Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: Ajin Cherian <itsajin@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Peter Smith <smithpb2250@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>, shveta malik <shveta.malik@gmail.com>
Date: 2023-12-01T03:19:09Z
Lists: pgsql-hackers 
On Thu, Nov 30, 2023 at 5:37 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Wed, Nov 29, 2023 at 8:17 PM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.com> wrote:
> >
> > This has been fixed.
> >
> > Best Regards,
> > Hou zj
>
> Thanks for addressing my comments. Some comments from my testing of patch v41
>
> 1. In my opinion, the second message "aborting the wait...moving to
> the next slot" does not hold much value. There might not even be a
> "next slot", there might be just one slot. I think the first LOG is
> enough to indicate that the sync-slot is waiting as it repeats this
> log till the slot catches up. I know these messages hold great value
> for debugging but in production,  "waiting..", "aborting the wait.."
> might not be as helpful, maybe change it to debug?
>
> 2023-11-30 05:13:49.811 EST [6115] LOG:  waiting for remote slot
> "sub1" LSN (0/3047A90) and catalog xmin (745) to pass local slot LSN
> (0/3047AC8) and catalog xmin (745)
> 2023-11-30 05:13:57.909 EST [6115] LOG:  aborting the wait for remote
> slot "sub1" and moving to the next slot, will attempt creating it
> again
> 2023-11-30 05:14:07.921 EST [6115] LOG:  waiting for remote slot
> "sub1" LSN (0/3047A90) and catalog xmin (745) to pass local slot LSN
> (0/3047AC8) and catalog xmin (745)
>

Sure, the message can be trimmed down. But I am not very sure if we
should convert it to DEBUG. It might be useful to know what exactly is
happening with this slot through the log file.Curious to know what
others think here?

>
> 2. If a slot on the standby is in the "i" state as it hasn't been
> synced and it was invalidated on the primary, should you continuously
> retry creating this invalidated slot on the standby?
>
> 2023-11-30 06:21:41.844 EST [10563] LOG:  waiting for remote slot
> "sub1" LSN (0/0) and catalog xmin (785) to pass local slot LSN
> (0/EED9330) and catalog xmin (785)
> 2023-11-30 06:21:41.845 EST [10563] WARNING:  slot "sub1" invalidated
> on the primary server, slot creation aborted
> 2023-11-30 06:21:51.892 EST [10563] LOG:  waiting for remote slot
> "sub1" LSN (0/0) and catalog xmin (785) to pass local slot LSN
> (0/EED9330) and catalog xmin (785)
> 2023-11-30 06:21:51.893 EST [10563] WARNING:  slot "sub1" invalidated
> on the primary server, slot creation aborted
>

No, it should not be synced after that. It should be marked as
invalidated and skipped. And perhaps the state should also be moved to
'r' as we are done with it; but since it is invalidated, it will not
be used further even if 'r'.

> 3. If creation of a slot on the standby fails for one slot because a
> slot of the same name exists, then thereafter no new sync slots are
> created on standby. Is this expected? I do see that previously created
> slots are kept up to date, just that no new slots are created after
> that.
>

yes, it is done so as per the suggestion/discussion in [1]. It is done
so that users can catch this issue at the earliest.

[1]: https://www.postgresql.org/message-id/CAA4eK1J5D-Z7dFa89acf7O%2BCa6Y9bygTpi52KAKVCg%2BPE%2BZfog%40mail.gmail.com

thanks
Shveta

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).