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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, 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: 2023-11-21T05:16:06Z
Lists: pgsql-hackers 
On Mon, Nov 20, 2023 at 6:51 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 11/20/23 11:59 AM, Amit Kapila wrote:
> > On Mon, Nov 20, 2023 at 3:17 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> On 11/18/23 11:45 AM, Amit Kapila wrote:
> >>> On Fri, Nov 17, 2023 at 5:18 PM Drouvot, Bertrand
> >>> <bertranddrouvot.pg@gmail.com> wrote:
> >>>>
> >>>> 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:
> >>>>>
> >>>>> 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.
> >>>>>
> >>>
> >>> I was thinking what if we just ignore creating such slots (which
> >>> require init state) in the first place? I think that can be
> >>> time-consuming in some cases but it will reduce the complexity and we
> >>> can always improve such cases later if we really encounter them in the
> >>> real world. I am not very sure that added complexity is worth
> >>> addressing this particular case, so I would like to know your and
> >>> others' opinions.
> >>>
> >>
> >> I'm not sure I understand your point. Are you saying that we should not create
> >> slots on the standby that are "currently" reported in a 'i' state? (so just keep
> >> the 'r' and 'n' states?)
> >>
> >
> > Yes.
> >
>
> As far the 'i' state here, from what I see, it is currently useful for:
>
> 1. Cascading standby to not sync slots with state = 'i' from
> the first standby.
> 2. Easily report Slots that did not catch up on the primary yet.
> 3. Avoid inactive slots to block "active" ones creation.
>
> So not creating those slots should not be an issue for 1. (sync are
> not needed on cascading standby as not created on the first standby yet)
> but is an issue for 2. (unless we provide another way to keep track and report
> such slots) and 3. (as I think we should still need to reserve WAL).
>
> I've a question: we'd still need to reserve WAL for those slots, no?
>
> If that's the case and if we don't call ReplicationSlotCreate() then ReplicationSlotReserveWal()
> would not work as  MyReplicationSlot would be NULL.
>

Yes, we need to reserve WAL to see if we can sync the slot. We are
currently creating an RS_EPHEMERAL slot and if we don't explicitly
persist it when we can't sync, then it will be dropped when we do
ReplicationSlotRelease() at the end of synchronize_one_slot(). So, the
loss is probably, the next time we again try to sync the slot, we need
to again create it and may need to wait for newer restart_lsn on
standby which could be avoided if we have the slot in 'i' state from
the previous run. I don't deny the importance of having 'i'
(initialized) state but was just trying to say that it has additional
code complexity. OTOH, having it may give better visibility to even
users about slots that are not active (say manually created slots on
the primary).

-- 
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).