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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>47.2. 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="47.1. Logical Decoding Examples" /><link rel="next" href="logicaldecoding-walsender.html" title="47.3. 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">47.2. Logical Decoding Concepts</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-example.html" title="47.1. Logical Decoding Examples">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logicaldecoding.html" title="Chapter 47. Logical Decoding">Up</a></td><th width="60%" align="center">Chapter 47. Logical Decoding</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-walsender.html" title="47.3. 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">47.2. 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">47.2.1. Logical Decoding</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-REPLICATION-SLOTS">47.2.2. Replication Slots</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-REPLICATION-SLOTS-SYNCHRONIZATION">47.2.3. Replication Slot Synchronization</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-EXPLANATION-OUTPUT-PLUGINS">47.2.4. Output Plugins</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-explanation.html#LOGICALDECODING-EXPLANATION-EXPORTED-SNAPSHOTS">47.2.5. Exported Snapshots</a></span></dt></dl></div><div class="sect2" id="LOGICALDECODING-EXPLANATION-LOG-DEC"><div class="titlepage"><div><div><h3 class="title">47.2.1. 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 28. 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">47.2.2. 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="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 might 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 48. 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 <code class="literal">catalog_xmin</code> 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 that 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="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="LOGICALDECODING-REPLICATION-SLOTS-SYNCHRONIZATION"><div class="titlepage"><div><div><h3 class="title">47.2.3. Replication Slot Synchronization <a href="#LOGICALDECODING-REPLICATION-SLOTS-SYNCHRONIZATION" class="id_link">#</a></h3></div></div></div><p>
     The logical replication slots on the primary can be synchronized to
     the hot standby by using the <code class="literal">failover</code> parameter of
     <a class="link" href="functions-admin.html#PG-CREATE-LOGICAL-REPLICATION-SLOT">
     <code class="function">pg_create_logical_replication_slot</code></a>, or by
     using the <a class="link" href="sql-createsubscription.html#SQL-CREATESUBSCRIPTION-PARAMS-WITH-FAILOVER">
     <code class="literal">failover</code></a> option of
     <code class="command">CREATE SUBSCRIPTION</code> during slot creation.
     Additionally, enabling <a class="link" href="runtime-config-replication.html#GUC-SYNC-REPLICATION-SLOTS">
     <code class="varname">sync_replication_slots</code></a> on the standby
     is required. By enabling <a class="link" href="runtime-config-replication.html#GUC-SYNC-REPLICATION-SLOTS">
     <code class="varname">sync_replication_slots</code></a>
     on the standby, the failover slots can be synchronized periodically in
     the slotsync worker. For the synchronization to work, it is mandatory to
     have a physical replication slot between the primary and the standby (i.e.,
     <a class="link" href="runtime-config-replication.html#GUC-PRIMARY-SLOT-NAME"><code class="varname">primary_slot_name</code></a>
     should be configured on the standby), and
     <a class="link" href="runtime-config-replication.html#GUC-HOT-STANDBY-FEEDBACK"><code class="varname">hot_standby_feedback</code></a>
     must be enabled on the standby. It is also necessary to specify a valid
     <code class="literal">dbname</code> in the
     <a class="link" href="runtime-config-replication.html#GUC-PRIMARY-CONNINFO"><code class="varname">primary_conninfo</code></a>.
     It's highly recommended that the said physical replication slot is named in
     <a class="link" href="runtime-config-replication.html#GUC-SYNCHRONIZED-STANDBY-SLOTS"><code class="varname">synchronized_standby_slots</code></a>
     list on the primary, to prevent the subscriber from consuming changes
     faster than the hot standby. Even when correctly configured, some latency
     is expected when sending changes to logical subscribers due to the waiting
     on slots named in
     <a class="link" href="runtime-config-replication.html#GUC-SYNCHRONIZED-STANDBY-SLOTS"><code class="varname">synchronized_standby_slots</code></a>.
     When <code class="varname">synchronized_standby_slots</code> is utilized, the
     primary server will not completely shut down until the corresponding
     standbys, associated with the physical replication slots specified
     in <code class="varname">synchronized_standby_slots</code>, have confirmed
     receiving the WAL up to the latest flushed position on the primary server.
    </p><div class="note"><h3 class="title">Note</h3><p>
      While enabling <a class="link" href="runtime-config-replication.html#GUC-SYNC-REPLICATION-SLOTS">
      <code class="varname">sync_replication_slots</code></a> allows for automatic
      periodic synchronization of failover slots, they can also be manually
      synchronized using the <a class="link" href="functions-admin.html#PG-SYNC-REPLICATION-SLOTS">
      <code class="function">pg_sync_replication_slots</code></a> function on the standby.
      However, this function is primarily intended for testing and debugging and
      should be used with caution. Unlike automatic synchronization, it does not
      include cyclic retries, making it more prone to synchronization failures,
      particularly during initial sync scenarios where the required WAL files
      or catalog rows for the slot might have already been removed or are at risk
      of being removed on the standby. In contrast, automatic synchronization
      via <code class="varname">sync_replication_slots</code> provides continuous slot
      updates, enabling seamless failover and supporting high availability.
      Therefore, it is the recommended method for synchronizing slots.
     </p></div><p>
     When slot synchronization is configured as recommended,
     and the initial synchronization is performed either automatically or
     manually via <code class="function">pg_sync_replication_slots</code>, the standby
     can persist the synchronized slot only if the following condition is met:
     The logical replication slot on the primary must retain WALs and system
     catalog rows that are still available on the standby. This ensures data
     integrity and allows logical replication to continue smoothly after
     promotion.
     If the required WALs or catalog rows have already been purged from the
     standby, the slot will not be persisted to avoid data loss. In such
     cases, the following log message may appear:
</p><pre class="programlisting">
LOG:  could not synchronize replication slot "failover_slot"
DETAIL:  Synchronization could lead to data loss, because the remote slot needs WAL at LSN 0/3003F28 and catalog xmin 754, but the standby has LSN 0/3003F28 and catalog xmin 756.
</pre><p>
     If the logical replication slot is actively used by a consumer, no
     manual intervention is needed; the slot will advance automatically,
     and synchronization will resume in the next cycle. However, if no
     consumer is configured, it is advisable to manually advance the slot
     on the primary using <a class="link" href="functions-admin.html#PG-LOGICAL-SLOT-GET-CHANGES">
     <code class="function">pg_logical_slot_get_changes</code></a> or
     <a class="link" href="functions-admin.html#PG-LOGICAL-SLOT-GET-BINARY-CHANGES">
     <code class="function">pg_logical_slot_get_binary_changes</code></a>,
     allowing synchronization to proceed.
    </p><p>
     The ability to resume logical replication after failover depends upon the
     <a class="link" href="view-pg-replication-slots.html" title="53.20. pg_replication_slots">pg_replication_slots</a>.<code class="structfield">synced</code>
     value for the synchronized slots on the standby at the time of failover.
     Only persistent slots that have attained synced state as true on the standby
     before failover can be used for logical replication after failover.
     Temporary synced slots cannot be used for logical decoding, therefore
     logical replication for those slots cannot be resumed. For example, if the
     synchronized slot could not become persistent on the standby due to a
     disabled subscription, then the subscription cannot be resumed after
     failover even when it is enabled.
    </p><p>
     To resume logical replication after failover from the synced logical
     slots, the subscription's 'conninfo' must be altered to point to the
     new primary server. This is done using
     <a class="link" href="sql-altersubscription.html#SQL-ALTERSUBSCRIPTION-PARAMS-CONNECTION"><code class="command">ALTER SUBSCRIPTION ... CONNECTION</code></a>.
     It is recommended that subscriptions are first disabled before promoting
     the standby and are re-enabled after altering the connection string.
    </p><div class="caution"><h3 class="title">Caution</h3><p>
      There is a chance that the old primary is up again during the promotion
      and if subscriptions are not disabled, the logical subscribers may
      continue to receive data from the old primary server even after promotion
      until the connection string is altered. This might result in data
      inconsistency issues, preventing the logical subscribers from being
      able to continue replication from the new primary server.
     </p></div></div><div class="sect2" id="LOGICALDECODING-EXPLANATION-OUTPUT-PLUGINS"><div class="titlepage"><div><div><h3 class="title">47.2.4. 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">47.2.5. 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="9.28.5. Snapshot Synchronization Functions">Section 9.28.5</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>
     Applications that do not require
     snapshot export may suppress it with the <code class="literal">SNAPSHOT 'nothing'</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="47.1. Logical Decoding Examples">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html" title="Chapter 47. Logical Decoding">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-walsender.html" title="47.3. Streaming Replication Protocol Interface">Next</a></td></tr><tr><td width="40%" align="left" valign="top">47.1. Logical Decoding Examples </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 47.3. Streaming Replication Protocol Interface</td></tr></table></div></body></html>