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>, 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: 2023-11-07T10:55:11Z
Lists: pgsql-hackers 
On Tue, Nov 7, 2023 at 3:51 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 10/31/23 10:37 AM, shveta malik wrote:
> > On Fri, Oct 27, 2023 at 8:43 PM Drouvot, Bertrand
> >>
> >> Agree with your test case, but in my case I was not using pub/sub.
> >>
> >> I was not clear, so when I said:
> >>
> >>>> - create logical_slot1 on the primary (and don't start using it)
> >>
> >> I meant don't start decoding from it (like using pg_recvlogical() or
> >> pg_logical_slot_get_changes()).
> >>
> >> By using pub/sub the "don't start using it" is not satisfied.
> >>
> >> My test case is:
> >>
> >> "
> >> SELECT * FROM pg_create_logical_replication_slot('logical_slot1', 'test_decoding', false, true, true);
> >> SELECT * FROM pg_create_logical_replication_slot('logical_slot2', 'test_decoding', false, true, true);
> >> pg_recvlogical -d postgres -S logical_slot2 --no-loop --start -f -
> >> "
> >>
> >
> > Okay, I am able to reproduce it now. Thanks for clarification. I have
> > tried to change the algorithm as per suggestion by Amit in [1]
> >
> > [1]: https://www.postgresql.org/message-id/CAA4eK1KBL0110gamQfc62X%3D5JV8-Qjd0dw0Mq0o07cq6kE%2Bq%3Dg%40mail.gmail.com
>
> Thanks!
>
> >
> > This is not full proof solution but optimization over first one. Now
> > in any sync-cycle, we take 2 attempts for slots-creation (if any slots
> > are available to be created). In first attempt, we do not wait
> > indefinitely on inactive slots, we wait only for a fixed amount of
> > time and if remote-slot is still behind, then we add that to the
> > pending list and move to the next slot. Once we are done with first
> > attempt, in second attempt, we go for the pending ones and now we wait
> > on each of them until the primary catches up.
>
> Aren't we "just" postponing the "issue"? I mean if there is really no activity
> on, say, the first created slot, then once we move to the second attempt then any newly
> created slot from that time would wait to be synced forever, no?
>

We have to wait at some point in time for such inactive slots and the
same is true even for manually created slots on standby. Do you have
any better ideas to deal with it?

> Looking at V30:
>
> +       /* Update lsns of slot to remote slot's current position */
> +       local_slot_update(remote_slot);
> +       ReplicationSlotPersist();
> +
> +       ereport(LOG, errmsg("created slot \"%s\" locally", remote_slot->name));
>
> I think this message is confusing as the slot has been created before it, here:
>
> +   else
> +   {
> +       TransactionId xmin_horizon = InvalidTransactionId;
> +       ReplicationSlot *slot;
> +
> +       ReplicationSlotCreate(remote_slot->name, true, RS_EPHEMERAL,
> +                             remote_slot->two_phase, false);
>
> So that it shows up in pg_replication_slots before this message is emitted (and that
> specially true/worst for non active slots).
>
> Maybe something like "newly locally created slot XXX has been synced..."?
>
> While at it, would that make sense to move
>
> +       slot->data.failover = true;
>
> once we stop waiting for this slot? I think that would avoid confusion if one
> query pg_replication_slots while we are still waiting for this slot to be synced,
> thoughts? (currently we can see pg_replication_slots.synced_slot set to true
> while we are still waiting).
>

The failover property of the slot is different from whether the slot
has been synced yet, so we can't change the location of marking it but
we can try to improve when to show that slot has been synced.

-- 
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).