   <date>1998-10-07</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   The <command>NOTIFY</command> command sends a notify event to each
   frontend application that has previously executed
   <command>LISTEN <replaceable class="parameter">notifyname</replaceable></command>
   for the specified notify condition in the current database.
  </para>
  <para>
   The information passed to the frontend for a notify event includes the notify
   condition name and the notifying backend process's PID.  It is up to the
   database designer to define the condition names that will be used in a given
   database and what each one means.
  </para>
  <para>
   Commonly, the notify condition 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 condition names
   to signal different sorts of changes to a single table.
  </para>
  <para>
   <command>NOTIFY</command> provides a simple form of signal or
   IPC (interprocess communication) mechanism for a collection of processes
   accessing the same <productname>Postgres</productname> database.
   Higher-level mechanisms can be built by using tables in the database to
   pass additional data (beyond a mere condition 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 we would like all the commands within it to have had no
   effect, including <command>NOTIFY</command>.  But it can be disconcerting if one
   is expecting the notify events to be delivered immediately.  Secondly, if
   a listening backend receives a notify signal while it is within a transaction,
   the notify event will not be delivered to its connected frontend until just
   after the transaction is completed (either committed or aborted).  Again, the
   reasoning is that if a notify were delivered within a transaction that was
   later aborted, one would want the notification to be undone somehow---but
   the backend cannot <quote>take back</quote> a notify once it has sent it to the frontend.
   So notify 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 condition name is signaled multiple times in quick
   succession, recipients may get only one notify event for several executions
   of <command>NOTIFY</command>.  So it is a bad idea to depend on the number
   of notifies 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 frontend that sends <command>NOTIFY</command> to be
   listening on the same notify name itself.  In that case it will get back a
   notify event, just like all the other listening frontends.  Depending on the
   application logic, this could result in useless work---for example,
   re-reading a database table to find the same updates that that frontend just
   wrote out.  In <productname>Postgres</productname> 6.4 and later, it is
   possible to avoid such extra work by noticing whether the notifying backend
   process's PID (supplied in the notify event message) is the same as one's own
   backend's PID (available from libpq).  When they are the same, the notify
   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>Postgres</productname> keeps self-notifies separate from notifies
   arriving from other backends, so you cannot miss an outside notify by ignoring
   your own notifies.)
  </para>

  <refsect2 id="R2-SQL-NOTIFY-3">
   <refsect2info>
    <date>1998-10-07</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    <replaceable class="PARAMETER">name</replaceable>
    can be any string valid as a name;
    it need not correspond to the name of any actual table.  If
    <replaceable class="PARAMETER">name</replaceable>
    is enclosed in double-quotes, it need not even be a syntactically
    valid name, but can be any string up to 31 characters long.
   </para>
   <para>
    In some previous releases of
    <productname>Postgres</productname>,
    <replaceable class="PARAMETER">name</replaceable>
    had to be enclosed in double-quotes when it did not correspond to any existing
    table name, even if syntactically valid as a name.  That is no longer required.
   </para>
   <para>
    In <productname>Postgres</productname> releases prior to 6.4, the backend
    PID delivered in a notify message was always the PID of the frontend's own
    backend.  So it was not possible to distinguish one's own notifies from other
    clients' notifies in those earlier releases.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-NOTIFY-2">
  <title>
   Usage
  </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 id="R1-SQL-NOTIFY-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-NOTIFY-4">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>NOTIFY</command> statement in
    <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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