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<!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>49.8. Synchronous Replication Support for Logical Decoding</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="logicaldecoding-writer.html" title="49.7. Logical Decoding Output Writers" /><link rel="next" href="logicaldecoding-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">49.8. Synchronous Replication Support for Logical Decoding</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-writer.html" title="49.7. Logical Decoding Output Writers">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Up</a></td><th width="60%" align="center">Chapter 49. Logical Decoding</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGICALDECODING-SYNCHRONOUS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">49.8. Synchronous Replication Support for Logical Decoding</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logicaldecoding-synchronous.html#id-1.8.14.14.2">49.8.1. Overview</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-synchronous.html#LOGICALDECODING-SYNCHRONOUS-CAVEATS">49.8.2. Caveats</a></span></dt></dl></div><div class="sect2" id="id-1.8.14.14.2"><div class="titlepage"><div><div><h3 class="title">49.8.1. Overview</h3></div></div></div><p>
Logical decoding can be used to build
<a class="link" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="27.2.8. Synchronous Replication">synchronous
replication</a> solutions with the same user interface as synchronous
replication for <a class="link" href="warm-standby.html#STREAMING-REPLICATION" title="27.2.5. Streaming Replication">streaming
replication</a>. To do this, the streaming replication interface
(see <a class="xref" href="logicaldecoding-walsender.html" title="49.3. Streaming Replication Protocol Interface">Section 49.3</a>) must be used to stream out
data. Clients have to send <code class="literal">Standby status update (F)</code>
(see <a class="xref" href="protocol-replication.html" title="55.4. Streaming Replication Protocol">Section 55.4</a>) messages, just like streaming
replication clients do.
</p><div class="note"><h3 class="title">Note</h3><p>
A synchronous replica receiving changes via logical decoding will work in
the scope of a single database. Since, in contrast to
that, <em class="parameter"><code>synchronous_standby_names</code></em> currently is
server wide, this means this technique will not work properly if more
than one database is actively used.
</p></div></div><div class="sect2" id="LOGICALDECODING-SYNCHRONOUS-CAVEATS"><div class="titlepage"><div><div><h3 class="title">49.8.2. Caveats</h3></div></div></div><p>
In synchronous replication setup, a deadlock can happen, if the transaction
has locked [user] catalog tables exclusively. See
<a class="xref" href="logicaldecoding-output-plugin.html#LOGICALDECODING-CAPABILITIES" title="49.6.2. Capabilities">Section 49.6.2</a> for information on user
catalog tables. This is because logical decoding of transactions can lock
catalog tables to access them. To avoid this users must refrain from taking
an exclusive lock on [user] catalog tables. This can happen in the following
ways:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Issuing an explicit <code class="command">LOCK</code> on <code class="structname">pg_class</code>
in a transaction.
</p></li><li class="listitem"><p>
Perform <code class="command">CLUSTER</code> on <code class="structname">pg_class</code> in
a transaction.
</p></li><li class="listitem"><p>
<code class="command">PREPARE TRANSACTION</code> after <code class="command">LOCK</code> command
on <code class="structname">pg_class</code> and allow logical decoding of two-phase
transactions.
</p></li><li class="listitem"><p>
<code class="command">PREPARE TRANSACTION</code> after <code class="command">CLUSTER</code>
command on <code class="structname">pg_trigger</code> and allow logical decoding of
two-phase transactions. This will lead to deadlock only when published table
have a trigger.
</p></li><li class="listitem"><p>
Executing <code class="command">TRUNCATE</code> on [user] catalog table in a
transaction.
</p></li></ul></div><p>
Note that these commands that can cause deadlock apply to not only explicitly
indicated system catalog tables above but also to any other [user] catalog
table.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logicaldecoding-writer.html" title="49.7. Logical Decoding Output Writers">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding">Next</a></td></tr><tr><td width="40%" align="left" valign="top">49.7. Logical Decoding Output Writers </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 49.9. Streaming of Large Transactions for Logical Decoding</td></tr></table></div></body></html>
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