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   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>47.8. 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="47.7. Logical Decoding Output Writers" /><link rel="next" href="logicaldecoding-streaming.html" title="47.9. 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">47.8. Synchronous Replication Support for Logical Decoding</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-writer.html" title="47.7. Logical Decoding Output Writers">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logicaldecoding.html" title="Chapter 47. Logical Decoding">Up</a></td><th width="60%" align="center">Chapter 47. Logical Decoding</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-streaming.html" title="47.9. 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">47.8. Synchronous Replication Support for Logical Decoding <a href="#LOGICALDECODING-SYNCHRONOUS" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logicaldecoding-synchronous.html#LOGICALDECODING-SYNCHRONOUS-OVERVIEW">47.8.1. Overview</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-synchronous.html#LOGICALDECODING-SYNCHRONOUS-CAVEATS">47.8.2. Caveats</a></span></dt></dl></div><div class="sect2" id="LOGICALDECODING-SYNCHRONOUS-OVERVIEW"><div class="titlepage"><div><div><h3 class="title">47.8.1. Overview <a href="#LOGICALDECODING-SYNCHRONOUS-OVERVIEW" class="id_link">#</a></h3></div></div></div><p>
   3      Logical decoding can be used to build
   4      <a class="link" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="26.2.8. Synchronous Replication">synchronous
   5      replication</a> solutions with the same user interface as synchronous
   6      replication for <a class="link" href="warm-standby.html#STREAMING-REPLICATION" title="26.2.5. Streaming Replication">streaming
   7      replication</a>.  To do this, the streaming replication interface
   8      (see <a class="xref" href="logicaldecoding-walsender.html" title="47.3. Streaming Replication Protocol Interface">Section 47.3</a>) must be used to stream out
   9      data. Clients have to send <code class="literal">Standby status update (F)</code>
  10      (see <a class="xref" href="protocol-replication.html" title="54.4. Streaming Replication Protocol">Section 54.4</a>) messages, just like streaming
  11      replication clients do.
  12     </p><div class="note"><h3 class="title">Note</h3><p>
  13       A synchronous replica receiving changes via logical decoding will work in
  14       the scope of a single database. Since, in contrast to
  15       that, <em class="parameter"><code>synchronous_standby_names</code></em> currently is
  16       server wide, this means this technique will not work properly if more
  17       than one database is actively used.
  18      </p></div></div><div class="sect2" id="LOGICALDECODING-SYNCHRONOUS-CAVEATS"><div class="titlepage"><div><div><h3 class="title">47.8.2. Caveats <a href="#LOGICALDECODING-SYNCHRONOUS-CAVEATS" class="id_link">#</a></h3></div></div></div><p>
  19      In synchronous replication setup, a deadlock can happen, if the transaction
  20      has locked [user] catalog tables exclusively. See
  21      <a class="xref" href="logicaldecoding-output-plugin.html#LOGICALDECODING-CAPABILITIES" title="47.6.2. Capabilities">Section 47.6.2</a> for information on user
  22      catalog tables. This is because logical decoding of transactions can lock
  23      catalog tables to access them. To avoid this users must refrain from taking
  24      an exclusive lock on [user] catalog tables. This can happen in the following
  25      ways:
  26
  27      </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
  28         Issuing an explicit <code class="command">LOCK</code> on <code class="structname">pg_class</code>
  29         in a transaction.
  30        </p></li><li class="listitem"><p>
  31         Perform <code class="command">CLUSTER</code> on <code class="structname">pg_class</code> in
  32         a transaction.
  33        </p></li><li class="listitem"><p>
  34         <code class="command">PREPARE TRANSACTION</code> after <code class="command">LOCK</code> command
  35         on <code class="structname">pg_class</code> and allow logical decoding of two-phase
  36         transactions.
  37        </p></li><li class="listitem"><p>
  38         <code class="command">PREPARE TRANSACTION</code> after <code class="command">CLUSTER</code>
  39         command on <code class="structname">pg_trigger</code> and allow logical decoding of
  40         two-phase transactions. This will lead to deadlock only when published table
  41         have a trigger.
  42        </p></li><li class="listitem"><p>
  43         Executing <code class="command">TRUNCATE</code> on [user] catalog table in a
  44         transaction.
  45        </p></li></ul></div><p>
  46
  47      Note that these commands can cause deadlocks not only for the system
  48      catalog tables listed above but for other catalog tables.
  49     </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="47.7. Logical Decoding Output Writers">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html" title="Chapter 47. Logical Decoding">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-streaming.html" title="47.9. Streaming of Large Transactions for Logical Decoding">Next</a></td></tr><tr><td width="40%" align="left" valign="top">47.7. Logical Decoding Output Writers </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 47.9. Streaming of Large Transactions for Logical Decoding</td></tr></table></div></body></html>