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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>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 18.4 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>
     Logical decoding can be used to build
     <a class="link" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="26.2.8. 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="26.2.5. Streaming Replication">streaming
     replication</a>.  To do this, the streaming replication interface
     (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
     data. Clients have to send <code class="literal">Standby status update (F)</code>
     (see <a class="xref" href="protocol-replication.html" title="54.4. Streaming Replication Protocol">Section 54.4</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">47.8.2. Caveats <a href="#LOGICALDECODING-SYNCHRONOUS-CAVEATS" class="id_link">#</a></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="47.6.2. Capabilities">Section 47.6.2</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 can cause deadlocks not only for the system
     catalog tables listed above but for other catalog tables.
    </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 18.4 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>