Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Peter Smith <smithpb2250@gmail.com>, shveta malik <shveta.malik@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-06-05T06:05:28Z
Lists: pgsql-hackers 
On Wed, Jun 5, 2024 at 7:52 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Attach the V4 doc patch which addressed Peter and Bertrand's comments.
>

Few comments:

1.
+       On the subscriber node, use the following SQL to identify
+       which slots should be synced to the standby that we plan to promote.
+<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
+           ));
+ slots
+-------
+ {sub1,sub2,sub3}

This should additionally say what exactly this SQL is doing to fetch
the required slots.

2.
If <varname>standby_slot_names</varname> is
+     not configured correctly, it is highly recommended to run this step after
+     the primary server is down, otherwise the results of the query can vary
+     due to the ongoing replication on the logical subscribers from the primary
+     server.
+    </para>
+     <substeps>
+      <step performance="required">
+       <para>
+        On the subscriber node, check the last replayed WAL.
+        This step needs to be run on any database that includes
failover enabled
+        subscriptions.
+<programlisting>
+test_sub=# SELECT
+               MAX(remote_lsn) AS remote_lsn_on_subscriber
+           FROM

If the 'standby_slot_names' is not configured then we can't ensure
that standby is always ahead because what if immediately after running
this query the additional WAL got synced to the subscriber before
standby? Now, as you mentioned users can first shutdown primary to
ensure that no additional WAL is sent to the subscriber. After that,
it is possible that one can use these complex queries to ensure that
the subscriber is behind the standby but it is better to encourage
users to use standby_slot_names to ensure the same. If at all we get
such use cases and or requirements then we can add such additional
steps after understanding the user's requirements. For now, we should
remove these additional steps.

-- 
With Regards,
Amit Kapila.

Commits

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

  2. Fix the review comments and a bug in the slot sync code.

  3. Fix a test in failover slots regression test.

  4. Fix the intermittent buildfarm failures in 040_standby_failover_slots_sync.

  5. Split XLogCtl->LogwrtResult into separate struct members

  6. Ensure that the sync slots reach a consistent state after promotion without losing data.

  7. Introduce a new GUC 'standby_slot_names'.

  8. Fix BF failure introduced by commit b3f6b14cf4.

  9. Fixups for commit 93db6cbda0.

  10. Fix BF failure in commit 93db6cbda0.

  11. Add a new slot sync worker to synchronize logical slots.

  12. Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

  13. Disable autovacuum on primary in 040_standby_failover_slots_sync test.

  14. Fix the incorrect format specifier used in commit 7a424ece48.

  15. Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

  16. Add more LOG and DEBUG messages for slot synchronization.

  17. Another try to fix BF failure introduced in commit ddd5f4f54a.

  18. Fix BF introduced in commit ddd5f4f54a.

  19. Add a slot synchronization function.

  20. Clean-ups for 776621a5e4 and 7329240437.

  21. Enhance libpqrcv APIs to support slot synchronization.

  22. Add a failover option to subscriptions.

  23. Allow setting failover property in the replication command.

  24. Allow to enable failover property for replication slots via SQL API.

  25. Add macros for looping through a List without a ListCell.

  26. Track conflict_reason in pg_replication_slots.

  27. Allow upgrades to preserve the full subscription's state.

  28. Add support for incremental backup.

  29. Simplify some logic in CreateReplicationSlot()

  30. Fix uninitialized access to InitialRunningXacts during decoding after ERROR.

  31. Flexible options for CREATE_REPLICATION_SLOT.

  32. Empty search_path in logical replication apply worker and walsender.

  33. Fix oldest xmin and LSN computation across repslots after advancing

  34. Require C99 (and thus MSCV 2013 upwards).