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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>31.7. Architecture</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="logical-replication-restrictions.html" title="31.6. Restrictions" /><link rel="next" href="logical-replication-monitoring.html" title="31.8. Monitoring" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">31.7. Architecture</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logical-replication-restrictions.html" title="31.6. Restrictions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logical-replication.html" title="Chapter 31. Logical Replication">Up</a></td><th width="60%" align="center">Chapter 31. Logical Replication</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication-monitoring.html" title="31.8. Monitoring">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGICAL-REPLICATION-ARCHITECTURE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">31.7. Architecture</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logical-replication-architecture.html#LOGICAL-REPLICATION-SNAPSHOT">31.7.1. Initial Snapshot</a></span></dt></dl></div><p>
Logical replication starts by copying a snapshot of the data on the
publisher database. Once that is done, changes on the publisher are sent
to the subscriber as they occur in real time. The subscriber applies data
in the order in which commits were made on the publisher so that
transactional consistency is guaranteed for the publications within any
single subscription.
</p><p>
Logical replication is built with an architecture similar to physical
streaming replication (see <a class="xref" href="warm-standby.html#STREAMING-REPLICATION" title="27.2.5. Streaming Replication">Section 27.2.5</a>). It is
implemented by <span class="quote">“<span class="quote">walsender</span>”</span> and <span class="quote">“<span class="quote">apply</span>”</span>
processes. The walsender process starts logical decoding (described
in <a class="xref" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Chapter 49</a>) of the WAL and loads the standard
logical decoding plugin (pgoutput). The plugin transforms the changes read
from WAL to the logical replication protocol
(see <a class="xref" href="protocol-logical-replication.html" title="55.5. Logical Streaming Replication Protocol">Section 55.5</a>) and filters the data
according to the publication specification. The data is then continuously
transferred using the streaming replication protocol to the apply worker,
which maps the data to local tables and applies the individual changes as
they are received, in correct transactional order.
</p><p>
The apply process on the subscriber database always runs with
<a class="link" href="runtime-config-client.html#GUC-SESSION-REPLICATION-ROLE"><code class="varname">session_replication_role</code></a>
set to <code class="literal">replica</code>. This means that, by default,
triggers and rules will not fire on a subscriber. Users can optionally choose to
enable triggers and rules on a table using the
<a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE</code></a> command
and the <code class="literal">ENABLE TRIGGER</code> and <code class="literal">ENABLE RULE</code>
clauses.
</p><p>
The logical replication apply process currently only fires row triggers,
not statement triggers. The initial table synchronization, however, is
implemented like a <code class="command">COPY</code> command and thus fires both row
and statement triggers for <code class="command">INSERT</code>.
</p><div class="sect2" id="LOGICAL-REPLICATION-SNAPSHOT"><div class="titlepage"><div><div><h3 class="title">31.7.1. Initial Snapshot</h3></div></div></div><p>
The initial data in existing subscribed tables are snapshotted and
copied in a parallel instance of a special kind of apply process.
This process will create its own replication slot and copy the existing
data. As soon as the copy is finished the table contents will become
visible to other backends. Once existing data is copied, the worker
enters synchronization mode, which ensures that the table is brought
up to a synchronized state with the main apply process by streaming
any changes that happened during the initial data copy using standard
logical replication. During this synchronization phase, the changes
are applied and committed in the same order as they happened on the
publisher. Once synchronization is done, control of the
replication of the table is given back to the main apply process where
replication continues as normal.
</p><div class="note"><h3 class="title">Note</h3><p>
The publication <code class="literal">publish</code> parameter only affects what
DML operations will be replicated. The initial data synchronization does
not take this parameter into account when copying the existing table data.
</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logical-replication-restrictions.html" title="31.6. Restrictions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logical-replication.html" title="Chapter 31. Logical Replication">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication-monitoring.html" title="31.8. Monitoring">Next</a></td></tr><tr><td width="40%" align="left" valign="top">31.6. Restrictions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 31.8. Monitoring</td></tr></table></div></body></html>
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