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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>49.10. Two-phase Commit 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-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding" /><link rel="next" href="replication-origins.html" title="Chapter 50. Replication Progress Tracking" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">49.10. Two-phase Commit Support for Logical Decoding</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding">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 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="replication-origins.html" title="Chapter 50. Replication Progress Tracking">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGICALDECODING-TWO-PHASE-COMMITS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">49.10. Two-phase Commit Support for Logical Decoding</h2></div></div></div><p>
With the basic output plugin callbacks (eg., <code class="function">begin_cb</code>,
<code class="function">change_cb</code>, <code class="function">commit_cb</code> and
<code class="function">message_cb</code>) two-phase commit commands like
<code class="command">PREPARE TRANSACTION</code>, <code class="command">COMMIT PREPARED</code>
and <code class="command">ROLLBACK PREPARED</code> are not decoded. While the
<code class="command">PREPARE TRANSACTION</code> is ignored,
<code class="command">COMMIT PREPARED</code> is decoded as a <code class="command">COMMIT</code>
and <code class="command">ROLLBACK PREPARED</code> is decoded as a
<code class="command">ROLLBACK</code>.
</p><p>
To support the streaming of two-phase commands, an output plugin needs to
provide additional callbacks. There are multiple two-phase commit callbacks
that are required, (<code class="function">begin_prepare_cb</code>,
<code class="function">prepare_cb</code>, <code class="function">commit_prepared_cb</code>,
<code class="function">rollback_prepared_cb</code> and
<code class="function">stream_prepare_cb</code>) and an optional callback
(<code class="function">filter_prepare_cb</code>).
</p><p>
If the output plugin callbacks for decoding two-phase commit commands are
provided, then on <code class="command">PREPARE TRANSACTION</code>, the changes of
that transaction are decoded, passed to the output plugin, and the
<code class="function">prepare_cb</code> callback is invoked. This differs from the
basic decoding setup where changes are only passed to the output plugin
when a transaction is committed. The start of a prepared transaction is
indicated by the <code class="function">begin_prepare_cb</code> callback.
</p><p>
When a prepared transaction is rolled back using the
<code class="command">ROLLBACK PREPARED</code>, then the
<code class="function">rollback_prepared_cb</code> callback is invoked and when the
prepared transaction is committed using <code class="command">COMMIT PREPARED</code>,
then the <code class="function">commit_prepared_cb</code> callback is invoked.
</p><p>
Optionally the output plugin can define filtering rules via
<code class="function">filter_prepare_cb</code> to decode only specific transaction
in two phases. This can be achieved by pattern matching on the
<em class="parameter"><code>gid</code></em> or via lookups using the
<em class="parameter"><code>xid</code></em>.
</p><p>
The users that want to decode prepared transactions need to be careful about
below mentioned points:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
If the prepared transaction has locked [user] catalog tables exclusively
then decoding prepare can block till the main transaction is committed.
</p></li><li class="listitem"><p>
The logical replication solution that builds distributed two phase commit
using this feature can deadlock if the prepared transaction has locked
[user] catalog tables exclusively. To avoid this users must refrain from
having locks on catalog tables (e.g. explicit <code class="command">LOCK</code> command)
in such transactions.
See <a class="xref" href="logicaldecoding-synchronous.html#LOGICALDECODING-SYNCHRONOUS-CAVEATS" title="49.8.2. Caveats">Section 49.8.2</a> for the details.
</p></li></ul></div><p>
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logicaldecoding-streaming.html" title="49.9. Streaming of Large Transactions for Logical Decoding">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="replication-origins.html" title="Chapter 50. Replication Progress Tracking">Next</a></td></tr><tr><td width="40%" align="left" valign="top">49.9. Streaming of Large Transactions for Logical Decoding </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 50. Replication Progress Tracking</td></tr></table></div></body></html>
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