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: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@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: 2023-09-25T08:44:04Z
Lists: pgsql-hackers 
Hi,

On 9/23/23 3:38 AM, Amit Kapila wrote:
> On Fri, Sep 22, 2023 at 6:01 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> Thanks for all the work that has been done on this feature, and sorry
>> to have been quiet on it for so long.
>>
>> On 9/18/23 12:22 PM, shveta malik wrote:
>>> On Wed, Sep 13, 2023 at 4:48 PM Hayato Kuroda (Fujitsu)
>>> <kuroda.hayato@fujitsu.com> wrote:
>>>> Right, but I wanted to know why it is needed. One motivation seemed to know the
>>>> WAL location of physical standby, but I thought that struct WalSnd.apply could
>>>> be also used. Is it bad to assume that the physical walsender always exists?
>>>>
>>>
>>> We do not plan to target this case where physical slot is not created
>>> between primary and physical-standby in the first draft.  In such a
>>> case, slot-synchronization will be skipped for the time being. We can
>>> extend this functionality (if needed) later.
>>>
>>
>> I do think it's needed to extend this functionality. Having physical slot
>> created sounds like a (too?) strong requirement as:
>>
>> - It has not been a requirement for Logical decoding on standby so that could sounds weird
>> to require it for sync slot (while it's not allowed to logical decode from sync slots)
>>
> 
> There is a difference here that we also need to prevent removal of
> rows required by sync_slots. That could be achieved by physical slot
> (and hot_standby_feedback). So, having a requirement to have physical
> slot doesn't sound too unreasonable to me. Otherwise, we need to
> invent some new mechanism of having some sort of placeholder slot to
> avoid removal of required rows. 

Thinking about it, I wonder if removal of required rows is even possible
given that:

- we don't allow to logical decode from a sync slot
- sync slot catalog_xmin <= its primary counter part catalog_xmin
- its primary counter part prevents rows removal thanks to its own catalog_xmin
- a sync slot is removed as soon as its primary counter part is removed

In that case I'm not sure how rows removal on the primary could lead to remove rows
required by a sync slot. Am I missing something? Do you have a scenario in mind?

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).