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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>30.5. 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 V1.79.1" /><link rel="prev" href="logical-replication-restrictions.html" title="30.4. Restrictions" /><link rel="next" href="logical-replication-monitoring.html" title="30.6. Monitoring" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">30.5. Architecture</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logical-replication-restrictions.html" title="30.4. Restrictions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logical-replication.html" title="Chapter 30. Logical Replication">Up</a></td><th width="60%" align="center">Chapter 30. Logical Replication</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication-monitoring.html" title="30.6. Monitoring">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LOGICAL-REPLICATION-ARCHITECTURE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">30.5. Architecture</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logical-replication-architecture.html#LOGICAL-REPLICATION-SNAPSHOT">30.5.1. 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="26.2.5. Streaming Replication">Section 26.2.5</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 48. Logical Decoding">Chapter 48</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="52.5. Logical Streaming Replication Protocol">Section 52.5</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
- <code class="varname">session_replication_role</code> set
- to <code class="literal">replica</code>, which produces the usual effects on triggers
- and constraints.
- </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">30.5.1. 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 temporary replication slot and
- copy the existing data. 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. Once the synchronization is done, the control
- of the replication of the table is given back to the main apply
- process where the replication continues as normal.
- </p></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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logical-replication.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication-monitoring.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">30.4. Restrictions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 30.6. Monitoring</td></tr></table></div></body></html>
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