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>, 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: 2023-11-07T14:28:11Z
Lists: pgsql-hackers 
Hi,

On 11/7/23 11:55 AM, Amit Kapila wrote:
> 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?
> 

What about:

- get rid of the second attempt and the pending_slot_list
- keep the wait_count and PrimaryCatchupWaitAttempt logic

so basically, get rid of:

    /*
     * Now sync the pending slots which were failed to be created in first
     * attempt.
     */
    foreach(cell, pending_slot_list)
    {
        RemoteSlot *remote_slot = (RemoteSlot *) lfirst(cell);

        /* Wait until the primary server catches up */
        PrimaryCatchupWaitAttempt = 0;

        synchronize_one_slot(wrconn, remote_slot, NULL);
    }

and the pending_slot_list list.

That way, for each slot that have not been created and synced yet:

- it will be created on the standby
- we will wait up to PrimaryCatchupWaitAttempt attempts
- the slot will be synced or removed on/from the standby

That way an inactive slot on the primary would not "block"
any other slots on the standby.

By "created" here I mean calling ReplicationSlotCreate() (not to be confused
with emitting "ereport(LOG, errmsg("created slot \"%s\" locally", remote_slot->name)); "
which is confusing as mentioned up-thread).

The problem I can see with this proposal is that the "sync" window waiting
for slot activity on the primary is "only" during the PrimaryCatchupWaitAttempt
attempts (as the slot will be dropped/recreated).

If we think this window is too short we could:

- increase it
or
- don't drop the slot once created (even if there is no activity
on the primary during PrimaryCatchupWaitAttempt attempts) so that
the next loop of attempts will compare with "older" LSN/xmin (as compare to
dropping and re-creating the slot). That way the window would be since the
initial slot creation.

Thoughts?

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).