Doc: Add the new section "Logical Replication Failover".

This aids the users to ensure that the failover marked slots are synced
to the standby and subscribers can continue replication even when the
publisher node goes down.

Author: Hou Zhijie, Shveta Malik, Amit Kapila
Reviewed-by: Peter Smith, Bertrand Drouvot
Discussion: https://postgr.es/m/OS0PR01MB57164D6F53FB4F6AD29AD9C594FB2@OS0PR01MB5716.jpnprd01.prod.outlook.com

doc/src/sgml/high-availability.sgml

@@ -1487,6 +1487,15 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
     Written administration procedures are advised.
    </para>
 
+   <para>
+    If you have opted for logical replication slot synchronization (see
+    <xref linkend="logicaldecoding-replication-slots-synchronization"/>),
+    then before switching to the standby server, it is recommended to check
+    if the logical slots synchronized on the standby server are ready
+    for failover. This can be done by following the steps described in
+    <xref linkend="logical-replication-failover"/>.
+   </para>
+
    <para>
     To trigger failover of a log-shipping standby server, run
     <command>pg_ctl promote</command> or call <function>pg_promote()</function>.

doc/src/sgml/logical-replication.sgml

@@ -687,6 +687,100 @@ ALTER SUBSCRIPTION
 
  </sect1>
 
+ <sect1 id="logical-replication-failover">
+  <title>Logical Replication Failover</title>
+
+  <para>
+   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 <literal>failover = true</literal> when creating subscriptions. See
+   <xref linkend="logicaldecoding-replication-slots-synchronization"/> for details.
+   Enabling the
+   <link linkend="sql-createsubscription-params-with-failover"><literal>failover</literal></link>
+   parameter ensures a seamless transition of those subscriptions after the
+   standby is promoted. They can continue subscribing to publications on the
+   new primary server without losing data. Note that in the case of
+   asynchronous replication, there remains a risk of data loss for transactions
+   committed on the former primary server but have yet to be replicated to the new
+   primary server.
+  </para>
+
+  <para>
+   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
+   <link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>.
+  </para>
+
+  <para>
+   To confirm that the standby server is indeed ready for failover, follow these
+   steps to verify that all necessary logical replication slots have been
+   synchronized to the standby server:
+  </para>
+
+  <procedure>
+   <step performance="required">
+    <para>
+     On the subscriber node, use the following SQL to identify which slots
+     should be synced to the standby that we plan to promote. This query will
+     return the relevant replication slots, including the main slots and table
+     synchronization slots associated with the failover-enabled subscriptions.
+     Note that the table sync slot should be synced to the standby server only
+     if the table copy is finished (See <xref linkend="catalog-pg-subscription-rel"/>).
+     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.
+<programlisting>
+test_sub=# SELECT
+               array_agg(slot_name) AS slots
+           FROM
+           ((
+               SELECT r.srsubid AS subid, 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
+           ) UNION (
+               SELECT s.oid AS subid, s.subslotname as slot_name
+               FROM pg_subscription s
+               WHERE s.subfailover
+           ))
+           WHERE slot_name IS NOT NULL;
+ slots
+-------
+ {sub1,sub2,sub3}
+(1 row)
+</programlisting></para>
+   </step>
+   <step performance="required">
+    <para>
+     Check that the logical replication slots identified above exist on
+     the standby server and are ready for failover.
+<programlisting>
+test_standby=# SELECT slot_name, (synced AND NOT temporary AND NOT conflicting) AS failover_ready
+               FROM pg_replication_slots
+               WHERE slot_name IN ('sub1','sub2','sub3');
+  slot_name  | failover_ready
+-------------+----------------
+  sub1       | t
+  sub2       | t
+  sub3       | t
+(3 rows)
+</programlisting></para>
+    </step>
+  </procedure>
+
+  <para>
+   If all the slots are present on the standby server and the result
+   (<literal>failover_ready</literal>) of the above SQL query is true, then
+   existing subscriptions can continue subscribing to publications now on the
+   new primary server without losing data.
+  </para>
+
+ </sect1>
+
  <sect1 id="logical-replication-row-filter">
   <title>Row Filters</title>