codekingpro/portable-devtools

1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>NOTIFY</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-move.html" title="MOVE" /><link rel="next" href="sql-prepare.html" title="PREPARE" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">NOTIFY</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-move.html" title="MOVE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-prepare.html" title="PREPARE">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-NOTIFY"><div class="titlepage"></div><a id="id-1.9.3.158.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">NOTIFY</span></h2><p>NOTIFY — generate a notification</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">3NOTIFY <em class="replaceable"><code>channel</code></em> [ , <em class="replaceable"><code>payload</code></em> ]4</pre></div><div class="refsect1" id="id-1.9.3.158.5"><h2>Description</h2><p>5   The <code class="command">NOTIFY</code> command sends a notification event together6   with an optional <span class="quote">“<span class="quote">payload</span>”</span> string to each client application that7   has previously executed8   <code class="command">LISTEN <em class="replaceable"><code>channel</code></em></code>9   for the specified channel name in the current database.10   Notifications are visible to all users.11  </p><p>12   <code class="command">NOTIFY</code> provides a simple13   interprocess communication mechanism for a collection of processes14   accessing the same <span class="productname">PostgreSQL</span> database.15   A payload string can be sent along with the notification, and16   higher-level mechanisms for passing structured data can be built by using17   tables in the database to pass additional data from notifier to listener(s).18  </p><p>19   The information passed to the client for a notification event includes the20   notification channel21   name, the notifying session's server process <acronym class="acronym">PID</acronym>, and the22   payload string, which is an empty string if it has not been specified.23  </p><p>24   It is up to the database designer to define the channel names that will25   be used in a given database and what each one means.26   Commonly, the channel name is the same as the name of some table in27   the database, and the notify event essentially means, <span class="quote">“<span class="quote">I changed this table,28   take a look at it to see what's new</span>”</span>.  But no such association is enforced by29   the <code class="command">NOTIFY</code> and <code class="command">LISTEN</code> commands.  For30   example, a database designer could use several different channel names31   to signal different sorts of changes to a single table.  Alternatively,32   the payload string could be used to differentiate various cases.33  </p><p>34   When <code class="command">NOTIFY</code> is used to signal the occurrence of changes35   to a particular table, a useful programming technique is to put the36   <code class="command">NOTIFY</code> in a statement trigger that is triggered by table updates.37   In this way, notification happens automatically when the table is changed,38   and the application programmer cannot accidentally forget to do it.39  </p><p>40   <code class="command">NOTIFY</code> interacts with SQL transactions in some important41   ways.  Firstly, if a <code class="command">NOTIFY</code> is executed inside a42   transaction, the notify events are not delivered until and unless the43   transaction is committed.  This is appropriate, since if the transaction44   is aborted, all the commands within it have had no45   effect, including <code class="command">NOTIFY</code>.  But it can be disconcerting if one46   is expecting the notification events to be delivered immediately.  Secondly, if47   a listening session receives a notification signal while it is within a transaction,48   the notification event will not be delivered to its connected client until just49   after the transaction is completed (either committed or aborted).  Again, the50   reasoning is that if a notification were delivered within a transaction that was51   later aborted, one would want the notification to be undone somehow —52   but53   the server cannot <span class="quote">“<span class="quote">take back</span>”</span> a notification once it has sent it to the client.54   So notification events are only delivered between transactions.  The upshot of this55   is that applications using <code class="command">NOTIFY</code> for real-time signaling56   should try to keep their transactions short.57  </p><p>58   If the same channel name is signaled multiple times with identical59   payload strings within the same transaction, only one instance of the60   notification event is delivered to listeners.61   On the other hand, notifications with distinct payload strings will62   always be delivered as distinct notifications. Similarly, notifications from63   different transactions will never get folded into one notification.64   Except for dropping later instances of duplicate notifications,65   <code class="command">NOTIFY</code> guarantees that notifications from the same66   transaction get delivered in the order they were sent.  It is also67   guaranteed that messages from different transactions are delivered in68   the order in which the transactions committed.69  </p><p>70   It is common for a client that executes <code class="command">NOTIFY</code>71   to be listening on the same notification channel itself.  In that case72   it will get back a notification event, just like all the other73   listening sessions.  Depending on the application logic, this could74   result in useless work, for example, reading a database table to75   find the same updates that that session just wrote out.  It is76   possible to avoid such extra work by noticing whether the notifying77   session's server process <acronym class="acronym">PID</acronym> (supplied in the78   notification event message) is the same as one's own session's79   <acronym class="acronym">PID</acronym> (available from <span class="application">libpq</span>).  When they80   are the same, the notification event is one's own work bouncing81   back, and can be ignored.82  </p></div><div class="refsect1" id="id-1.9.3.158.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>channel</code></em></span></dt><dd><p>83      Name of the notification channel to be signaled (any identifier).84     </p></dd><dt><span class="term"><em class="replaceable"><code>payload</code></em></span></dt><dd><p>85      The <span class="quote">“<span class="quote">payload</span>”</span> string to be communicated along with the86      notification.  This must be specified as a simple string literal.87      In the default configuration it must be shorter than 8000 bytes.88      (If binary data or large amounts of information need to be communicated,89      it's best to put it in a database table and send the key of the record.)90     </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.158.7"><h2>Notes</h2><p>91   There is a queue that holds notifications that have been sent but not92   yet processed by all listening sessions.  If this queue becomes full,93   transactions calling <code class="command">NOTIFY</code> will fail at commit.94   The queue is quite large (8GB in a standard installation) and should be95   sufficiently sized for almost every use case. However, no cleanup can take96   place if a session executes <code class="command">LISTEN</code> and then enters a97   transaction for a very long time. Once the queue is half full you will see98   warnings in the log file pointing you to the session that is preventing99   cleanup. In this case you should make sure that this session ends its100   current transaction so that cleanup can proceed.101  </p><p>102   The function <code class="function">pg_notification_queue_usage</code> returns the103   fraction of the queue that is currently occupied by pending notifications.104   See <a class="xref" href="functions-info.html" title="9.26. System Information Functions and Operators">Section 9.26</a> for more information.105  </p><p>106   A transaction that has executed <code class="command">NOTIFY</code> cannot be107   prepared for two-phase commit.108  </p><div class="refsect2" id="id-1.9.3.158.7.5"><h3>pg_notify</h3><a id="id-1.9.3.158.7.5.2" class="indexterm"></a><p>109    To send a notification you can also use the function110    <code class="literal"><code class="function">pg_notify</code>(<code class="type">text</code>,111    <code class="type">text</code>)</code>. The function takes the channel name as the112    first argument and the payload as the second. The function is much easier113    to use than the <code class="command">NOTIFY</code> command if you need to work with114    non-constant channel names and payloads.115   </p></div></div><div class="refsect1" id="id-1.9.3.158.8"><h2>Examples</h2><p>116   Configure and execute a listen/notify sequence from117   <span class="application">psql</span>:118 119</p><pre class="programlisting">120LISTEN virtual;121NOTIFY virtual;122Asynchronous notification "virtual" received from server process with PID 8448.123NOTIFY virtual, 'This is the payload';124Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.125 126LISTEN foo;127SELECT pg_notify('fo' || 'o', 'pay' || 'load');128Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.129</pre></div><div class="refsect1" id="id-1.9.3.158.9"><h2>Compatibility</h2><p>130   There is no <code class="command">NOTIFY</code> statement in the SQL131   standard.132  </p></div><div class="refsect1" id="id-1.9.3.158.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-listen.html" title="LISTEN"><span class="refentrytitle">LISTEN</span></a>, <a class="xref" href="sql-unlisten.html" title="UNLISTEN"><span class="refentrytitle">UNLISTEN</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-move.html" title="MOVE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-prepare.html" title="PREPARE">Next</a></td></tr><tr><td width="40%" align="left" valign="top">MOVE </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> PREPARE</td></tr></table></div></body></html>