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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>29.3. Logical Replication Failover</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="logical-replication-subscription.html" title="29.2. Subscription" /><link rel="next" href="logical-replication-row-filter.html" title="29.4. Row Filters" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">29.3. Logical Replication Failover</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logical-replication-subscription.html" title="29.2. Subscription">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logical-replication.html" title="Chapter 29. Logical Replication">Up</a></td><th width="60%" align="center">Chapter 29. Logical Replication</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="logical-replication-row-filter.html" title="29.4. Row Filters">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGICAL-REPLICATION-FAILOVER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">29.3. Logical Replication Failover <a href="#LOGICAL-REPLICATION-FAILOVER" class="id_link">#</a></h2></div></div></div><p>
   To allow subscriber nodes to continue replicating data from the publisher
   node even when the publisher node goes down, there must be a physical standby
   corresponding to the publisher node. The logical slots on the primary server
   corresponding to the subscriptions can be synchronized to the standby server by
   specifying <code class="literal">failover = true</code> when creating subscriptions. See
   <a class="xref" href="logicaldecoding-explanation.html#LOGICALDECODING-REPLICATION-SLOTS-SYNCHRONIZATION" title="47.2.3. Replication Slot Synchronization">Section 47.2.3</a> for details.
   Enabling the
   <a class="link" href="sql-createsubscription.html#SQL-CREATESUBSCRIPTION-PARAMS-WITH-FAILOVER"><code class="literal">failover</code></a>
   parameter ensures a seamless transition of those subscriptions after the
   standby is promoted. They can continue subscribing to publications on the
   new primary server.
  </p><p>
   Because the slot synchronization logic copies asynchronously, it is
   necessary to confirm that replication slots have been synced to the standby
   server before the failover happens. To ensure a successful failover, the
   standby server must be ahead of the subscriber. This can be achieved by
   configuring
   <a class="link" href="runtime-config-replication.html#GUC-SYNCHRONIZED-STANDBY-SLOTS"><code class="varname">synchronized_standby_slots</code></a>.
  </p><p>
   To confirm that the standby server is indeed ready for failover for a given subscriber, follow these
   steps to verify that all the logical replication slots required by that subscriber have been
   synchronized to the standby server:
  </p><div class="procedure"><ol class="procedure" type="1"><li class="step"><p>
     On the subscriber node, use the following SQL to identify which replication
     slots should be synced to the standby that we plan to promote. This query
     will return the relevant replication slots associated with the
     failover-enabled subscriptions.
</p><pre class="programlisting">
/* sub # */ SELECT
               array_agg(quote_literal(s.subslotname)) AS slots
           FROM  pg_subscription s
           WHERE s.subfailover AND
                 s.subslotname IS NOT NULL;
 slots
-------
 {'sub1','sub2','sub3'}
(1 row)
</pre></li><li class="step"><p>
     On the subscriber node, use the following SQL to identify which table
     synchronization slots should be synced to the standby that we plan to promote.
     This query needs to be run on each database that includes the failover-enabled
     subscription(s). Note that the table sync slot should be synced to the standby
     server only if the table copy is finished
     (See <a class="xref" href="catalog-pg-subscription-rel.html" title="52.55. pg_subscription_rel">Section 52.55</a>).
     We don't need to ensure that the table sync slots are synced in other scenarios
     as they will either be dropped or re-created on the new primary server in those
     cases.
</p><pre class="programlisting">
/* sub # */ SELECT
               array_agg(quote_literal(slot_name)) AS slots
           FROM
           (
               SELECT CONCAT('pg_', srsubid, '_sync_', srrelid, '_', ctl.system_identifier) AS slot_name
               FROM pg_control_system() ctl, pg_subscription_rel r, pg_subscription s
               WHERE r.srsubstate = 'f' AND s.oid = r.srsubid AND s.subfailover
           );
 slots
-------
 {'pg_16394_sync_16385_7394666715149055164'}
(1 row)
</pre></li><li class="step"><p>
     Check that the logical replication slots identified above exist on
     the standby server and are ready for failover.
</p><pre class="programlisting">
/* standby # */ SELECT slot_name, (synced AND NOT temporary AND invalidation_reason IS NULL) AS failover_ready
               FROM pg_replication_slots
               WHERE slot_name IN
                   ('sub1','sub2','sub3', 'pg_16394_sync_16385_7394666715149055164');
  slot_name                                 | failover_ready
--------------------------------------------+----------------
  sub1                                      | t
  sub2                                      | t
  sub3                                      | t
  pg_16394_sync_16385_7394666715149055164   | t
(4 rows)
</pre></li></ol></div><p>
   If all the slots are present on the standby server and the result
   (<code class="literal">failover_ready</code>) of the above SQL query is true, then
   existing subscriptions can continue subscribing to publications on the new
   primary server.
  </p><p>
   The first two steps in the above procedure are meant for a
   <span class="productname">PostgreSQL</span> subscriber. It is recommended to run
   these steps on each subscriber node, that will be served by the designated
   standby after failover, to obtain the complete list of replication
   slots. This list can then be verified in Step 3 to ensure failover readiness.
   Non-<span class="productname">PostgreSQL</span> subscribers, on the other hand, may
   use their own methods to identify the replication slots used by their
   respective subscriptions.
  </p><p>
   In some cases, such as during a planned failover, it is necessary to confirm
   that all subscribers, whether <span class="productname">PostgreSQL</span> or
   non-<span class="productname">PostgreSQL</span>, will be able to continue
   replication after failover to a given standby server. In such cases, use the
   following SQL, instead of performing the first two steps above, to identify
   which replication slots on the primary need to be synced to the standby that
   is intended for promotion. This query returns the relevant replication slots
   associated with all the failover-enabled subscriptions.
  </p><p>
</p><pre class="programlisting">
/* primary # */ SELECT array_agg(quote_literal(r.slot_name)) AS slots
               FROM pg_replication_slots r
               WHERE r.failover AND NOT r.temporary;
 slots
-------
 {'sub1','sub2','sub3', 'pg_16394_sync_16385_7394666715149055164'}
(1 row)
</pre></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logical-replication-subscription.html" title="29.2. Subscription">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logical-replication.html" title="Chapter 29. Logical Replication">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication-row-filter.html" title="29.4. Row Filters">Next</a></td></tr><tr><td width="40%" align="left" valign="top">29.2. Subscription </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"> 29.4. Row Filters</td></tr></table></div></body></html>