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: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@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>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: 2024-02-20T13:25:48Z
Lists: pgsql-hackers

Attachments

On Tue, Feb 20, 2024 at 6:19 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> Thank you for the explanation. It makes sense to me to move the check.
>
> As for ValidateSlotSyncParams() called by SlotSyncWorkerAllowed(), I
> have two comments:
>
> 1. The error messages are not very descriptive and seem not to match
> other messages the postmaster says. When starting the standby server
> with misconfiguration about the slotsync, I got the following messages
> from the postmaster:
>
> 2024-02-20 17:01:16.356 JST [456741] LOG:  database system is ready to
> accept read-only connections
> 2024-02-20 17:01:16.358 JST [456741] LOG:  bad configuration for slot
> synchronization
> 2024-02-20 17:01:16.358 JST [456741] HINT:  "hot_standby_feedback"
> must be enabled.
>
> It says "bad configuration" but is still working, and does not say
> further information such as whether it skipped to start the slotsync
> worker etc. I think these messages could work for the slotsync worker
> but we might want to have more descriptive messages for the
> postmaster. For example, "skipped starting slot sync worker because
> hot_standby_feedback is disabled".
>

We are planning to change it to something like:"slot synchronization
requires hot_standby_feedback to be enabled". See [1]

> 2. If the wal_level is not logical, the server will need to restart
> anyway to change the wal_level and have the slotsync worker work. Does
> it make sense to have the postmaster exit if the wal_level is not
> logical and sync_replication_slots is enabled? For instance, we have
> similar checks in PostmsaterMain():
>
>     if (summarize_wal && wal_level == WAL_LEVEL_MINIMAL)
>         ereport(ERROR,
>                 (errmsg("WAL cannot be summarized when wal_level is
> \"minimal\"")));
>

+1. I think giving an error in this case makes sense.

Miscellaneous comments:
========================
1.
+void
+ShutDownSlotSync(void)
+{
+ SpinLockAcquire(&SlotSyncCtx->mutex);
+
+ SlotSyncCtx->stopSignaled = true;

This flag is never reset back. I think we should reset this once the
promotion is complete. Though offhand, I don't see any problem with
this but it doesn't look clean and can be a source of bugs in the
future.

2.
+char *
+CheckDbnameInConninfo(void)
 {
  char    *dbname;

Let's name this function as CheckAndGetDbnameFromConninfo().

Apart from the above, I have made cosmetic changes in the attached.

[1] - https://www.postgresql.org/message-id/CAJpy0uBWomyAjP0zyFdzhGxn%2BXsAb2OdJA%2BKfNyZRv2nV6PD9g%40mail.gmail.com

-- 
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).