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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 15.5 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 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 31.8. Monitoring</td></tr></table></div></body></html>
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