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: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-12-06T05:26:47Z
Lists: pgsql-hackers 
On Tue, Dec 5, 2023 at 7:38 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 12/5/23 12:32 PM, Amit Kapila wrote:
> > On Tue, Dec 5, 2023 at 10:38 AM shveta malik <shveta.malik@gmail.com> wrote:
> >>
> >> On Mon, Dec 4, 2023 at 10:07 PM Drouvot, Bertrand
> >> <bertranddrouvot.pg@gmail.com> wrote:
> >>>>
> >>>
> >>> Maybe another option could be to have the walreceiver a way to let the slot sync
> >>> worker knows that it (the walreceiver) was not able to start due to non existing
> >>> replication slot on the primary? (that way we'd avoid the slot sync worker having
> >>> to talk to the primary).
> >>
> >> Few points:
> >> 1) I think if we do it, we should do it in generic way i.e. slotsync
> >> worker should go to no-op if walreceiver is not able to start due to
> >> any reason and not only due to invalid primary_slot_name.
> >> 2) Secondly, slotsync worker needs to make sure it has synced the
> >> slots so far i.e. worker should not go to no-op immediately on seeing
> >> missing WalRcv process if there are pending slots to be synced.
> >>
> >
> > Won't it be better to just ping and check the validity of
> > 'primary_slot_name' at the start of slot-sync and if it is changed
> > anytime? I think it would be better to avoid adding dependency on
> > walreciever state as that sounds like needless complexity.
>
> I think the overall extra complexity is linked to the fact that we first
> want to ensure that the slots are in sync before shutting down the
> sync slot worker.
>
> I think than talking to the primary or relying on the walreceiver state
> is "just" what would trigger the decision to shutdown the sync slot worker.
>
> Relying on the walreceiver state looks better to me (as it avoids possibly
> useless round trips with the primary).
>

But the round trip will only be once in the beginning and if the user
changes the GUC primary-slot_name which shouldn't be that often.

> Also the walreceiver could be down for multiple reasons, and I think there
> is no point of having a sync slot worker running if the slots are in sync and
> there is no walreceiver running (even if primary_slot_name is a valid one).
>

I feel that is indirectly relying on the fact that the primary won't
advance logical slots unless physical standby has consumed data. Now,
it is possible that slot-sync worker lags behind and still needs to
sync more data for slots in which it makes sense for slot-sync worker
to be alive. I think we can try to avoid checking walreceiver status
till we can get more data to avoid the problem I mentioned but it
doesn't sound like a clean way to achieve our purpose.

-- 
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).