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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>49.2. Logical Decoding Concepts</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-example.html" title="49.1. Logical Decoding Examples" /><link rel="next" href="logicaldecoding-walsender.html" title="49.3. Streaming Replication Protocol Interface" /></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">49.2. Logical Decoding Concepts</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-example.html" title="49.1. Logical Decoding Examples">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 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-walsender.html" title="49.3. Streaming Replication Protocol Interface">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LOGICALDECODING-EXPLANATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">49.2. Logical Decoding Concepts</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logicaldecoding-explanation.html#id-1.8.14.8.2">49.2.1. Logical Decoding</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-REPLICATION-SLOTS">49.2.2. Replication Slots</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#id-1.8.14.8.4">49.2.3. Output Plugins</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#id-1.8.14.8.5">49.2.4. Exported Snapshots</a></span></dt></dl></div><div class="sect2" id="id-1.8.14.8.2"><div class="titlepage"><div><div><h3 class="title">49.2.1. Logical Decoding</h3></div></div></div><a id="id-1.8.14.8.2.2" class="indexterm"></a><p>
Logical decoding is the process of extracting all persistent changes
to a database's tables into a coherent, easy to understand format which
can be interpreted without detailed knowledge of the database's internal
state.
</p><p>
In <span class="productname">PostgreSQL</span>, logical decoding is implemented
by decoding the contents of the <a class="link" href="wal.html" title="Chapter 30. Reliability and the Write-Ahead Log">write-ahead
log</a>, which describe changes on a storage level, into an
application-specific form such as a stream of tuples or SQL statements.
</p></div><div class="sect2" id="LOGICALDECODING-REPLICATION-SLOTS"><div class="titlepage"><div><div><h3 class="title">49.2.2. Replication Slots</h3></div></div></div><a id="id-1.8.14.8.3.2" class="indexterm"></a><p>
In the context of logical replication, a slot represents a stream of
changes that can be replayed to a client in the order they were made on
the origin server. Each slot streams a sequence of changes from a single
database.
</p><div class="note"><h3 class="title">Note</h3><p><span class="productname">PostgreSQL</span> also has streaming replication slots
(see <a class="xref" href="warm-standby.html#STREAMING-REPLICATION" title="26.2.5. Streaming Replication">Section 26.2.5</a>), but they are used somewhat
differently there.
</p></div><p>
A replication slot has an identifier that is unique across all databases
in a <span class="productname">PostgreSQL</span> cluster. Slots persist
independently of the connection using them and are crash-safe.
</p><p>
A logical slot will emit each change just once in normal operation.
The current position of each slot is persisted only at checkpoint, so in
the case of a crash the slot may return to an earlier LSN, which will
then cause recent changes to be resent when the server restarts.
Logical decoding clients are responsible for avoiding ill effects from
handling the same message more than once. Clients may wish to record
the last LSN they saw when decoding and skip over any repeated data or
(when using the replication protocol) request that decoding start from
that LSN rather than letting the server determine the start point.
The Replication Progress Tracking feature is designed for this purpose,
refer to <a class="link" href="replication-origins.html" title="Chapter 50. Replication Progress Tracking">replication origins</a>.
</p><p>
Multiple independent slots may exist for a single database. Each slot has
its own state, allowing different consumers to receive changes from
different points in the database change stream. For most applications, a
separate slot will be required for each consumer.
</p><p>
A logical replication slot knows nothing about the state of the
receiver(s). It's even possible to have multiple different receivers using
the same slot at different times; they'll just get the changes following
on from when the last receiver stopped consuming them. Only one receiver
may consume changes from a slot at any given time.
</p><div class="caution"><h3 class="title">Caution</h3><p>
Replication slots persist across crashes and know nothing about the state
of their consumer(s). They will prevent removal of required resources
even when there is no connection using them. This consumes storage
because neither required WAL nor required rows from the system catalogs
can be removed by <code class="command">VACUUM</code> as long as they are required by a replication
slot. In extreme cases this could cause the database to shut down to prevent
transaction ID wraparound (see <a class="xref" href="routine-vacuuming.html#VACUUM-FOR-WRAPAROUND" title="24.1.5. Preventing Transaction ID Wraparound Failures">Section 24.1.5</a>).
So if a slot is no longer required it should be dropped.
</p></div></div><div class="sect2" id="id-1.8.14.8.4"><div class="titlepage"><div><div><h3 class="title">49.2.3. Output Plugins</h3></div></div></div><p>
Output plugins transform the data from the write-ahead log's internal
representation into the format the consumer of a replication slot desires.
</p></div><div class="sect2" id="id-1.8.14.8.5"><div class="titlepage"><div><div><h3 class="title">49.2.4. Exported Snapshots</h3></div></div></div><p>
When a new replication slot is created using the streaming replication
interface (see <a class="xref" href="protocol-replication.html#PROTOCOL-REPLICATION-CREATE-SLOT">CREATE_REPLICATION_SLOT</a>), a
snapshot is exported
(see <a class="xref" href="functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION" title="9.26.5. Snapshot Synchronization Functions">Section 9.26.5</a>), which will show
exactly the state of the database after which all changes will be
included in the change stream. This can be used to create a new replica by
using <a class="link" href="sql-set-transaction.html" title="SET TRANSACTION"><code class="literal">SET TRANSACTION
SNAPSHOT</code></a> to read the state of the database at the moment
the slot was created. This transaction can then be used to dump the
database's state at that point in time, which afterwards can be updated
using the slot's contents without losing any changes.
</p><p>
Creation of a snapshot is not always possible. In particular, it will
fail when connected to a hot standby. Applications that do not require
snapshot export may suppress it with the <code class="literal">NOEXPORT_SNAPSHOT</code>
option.
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