Documentation for sequence synchronization feature.

Documentation for sequence synchronization feature.

Author: vigneshwaran-c

doc/src/sgml/catalogs.sgml

@@ -8155,16 +8155,19 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
   </indexterm>
 
   <para>
-   The catalog <structname>pg_subscription_rel</structname> contains the
-   state for each replicated relation in each subscription.  This is a
-   many-to-many mapping.
+   The catalog <structname>pg_subscription_rel</structname> stores the
+   state of each replicated table and sequence for each subscription.  This
+   is a many-to-many mapping.
   </para>
 
   <para>
-   This catalog only contains tables known to the subscription after running
-   either <link linkend="sql-createsubscription"><command>CREATE SUBSCRIPTION</command></link> or
-   <link linkend="sql-altersubscription"><command>ALTER SUBSCRIPTION ... REFRESH
-   PUBLICATION</command></link>.
+   This catalog only contains tables and sequences known to the subscription
+   after running
+   <link linkend="sql-createsubscription"><command>CREATE SUBSCRIPTION</command></link> or
+   <link linkend="sql-altersubscription-params-refresh-publication">
+   <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION</command></link> or
+   <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+   <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>.
   </para>
 
   <table>
@@ -8198,7 +8201,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
-       Reference to relation
+       Reference to table or sequence
       </para></entry>
      </row>
 
@@ -8207,12 +8210,20 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        <structfield>srsubstate</structfield> <type>char</type>
       </para>
       <para>
-       State code:
+       State code for the table or sequence.
+      </para>
+      <para>
+       State codes for tables:
        <literal>i</literal> = initialize,
        <literal>d</literal> = data is being copied,
        <literal>f</literal> = finished table copy,
        <literal>s</literal> = synchronized,
        <literal>r</literal> = ready (normal replication)
+      </para>
+      <para>
+       State codes for sequences:
+       <literal>i</literal> = initialize,
+       <literal>r</literal> = ready
       </para></entry>
      </row>
 

doc/src/sgml/config.sgml

@@ -5184,9 +5184,9 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         is taken into account.
        </para>
        <para>
-        In logical replication, this parameter also limits how often a failing
-        replication apply worker or table synchronization worker will be
-        respawned.
+        In logical replication, this parameter also limits how quickly a
+        failing replication apply worker or table synchronization worker or
+        sequence synchronization worker will be respawned.
        </para>
       </listitem>
      </varlistentry>
@@ -5327,8 +5327,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       <listitem>
        <para>
         Specifies maximum number of logical replication workers. This includes
-        leader apply workers, parallel apply workers, and table synchronization
-        workers.
+        leader apply workers, parallel apply workers, table synchronization
+        workers and a sequence synchronization worker.
        </para>
        <para>
         Logical replication workers are taken from the pool defined by
@@ -5351,10 +5351,12 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
        <para>
         Maximum number of synchronization workers per subscription. This
         parameter controls the amount of parallelism of the initial data copy
-        during the subscription initialization or when new tables are added.
+        during the subscription initialization or when new tables or sequences
+        are added.
        </para>
        <para>
-        Currently, there can be only one synchronization worker per table.
+        Currently, there can be only one table synchronization worker per table
+        and one sequence synchronization worker to synchronize all sequences.
        </para>
        <para>
         The synchronization workers are taken from the pool defined by

doc/src/sgml/logical-replication.sgml

@@ -102,16 +102,20 @@
    A <firstterm>publication</firstterm> can be defined on any physical
    replication primary.  The node where a publication is defined is referred to
    as <firstterm>publisher</firstterm>.  A publication is a set of changes
-   generated from a table or a group of tables, and might also be described as
-   a change set or replication set.  Each publication exists in only one database.
+   generated from a table or a group of tables or the current state of all
+   sequences, and might also be described as a change set or replication set.
+   Each publication exists in only one database.
   </para>
 
   <para>
    Publications are different from schemas and do not affect how the table is
    accessed.  Each table can be added to multiple publications if needed.
-   Publications may currently only contain tables and all tables in schema.
-   Objects must be added explicitly, except when a publication is created for
-   <literal>ALL TABLES</literal>.
+   Publications may currently only contain tables or sequences. Objects must be
+   added explicitly, except when a publication is created using
+   <literal>FOR TABLES IN SCHEMA</literal>, or <literal>FOR ALL TABLES</literal>,
+   or <literal>FOR ALL SEQUENCES</literal>. Unlike tables, the current state of
+   sequences may be synchronized at any time. For more information, refer to
+   <xref linkend="logical-replication-sequences"/>.
   </para>
 
   <para>
@@ -1711,6 +1715,204 @@ Publications:
   </note>
  </sect1>
 
+ <sect1 id="logical-replication-sequences">
+  <title>Replicating Sequences</title>
+
+  <para>
+   To replicate sequences from a publisher to a subscriber, first publish them
+   using <link linkend="sql-createpublication-params-for-all-sequences">
+   <command>CREATE PUBLICATION ... FOR ALL SEQUENCES</command></link>.
+  </para>
+
+  <para>
+   At the subscriber side:
+   <itemizedlist>
+    <listitem>
+     <para>
+      use <link linkend="sql-createsubscription"><command>CREATE SUBSCRIPTION</command></link>
+      to initially synchronize the published sequences.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      use <link linkend="sql-altersubscription-params-refresh-publication">
+      <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION</command></link>
+      to synchronize only newly added sequences.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      use <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+      <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>
+      to re-synchronize all sequences.
+     </para>
+    </listitem>
+   </itemizedlist>
+  </para>
+
+  <para>
+   A new <firstterm>sequence synchronization worker</firstterm> will be started
+   after executing any of the above subscriber commands, and will exit once the
+   sequences are synchronized.
+  </para>
+  <para>
+   The ability to launch a sequence synchronization worker is limited by the
+   <link linkend="guc-max-sync-workers-per-subscription">
+   <varname>max_sync_workers_per_subscription</varname></link>
+   configuration.
+  </para>
+
+  <sect2 id="sequence-definition-mismatches">
+   <title>Sequence Definition Mismatches</title>
+   <warning>
+    <para>
+     During sequence synchronization, the sequence definitions of the publisher
+     and the subscriber are compared. A WARNING is logged listing all differing
+     sequences before the process exits. The apply worker detects the failure
+     and repeatedly respawns the sequence synchronization worker to continue
+     the synchronization process until all differences are resolved. See also
+     <link linkend="guc-wal-retrieve-retry-interval"><varname>wal_retrieve_retry_interval</varname></link>.
+    </para>
+   </warning>
+   <para>
+    To resolve this, use
+    <link linkend="sql-altersequence"><command>ALTER SEQUENCE</command></link>
+    to align the subscriber's sequence parameters with those of the publisher.
+    Then, execute <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+    <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>.
+   </para>
+  </sect2>
+
+  <sect2 id="sequences-out-of-sync">
+   <title>Refreshing Stale Sequences</title>
+   <para>
+    Subscriber side sequence values may frequently become out of sync due to
+    updates on the publisher.
+   </para>
+   <para>
+    To verify, compare the sequence values between the publisher and
+    subscriber, and if necessary, execute
+    <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+    <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>.
+   </para>
+  </sect2>
+
+  <sect2 id="logical-replication-sequences-examples">
+   <title>Examples</title>
+
+   <para>
+    Create some sequences on the publisher.
+<programlisting>
+test_pub=# CREATE SEQUENCE s1 START WITH 10 INCREMENT BY 1;
+CREATE SEQUENCE
+test_pub=# CREATE SEQUENCE s2 START WITH 100 INCREMENT BY 10;
+CREATE SEQUENCE
+</programlisting></para>
+
+   <para>
+    Create the same sequences on the subscriber.
+<programlisting>
+test_sub=# CREATE SEQUENCE s1 START WITH 10 INCREMENT BY 1
+CREATE SEQUENCE
+test_sub=# CREATE SEQUENCE s2 START WITH 100 INCREMENT BY 10;
+CREATE SEQUENCE
+</programlisting></para>
+
+   <para>
+    Update the sequences at the publisher side few times.
+<programlisting>
+test_pub=# SELECT nextval('s1');
+ nextval
+---------
+      10
+(1 row)
+test_pub=# SELECT NEXTVAL('s1');
+ nextval
+---------
+      11
+(1 row)
+test_pub=# SELECT nextval('s2');
+ nextval
+---------
+     100
+(1 row)
+test_pub=# SELECT nextval('s2');
+ nextval
+---------
+     110
+(1 row)
+</programlisting></para>
+
+   <para>
+    Create a publication for the sequences.
+<programlisting>
+test_pub=# CREATE PUBLICATION pub1 FOR ALL SEQUENCES;
+CREATE PUBLICATION
+</programlisting></para>
+
+   <para>
+    Subscribe to the publication.
+<programlisting>
+test_sub=# CREATE SUBSCRIPTION sub1
+test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub1'
+test_sub-# PUBLICATION pub1;
+CREATE SUBSCRIPTION
+</programlisting></para>
+
+   <para>
+    Observe that initial sequence values are synchronized.
+<programlisting>
+test_sub=# SELECT * FROM s1;
+ last_value | log_cnt | is_called
+------------+---------+-----------
+         11 |      31 | t
+(1 row)
+
+test_sub=# SELECT * FROM s2;
+ last_value | log_cnt | is_called
+------------+---------+-----------
+        110 |      31 | t
+(1 row)
+</programlisting></para>
+
+   <para>
+    Update the sequences at the publisher side.
+<programlisting>
+test_pub=# SELECT nextval('s1');
+ nextval
+---------
+      12
+(1 row)
+test_pub=# SELECT nextval('s2');
+ nextval
+---------
+     120
+(1 row)
+</programlisting></para>
+
+   <para>
+    Re-synchronize all the sequences at the subscriber side using
+    <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+    <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>.
+<programlisting>
+test_sub=# ALTER SUBSCRIPTION sub1 REFRESH PUBLICATION SEQUENCES;
+ALTER SUBSCRIPTION
+
+test_sub=# SELECT * FROM s1;
+ last_value | log_cnt | is_called
+------------+---------+-----------
+         12 |      30 | t
+(1 row)
+
+test_sub=# SELECT * FROM s2
+ last_value | log_cnt | is_called
+------------+---------+-----------
+        120 |      30 | t
+(1 row)
+</programlisting></para>
+  </sect2>
+ </sect1>
+
  <sect1 id="logical-replication-conflicts">
   <title>Conflicts</title>
 
@@ -2040,16 +2242,19 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
 
    <listitem>
     <para>
-     Sequence data is not replicated.  The data in serial or identity columns
-     backed by sequences will of course be replicated as part of the table,
-     but the sequence itself would still show the start value on the
-     subscriber.  If the subscriber is used as a read-only database, then this
-     should typically not be a problem.  If, however, some kind of switchover
-     or failover to the subscriber database is intended, then the sequences
-     would need to be updated to the latest values, either by copying the
-     current data from the publisher (perhaps
-     using <command>pg_dump</command>) or by determining a sufficiently high
-     value from the tables themselves.
+     Incremental sequence changes are not replicated.  Although the data in
+     serial or identity columns backed by sequences will be replicated as part
+     of the table, the sequences themselves do not replicate ongoing changes.
+     On the subscriber, a sequence will retain the last value it synchronized
+     from the publisher. If the subscriber is used as a read-only database,
+     then this should typically not be a problem.  If, however, some kind of
+     switchover or failover to the subscriber database is intended, then the
+     sequences would need to be updated to the latest values, either by
+     executing <link linkend="sql-altersubscription-params-refresh-publication-sequences">
+     <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES</command></link>
+     or by copying the current data from the publisher (perhaps using
+     <command>pg_dump</command>) or by determining a sufficiently high value
+     from the tables themselves.
     </para>
    </listitem>
 
@@ -2367,8 +2572,8 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
    <para>
     <link linkend="guc-max-logical-replication-workers"><varname>max_logical_replication_workers</varname></link>
     must be set to at least the number of subscriptions (for leader apply
-    workers), plus some reserve for the table synchronization workers and
-    parallel apply workers.
+    workers), plus some reserve for the parallel apply workers, table synchronization workers, and a sequence
+    synchronization worker.
    </para>
 
    <para>
@@ -2381,8 +2586,9 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
 
    <para>
     <link linkend="guc-max-sync-workers-per-subscription"><varname>max_sync_workers_per_subscription</varname></link>
-     controls the amount of parallelism of the initial data copy during the
-     subscription initialization or when new tables are added.
+     controls how many tables can be synchronized in parallel during
+     subscription initialization or when new tables are added. One additional
+     worker is also needed for sequence synchronization.
    </para>
 
    <para>