Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-11-17T11:48:49Z
Lists: pgsql-hackers 
Hi,

On 11/17/23 2:46 AM, Zhijie Hou (Fujitsu) wrote:
> On Tuesday, November 14, 2023 10:27 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:
>> On 11/13/23 2:57 PM, Zhijie Hou (Fujitsu) wrote:
>>> On Friday, November 10, 2023 4:16 PM Drouvot, Bertrand
>> <bertranddrouvot.pg@gmail.com> wrote:
>>>> Yeah good point, agree to just error out in all the case then (if we
>>>> discard the sync_ reserved wording proposal, which seems to be the
>>>> case as probably not worth the extra work).
>>>
>>> Thanks for the discussion!
>>>
>>> Here is the V33 patch set which includes the following changes:
>>
>> Thanks for working on it!
>>
>>>
>>> 1) Drop slots with state 'i' in promotion flow after we shut down WalReceiver.
>>
>> @@ -3557,10 +3558,15 @@ WaitForWALToBecomeAvailable(XLogRecPtr
>> RecPtr, bool randAccess,
>>                        * this only after failure, so when you promote, we still
>>                        * finish replaying as much as we can from archive and
>>                        * pg_wal before failover.
>> +                    *
>> +                    * Drop the slots for which sync is initiated but not yet
>> +                    * completed i.e. they are still waiting for the primary
>> +                    * server to catch up.
>>                        */
>>                       if (StandbyMode && CheckForStandbyTrigger())
>>                       {
>>                           XLogShutdownWalRcv();
>> +                       slotsync_drop_initiated_slots();
>>                           return XLREAD_FAIL;
>>                       }
>>
>> I had a closer look and it seems this is not located at the right place.
>>
>> Indeed, it's added here:
>>
>> switch (currentSource)
>> {
>> 	case XLOG_FROM_ARCHIVE:
>> 	case XLOG_FROM_PG_WAL:
>>
>> While in our case we are in
>>
>> 	case XLOG_FROM_STREAM:
>>
>> So I think we should move slotsync_drop_initiated_slots() in the
>> XLOG_FROM_STREAM case. Maybe before shutting down the sync slot worker?
>> (the TODO item number 2 you mentioned up-thread)
> 
> Thanks for the comment.
> 
> I feel the WaitForWALToBecomeAvailable may not be the best place to shutdown
> slotsync worker and drop slots. There could be other reasons(other than
> promotion) as mentioned in comments in case XLOG_FROM_STREAM to reach the code
> there. I thought if the intention is to stop slotsync workers on promotion,
> maybe FinishWalRecovery() is a better place to do it as it's indicating the end
> of recovery and XLogShutdownWalRcv is also called in it.

I can see that slotsync_drop_initiated_slots() has been moved in FinishWalRecovery()
in v35. That looks ok.
> 
> And I feel we'd better drop the slots after shutting down the slotsync workers,
> because otherwise the slotsync workers could create the dropped slot again in
> rare cases.

Yeah, agree and I can see that it's done that way in v35.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).