Improve log messages and docs for slot synchronization.

Amit Kapila · Amit Kapila · commit 1546e17f9d06 · 2025-06-19T09:48:08.000+05:30
Improve the clarity of LOG messages when a failover logical slot synchronization fails, making the reasons more explicit for easier debugging. Update the documentation to outline scenarios where slot synchronization can fail, especially during the initial sync, and emphasize that pg_sync_replication_slot() is primarily intended for testing and debugging purposes. We also discussed improving the functionality of pg_sync_replication_slot() so that it can be used reliably, but we would take up that work for next version after some more discussion and review. Reported-by: Suraj Kharage <suraj.kharage@enterprisedb.com> Author: shveta malik <shveta.malik@gmail.com> Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com> Reviewed-by: Peter Smith <smithpb2250@gmail.com> Reviewed-by: Amit Kapila <amit.kapila16@gmail.com> Backpatch-through: 17, where it was introduced Discussion: https://postgr.es/m/CAF1DzPWTcg+m+x+oVVB=y4q9=PYYsL_mujVp7uJr-_oUtWNGbA@mail.gmail.com
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -29698,7 +29698,7 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
       </row>
 
       <row>
-       <entry role="func_table_entry"><para role="func_signature">
+       <entry id="pg-logical-slot-get-binary-changes" role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>pg_logical_slot_get_binary_changes</primary>
         </indexterm>
@@ -29970,7 +29970,9 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
         standby server. Temporary synced slots, if any, cannot be used for
         logical decoding and must be dropped after promotion. See
         <xref linkend="logicaldecoding-replication-slots-synchronization"/> for details.
-        Note that this function cannot be executed if
+        Note that this function is primarily intended for testing and
+        debugging purposes and should be used with caution. Additionaly,
+        this function cannot be executed if
         <link linkend="guc-sync-replication-slots"><varname>
         sync_replication_slots</varname></link> is enabled and the slotsync
         worker is already running to perform the synchronization of slots.
diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml
@@ -370,10 +370,10 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      <function>pg_create_logical_replication_slot</function></link>, or by
      using the <link linkend="sql-createsubscription-params-with-failover">
      <literal>failover</literal></link> option of
-     <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. By setting <link linkend="guc-sync-replication-slots">
+     <command>CREATE SUBSCRIPTION</command> during slot creation.
+     Additionally, enabling <link linkend="guc-sync-replication-slots">
+     <varname>sync_replication_slots</varname></link> on the standby
+     is required. By enabling <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
@@ -398,6 +398,52 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      receiving the WAL up to the latest flushed position on the primary server.
     </para>
 
+    <note>
+     <para>
+      While enabling <link linkend="guc-sync-replication-slots">
+      <varname>sync_replication_slots</varname></link> allows for automatic
+      periodic synchronization of failover slots, they can also be manually
+      synchronized using the <link linkend="pg-sync-replication-slots">
+      <function>pg_sync_replication_slots</function></link> 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 may have already been removed or are at risk
+      of being removed on the standby. In contrast, automatic synchronization
+      via <varname>sync_replication_slots</varname> provides continuous slot
+      updates, enabling seamless failover and supporting high availability.
+      Therefore, it is the recommended method for synchronizing slots.
+     </para>
+    </note>
+
+    <para>
+     When slot synchronization is configured as recommended,
+     and the initial synchronization is performed either automatically or
+     manually via pg_sync_replication_slot, 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:
+<programlisting>
+     LOG:  could not synchronize replication slot "failover_slot"
+     DETAIL:  Synchronization could lead to data loss as the remote slot needs WAL at LSN 0/3003F28 and catalog xmin 754, but the standby has LSN 0/3003F28 and catalog xmin 756
+</programlisting>
+     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 <link linkend="pg-logical-slot-get-changes">
+     <function>pg_logical_slot_get_changes</function></link> or
+     <link linkend="pg-logical-slot-get-binary-changes">
+     <function>pg_logical_slot_get_binary_changes</function></link>,
+     allowing synchronization to proceed.
+    </para>
+
     <para>
      The ability to resume logical replication after failover depends upon the
      <link linkend="view-pg-replication-slots">pg_replication_slots</link>.<structfield>synced</structfield>
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
@@ -211,9 +211,9 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		 * impact the users, so we used DEBUG1 level to log the message.
 		 */
 		ereport(slot->data.persistency == RS_TEMPORARY ? LOG : DEBUG1,
-				errmsg("could not synchronize replication slot \"%s\" because remote slot precedes local slot",
+				errmsg("could not synchronize replication slot \"%s\"",
 					   remote_slot->name),
-				errdetail("The remote slot has LSN %X/%X and catalog xmin %u, but the local slot has LSN %X/%X and catalog xmin %u.",
+				errdetail("Synchronization could lead to data loss as the remote slot needs WAL at LSN %X/%X and catalog xmin %u, but the standby has LSN %X/%X and catalog xmin %u.",
 						  LSN_FORMAT_ARGS(remote_slot->restart_lsn),
 						  remote_slot->catalog_xmin,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
@@ -593,7 +593,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	{
 		ereport(LOG,
 				errmsg("could not synchronize replication slot \"%s\"", remote_slot->name),
-				errdetail("Logical decoding could not find consistent point from local slot's LSN %X/%X.",
+				errdetail("Synchronization could lead to data loss as standby could not build a consistent snapshot to decode WALs at LSN %X/%X.",
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn)));
 
 		return false;
