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diff --git a/doc/src/sgml/html/sql-notify.html b/doc/src/sgml/html/sql-notify.html new file mode 100644 index 0000000..9852412 --- /dev/null +++ b/doc/src/sgml/html/sql-notify.html @@ -0,0 +1,132 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 15.4 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"> +NOTIFY <em class="replaceable"><code>channel</code></em> [ , <em class="replaceable"><code>payload</code></em> ] +</pre></div><div class="refsect1" id="id-1.9.3.158.5"><h2>Description</h2><p> + The <code class="command">NOTIFY</code> command sends a notification event together + with an optional <span class="quote">“<span class="quote">payload</span>”</span> string to each client application that + has previously executed + <code class="command">LISTEN <em class="replaceable"><code>channel</code></em></code> + for the specified channel name in the current database. + Notifications are visible to all users. + </p><p> + <code class="command">NOTIFY</code> provides a simple + interprocess communication mechanism for a collection of processes + accessing the same <span class="productname">PostgreSQL</span> 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). + </p><p> + The information passed to the client for a notification event includes the + notification channel + name, the notifying session's server process <acronym class="acronym">PID</acronym>, and the + payload string, which is an empty string if it has not been specified. + </p><p> + 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, <span class="quote">“<span class="quote">I changed this table, + take a look at it to see what's new</span>”</span>. But no such association is enforced by + the <code class="command">NOTIFY</code> and <code class="command">LISTEN</code> 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. + </p><p> + When <code class="command">NOTIFY</code> is used to signal the occurrence of changes + to a particular table, a useful programming technique is to put the + <code class="command">NOTIFY</code> 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. + </p><p> + <code class="command">NOTIFY</code> interacts with SQL transactions in some important + ways. Firstly, if a <code class="command">NOTIFY</code> 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 <code class="command">NOTIFY</code>. 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 <span class="quote">“<span class="quote">take back</span>”</span> 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 <code class="command">NOTIFY</code> for real-time signaling + should try to keep their transactions short. + </p><p> + 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, + <code class="command">NOTIFY</code> 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. + </p><p> + It is common for a client that executes <code class="command">NOTIFY</code> + 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 class="acronym">PID</acronym> (supplied in the + notification event message) is the same as one's own session's + <acronym class="acronym">PID</acronym> (available from <span class="application">libpq</span>). When they + are the same, the notification event is one's own work bouncing + back, and can be ignored. + </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> + Name of the notification channel to be signaled (any identifier). + </p></dd><dt><span class="term"><em class="replaceable"><code>payload</code></em></span></dt><dd><p> + The <span class="quote">“<span class="quote">payload</span>”</span> 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.) + </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.158.7"><h2>Notes</h2><p> + 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 <code class="command">NOTIFY</code> 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 <code class="command">LISTEN</code> 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. + </p><p> + The function <code class="function">pg_notification_queue_usage</code> returns the + fraction of the queue that is currently occupied by pending notifications. + See <a class="xref" href="functions-info.html" title="9.26. System Information Functions and Operators">Section 9.26</a> for more information. + </p><p> + A transaction that has executed <code class="command">NOTIFY</code> cannot be + prepared for two-phase commit. + </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> + To send a notification you can also use the function + <code class="literal"><code class="function">pg_notify</code>(<code class="type">text</code>, + <code class="type">text</code>)</code>. 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 <code class="command">NOTIFY</code> command if you need to work with + non-constant channel names and payloads. + </p></div></div><div class="refsect1" id="id-1.9.3.158.8"><h2>Examples</h2><p> + Configure and execute a listen/notify sequence from + <span class="application">psql</span>: + +</p><pre class="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. +</pre></div><div class="refsect1" id="id-1.9.3.158.9"><h2>Compatibility</h2><p> + There is no <code class="command">NOTIFY</code> statement in the SQL + standard. + </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 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> PREPARE</td></tr></table></div></body></html>
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