Add a new slot sync worker to synchronize logical slots.

By enabling slot synchronization, all the failover logical replication
slots on the primary (assuming configurations are appropriate) are
automatically created on the physical standbys and are synced
periodically. The slot sync worker on the standby server pings the primary
server at regular intervals to get the necessary failover logical slots
information and create/update the slots locally. The slots that no longer
require synchronization are automatically dropped by the worker.

The nap time of the worker is tuned according to the activity on the
primary. The slot sync worker waits for some time before the next
synchronization, with the duration varying based on whether any slots were
updated during the last cycle.

A new parameter sync_replication_slots enables or disables this new
process.

On promotion, the slot sync worker is shut down by the startup process to
drop any temporary slots acquired by the slot sync worker and to prevent
the worker from trying to fetch the failover slots.

A functionality to allow logical walsenders to wait for the physical will
be done in a subsequent commit.

Author: Shveta Malik, Hou Zhijie based on design inputs by Masahiko Sawada and Amit Kapila
Reviewed-by: Masahiko Sawada, Bertrand Drouvot, Peter Smith, Dilip Kumar, Ajin Cherian, Nisha Moond, Kuroda Hayato, Amit Kapila
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com

2 files changed, 22 insertions, 1 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index ffd711b7f21..ff184003feb 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4943,6 +4943,24 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-sync-replication-slots" xreflabel="sync_replication_slots">
+      <term><varname>sync_replication_slots</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>sync_replication_slots</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        It enables a physical standby to synchronize logical failover slots
+        from the primary server so that logical subscribers can resume
+        replication from the new primary server after failover.
+       </para>
+       <para>
+        It is disabled by default. This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+      </listitem>
+     </varlistentry>
      </variablelist>
     </sect2>
 
diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml
index eceaaaa2735..930c0fa8a6d 100644
--- a/doc/src/sgml/logicaldecoding.sgml
+++ b/doc/src/sgml/logicaldecoding.sgml
@@ -373,7 +373,10 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      <command>CREATE SUBSCRIPTION</command> during slot creation, and then calling
      <link linkend="pg-sync-replication-slots">
      <function>pg_sync_replication_slots</function></link>
-     on the standby. For the synchronization to work, it is mandatory to
+     on the standby. By setting <link linkend="guc-sync-replication-slots">
+     <varname>sync_replication_slots</varname></link>
+     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 aka
      <link linkend="guc-primary-slot-name"><varname>primary_slot_name</varname></link>
      should be configured on the standby, and