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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:19:15 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:19:15 +0000
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tree657d8194422a5daccecfd42d654b8a245ef7b4c8 /doc/src/sgml/html/libpq-notify.html
parentInitial commit. (diff)
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Adding upstream version 13.4.upstream/13.4upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<?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>33.8. Asynchronous Notification</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 V1.79.1" /><link rel="prev" href="libpq-fastpath.html" title="33.7. The Fast-Path Interface" /><link rel="next" href="libpq-copy.html" title="33.9. Functions Associated with the COPY Command" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">33.8. Asynchronous Notification</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-fastpath.html" title="33.7. The Fast-Path Interface">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq — C Library">Up</a></td><th width="60%" align="center">Chapter 33. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> — C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-copy.html" title="33.9. Functions Associated with the COPY Command">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-NOTIFY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">33.8. Asynchronous Notification</h2></div></div></div><a id="id-1.7.3.15.2" class="indexterm"></a><p>
+ <span class="productname">PostgreSQL</span> offers asynchronous notification
+ via the <code class="command">LISTEN</code> and <code class="command">NOTIFY</code>
+ commands. A client session registers its interest in a particular
+ notification channel with the <code class="command">LISTEN</code> command (and
+ can stop listening with the <code class="command">UNLISTEN</code> command). All
+ sessions listening on a particular channel will be notified
+ asynchronously when a <code class="command">NOTIFY</code> command with that
+ channel name is executed by any session. A <span class="quote">“<span class="quote">payload</span>”</span> string can
+ be passed to communicate additional data to the listeners.
+ </p><p>
+ <span class="application">libpq</span> applications submit
+ <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>,
+ and <code class="command">NOTIFY</code> commands as
+ ordinary SQL commands. The arrival of <code class="command">NOTIFY</code>
+ messages can subsequently be detected by calling
+ <code class="function" id="LIBPQ-PQNOTIFIES">PQnotifies</code>.<a id="id-1.7.3.15.4.7" class="indexterm"></a>
+ </p><p>
+ The function <code class="function">PQnotifies</code> returns the next notification
+ from a list of unhandled notification messages received from the server.
+ It returns a null pointer if there are no pending notifications. Once a
+ notification is returned from <code class="function">PQnotifies</code>, it is considered
+ handled and will be removed from the list of notifications.
+
+</p><pre class="synopsis">
+PGnotify *PQnotifies(PGconn *conn);
+
+typedef struct pgNotify
+{
+ char *relname; /* notification channel name */
+ int be_pid; /* process ID of notifying server process */
+ char *extra; /* notification payload string */
+} PGnotify;
+</pre><p>
+
+ After processing a <code class="structname">PGnotify</code> object returned
+ by <code class="function">PQnotifies</code>, be sure to free it with
+ <a class="xref" href="libpq-misc.html#LIBPQ-PQFREEMEM"><code class="function">PQfreemem</code></a>. It is sufficient to free the
+ <code class="structname">PGnotify</code> pointer; the
+ <code class="structfield">relname</code> and <code class="structfield">extra</code>
+ fields do not represent separate allocations. (The names of these fields
+ are historical; in particular, channel names need not have anything to
+ do with relation names.)
+ </p><p>
+ <a class="xref" href="libpq-example.html#LIBPQ-EXAMPLE-2" title="Example 33.2. libpq Example Program 2">Example 33.2</a> gives a sample program that illustrates
+ the use of asynchronous notification.
+ </p><p>
+ <code class="function">PQnotifies</code> does not actually read data from the
+ server; it just returns messages previously absorbed by another
+ <span class="application">libpq</span> function. In ancient releases of
+ <span class="application">libpq</span>, the only way to ensure timely receipt
+ of <code class="command">NOTIFY</code> messages was to constantly submit commands, even
+ empty ones, and then check <code class="function">PQnotifies</code> after each
+ <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>. While this still works, it is deprecated
+ as a waste of processing power.
+ </p><p>
+ A better way to check for <code class="command">NOTIFY</code> messages when you have no
+ useful commands to execute is to call
+ <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>
+ </a>, then check
+ <code class="function">PQnotifies</code>. You can use
+ <code class="function">select()</code> to wait for data to arrive from the
+ server, thereby using no <acronym class="acronym">CPU</acronym> power unless there is
+ something to do. (See <a class="xref" href="libpq-status.html#LIBPQ-PQSOCKET"><code class="function">PQsocket</code></a> to obtain the file
+ descriptor number to use with <code class="function">select()</code>.) Note that
+ this will work OK whether you submit commands with
+ <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>/<a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> or
+ simply use <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>. You should, however, remember
+ to check <code class="function">PQnotifies</code> after each
+ <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> or <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>, to
+ see if any notifications came in during the processing of the command.
+ </p></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-fastpath.html" title="33.7. The Fast-Path Interface">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq — C Library">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-copy.html" title="33.9. Functions Associated with the COPY Command">Next</a></td></tr><tr><td width="40%" align="left" valign="top">33.7. The Fast-Path Interface </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 33.9. Functions Associated with the <code xmlns="http://www.w3.org/1999/xhtml" class="command">COPY</code> Command</td></tr></table></div></body></html> \ No newline at end of file
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