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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>Chapter 50. Replication Progress Tracking</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-two-phase-commits.html" title="49.10. Two-phase Commit Support for Logical Decoding" /><link rel="next" href="archive-modules.html" title="Chapter 51. Archive Modules" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 50. Replication Progress Tracking</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-two-phase-commits.html" title="49.10. Two-phase Commit Support for Logical Decoding">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="server-programming.html" title="Part V. Server Programming">Up</a></td><th width="60%" align="center">Part V. Server Programming</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="archive-modules.html" title="Chapter 51. Archive Modules">Next</a></td></tr></table><hr /></div><div class="chapter" id="REPLICATION-ORIGINS"><div class="titlepage"><div><div><h2 class="title">Chapter 50. Replication Progress Tracking</h2></div></div></div><a id="id-1.8.15.2" class="indexterm"></a><a id="id-1.8.15.3" class="indexterm"></a><p>
Replication origins are intended to make it easier to implement
logical replication solutions on top
of <a class="link" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">logical decoding</a>.
They provide a solution to two common problems:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>How to safely keep track of replication progress</p></li><li class="listitem"><p>How to change replication behavior based on the
origin of a row; for example, to prevent loops in bi-directional
replication setups</p></li></ul></div><p>
</p><p>
Replication origins have just two properties, a name and an ID. The name,
which is what should be used to refer to the origin across systems, is
free-form <code class="type">text</code>. It should be used in a way that makes conflicts
between replication origins created by different replication solutions
unlikely; e.g., by prefixing the replication solution's name to it.
The ID is used only to avoid having to store the long version
in situations where space efficiency is important. It should never be shared
across systems.
</p><p>
Replication origins can be created using the function
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-CREATE"><code class="function">pg_replication_origin_create()</code></a>;
dropped using
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-DROP"><code class="function">pg_replication_origin_drop()</code></a>;
and seen in the
<a class="link" href="catalog-pg-replication-origin.html" title="53.44. pg_replication_origin"><code class="structname">pg_replication_origin</code></a>
system catalog.
</p><p>
One nontrivial part of building a replication solution is to keep track of
replay progress in a safe manner. When the applying process, or the whole
cluster, dies, it needs to be possible to find out up to where data has
successfully been replicated. Naive solutions to this, such as updating a
row in a table for every replayed transaction, have problems like run-time
overhead and database bloat.
</p><p>
Using the replication origin infrastructure a session can be
marked as replaying from a remote node (using the
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-SESSION-SETUP"><code class="function">pg_replication_origin_session_setup()</code></a>
function). Additionally the <acronym class="acronym">LSN</acronym> and commit
time stamp of every source transaction can be configured on a per
transaction basis using
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-XACT-SETUP"><code class="function">pg_replication_origin_xact_setup()</code></a>.
If that's done replication progress will persist in a crash safe
manner. Replay progress for all replication origins can be seen in the
<a class="link" href="view-pg-replication-origin-status.html" title="54.18. pg_replication_origin_status">
<code class="structname">pg_replication_origin_status</code>
</a> view. An individual origin's progress, e.g., when resuming
replication, can be acquired using
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-PROGRESS"><code class="function">pg_replication_origin_progress()</code></a>
for any origin or
<a class="link" href="functions-admin.html#PG-REPLICATION-ORIGIN-SESSION-PROGRESS"><code class="function">pg_replication_origin_session_progress()</code></a>
for the origin configured in the current session.
</p><p>
In replication topologies more complex than replication from exactly one
system to one other system, another problem can be that it is hard to avoid
replicating replayed rows again. That can lead both to cycles in the
replication and inefficiencies. Replication origins provide an optional
mechanism to recognize and prevent that. When configured using the functions
referenced in the previous paragraph, every change and transaction passed to
output plugin callbacks (see <a class="xref" href="logicaldecoding-output-plugin.html" title="49.6. Logical Decoding Output Plugins">Section 49.6</a>)
generated by the session is tagged with the replication origin of the
generating session. This allows treating them differently in the output
plugin, e.g., ignoring all but locally-originating rows. Additionally
the <a class="link" href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-FILTER-ORIGIN" title="49.6.4.7. Origin Filter Callback">
<code class="function">filter_by_origin_cb</code></a> callback can be used
to filter the logical decoding change stream based on the
source. While less flexible, filtering via that callback is
considerably more efficient than doing it in the output plugin.
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