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diff --git a/doc/src/sgml/html/logicaldecoding-explanation.html b/doc/src/sgml/html/logicaldecoding-explanation.html new file mode 100644 index 0000000..d1e667a --- /dev/null +++ b/doc/src/sgml/html/logicaldecoding-explanation.html @@ -0,0 +1,101 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 id="docContent" class="container-fluid col-10"><div 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 16.2 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 /></div><div class="sect1" id="LOGICALDECODING-EXPLANATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">49.2. Logical Decoding Concepts <a href="#LOGICALDECODING-EXPLANATION" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-EXPLANATION-LOG-DEC">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#LOGICALDECODING-EXPLANATION-OUTPUT-PLUGINS">49.2.3. Output Plugins</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-EXPLANATION-EXPORTED-SNAPSHOTS">49.2.4. Exported Snapshots</a></span></dt></dl></div><div class="sect2" id="LOGICALDECODING-EXPLANATION-LOG-DEC"><div class="titlepage"><div><div><h3 class="title">49.2.1. Logical Decoding <a href="#LOGICALDECODING-EXPLANATION-LOG-DEC" class="id_link">#</a></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 <a href="#LOGICALDECODING-REPLICATION-SLOTS" class="id_link">#</a></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="27.2.5. Streaming Replication">Section 27.2.5</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 sent again 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><p> + A logical replication slot can also be created on a hot standby. To prevent + <code class="command">VACUUM</code> from removing required rows from the system + catalogs, <code class="varname">hot_standby_feedback</code> should be set on the + standby. In spite of that, if any required rows get removed, the slot gets + invalidated. It's highly recommended to use a physical slot between the + primary and the standby. Otherwise, <code class="varname">hot_standby_feedback</code> + will work but only while the connection is alive (for example a node + restart would break it). Then, the primary may delete system catalog rows + that could be needed by the logical decoding on the standby (as it does + not know about the catalog_xmin on the standby). Existing logical slots + on standby also get invalidated if <code class="varname">wal_level</code> on the + primary is reduced to less than <code class="literal">logical</code>. + This is done as soon as the standby detects such a change in the WAL stream. + It means that, for walsenders which are lagging (if any), some WAL records up + to the <code class="varname">wal_level</code> parameter change on the primary won't be + decoded. + </p><p> + Creation of a logical slot requires information about all the currently + running transactions. On the primary, this information is available + directly, but on a standby, this information has to be obtained from + primary. Thus, slot creation may need to wait for some activity to happen + on the primary. If the primary is idle, creating a logical slot on + standby may take noticeable time. This can be sped up by calling the + <code class="function">pg_log_standby_snapshot</code> function on the primary. + </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="25.1.5. Preventing Transaction ID Wraparound Failures">Section 25.1.5</a>). + So if a slot is no longer required it should be dropped. + </p></div></div><div class="sect2" id="LOGICALDECODING-EXPLANATION-OUTPUT-PLUGINS"><div class="titlepage"><div><div><h3 class="title">49.2.3. Output Plugins <a href="#LOGICALDECODING-EXPLANATION-OUTPUT-PLUGINS" class="id_link">#</a></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="LOGICALDECODING-EXPLANATION-EXPORTED-SNAPSHOTS"><div class="titlepage"><div><div><h3 class="title">49.2.4. Exported Snapshots <a href="#LOGICALDECODING-EXPLANATION-EXPORTED-SNAPSHOTS" class="id_link">#</a></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-REPLICATION-SLOT">CREATE_REPLICATION_SLOT</a>), a + snapshot is exported + (see <a class="xref" href="functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION" title="9.27.5. Snapshot Synchronization Functions">Section 9.27.5</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. + </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logicaldecoding-example.html" title="49.1. Logical Decoding Examples">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-walsender.html" title="49.3. 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