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Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Amit Kapila <amit.kapila16@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>, shveta malik <shveta.malik@gmail.com>
Date: 2023-10-27T10:34:16Z
Lists: pgsql-hackers

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On Fri, Oct 27, 2023 at 3:26 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Wed, Oct 25, 2023 at 3:15 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > Hi,
> >
> > On 10/25/23 5:00 AM, shveta malik wrote:
> > > On Tue, Oct 24, 2023 at 11:54 AM Drouvot, Bertrand
> > > <bertranddrouvot.pg@gmail.com> wrote:
> > >>
> > >> Hi,
> > >>
> > >> On 10/23/23 2:56 PM, shveta malik wrote:
> > >>> On Mon, Oct 23, 2023 at 5:52 PM Drouvot, Bertrand
> > >>> <bertranddrouvot.pg@gmail.com> wrote:
> > >>
> > >>>> We are waiting for DEFAULT_NAPTIME_PER_CYCLE (3 minutes) before checking if there
> > >>>> is new synced slot(s) to be created on the standby. Do we want to keep this behavior
> > >>>> for V1?
> > >>>>
> > >>>
> > >>> I think for the slotsync workers case, we should reduce the naptime in
> > >>> the launcher to say 30sec and retain the default one of 3mins for
> > >>> subscription apply workers. Thoughts?
> > >>>
> > >>
> > >> Another option could be to keep DEFAULT_NAPTIME_PER_CYCLE and create a new
> > >> API on the standby that would refresh the list of sync slot at wish, thoughts?
> > >>
> > >
> > > Do you mean API to refresh list of DBIDs rather than sync-slots?
> > > As per current design, launcher gets DBID lists for all the failover
> > > slots from the primary at intervals of DEFAULT_NAPTIME_PER_CYCLE.
> >
> > I mean an API to get a newly created slot on the primary being created/synced on
> > the standby at wish.
> >
> > Also let's imagine this scenario:
> >
> > - create logical_slot1 on the primary (and don't start using it)
> >
> > Then on the standby we'll get things like:
> >
> > 2023-10-25 08:33:36.897 UTC [740298] LOG:  waiting for remote slot "logical_slot1" LSN (0/C00316A0) and catalog xmin (752) to pass local slot LSN (0/C0049530) and and catalog xmin (754)
> >
> > That's expected and due to the fact that ReplicationSlotReserveWal() does set the slot
> > restart_lsn to a value < at the corresponding restart_lsn slot on the primary.
> >
> > - create logical_slot2 on the primary (and start using it)
> >
> > Then logical_slot2 won't be created/synced on the standby until there is activity on logical_slot1 on the primary
> > that would produce things like:
> > 2023-10-25 08:41:35.508 UTC [740298] LOG:  wait over for remote slot "logical_slot1" as its LSN (0/C005FFD8) and catalog xmin (756) has now passed local slot LSN (0/C0049530) and catalog xmin (754)
>
>
> Slight correction to above. As soon as we start activity on
> logical_slot2, it will impact all the slots on primary, as the WALs
> are consumed by all the slots. So even if there is activity on
> logical_slot2, logical_slot1 creation on standby will be unblocked and
> it will then move to logical_slot2 creation. eg:
>
> --on standby:
> 2023-10-27 15:15:46.069 IST [696884] LOG:  waiting for remote slot
> "mysubnew1_1" LSN (0/3C97970) and catalog xmin (756) to pass local
> slot LSN (0/3C979A8) and and catalog xmin (756)
>
> on primary:
> newdb1=# select now();
>                now
> ----------------------------------
>  2023-10-27 15:15:51.504835+05:30
> (1 row)
>
> --activity on mysubnew1_3
> newdb1=# insert into tab1_3 values(1);
> INSERT 0 1
> newdb1=# select now();
>                now
> ----------------------------------
>  2023-10-27 15:15:54.651406+05:30
>
>
> --on standby, mysubnew1_1 is unblocked.
> 2023-10-27 15:15:56.223 IST [696884] LOG:  wait over for remote slot
> "mysubnew1_1" as its LSN (0/3C97A18) and catalog xmin (757) has now
> passed local slot LSN (0/3C979A8) and catalog xmin (756)
>
> My Setup:
> mysubnew1_1 -->mypubnew1_1 -->tab1_1
> mysubnew1_3 -->mypubnew1_3-->tab1_3
>
> thanks
> Shveta

PFA v26 patches. The changes are:

1) 'Failover' in the main slot is now set when the table
synchronization phase is finished. So even when failover is enabled
for a subscription, the internal failover state remains temporarily
“pending” until the initialization phase completes.

2) If the standby is down, but standby_slot_names has that slot name,
we emit a warning now while waiting for that standby.

3) Fixed bug where pg_logical_slot_get_changes was resetting failover
property of slot. Thanks Ajin for providing the fix.

4) Fixed bug where standby_slot_names_list was not initialized for
non-walsender cases making pg_logical_slot_get_changes() to proceed
w/o waiting for standbys.

5) Fixed a bug where standby_slot_names_list was freed (due to free of
per_query context in non-walsender cases) but was not nullified and
thus next call was using this freed pointer and was crashing.

6) Improved wait_for_primary_slot_catchup(), we now fetch
remote-conflicting(invalidation) too and abort the wait and slot
creation if the slot on primary is invalidated.

7) Slot-sync workers now wait for cascading standby's confirmation
before updating logical synced slots on first standby.

First 5 changes are in patch001, 6th one is in patch002. For 7th, I
have created a new patch (003) to separate out the additional changes
needed for cascading standbys.

==========

Open questions regarding change for pt 1 above:
a) I think we should restrict the 'alter-sub set failover' when
failover-state is currently in 'p' (pending) state i.e. table-sync is
going over. Once table-sync is over, then toggle of 'failover' should
be allowed using alter-subscription.

b) Currently I have restricted  'alter subscription.. refresh
publication with copy=true' when failover=true (on a similar line of
two-phase). The reason being, refresh with copy=true will go for
table-sync again and since failover was set in main-slot after
table-sync was done, it will need going through the same transition of
'p' to 'e' for main slot making it unsyncable for that time. Should it
be allowed?
Currently:
newdb1=# ALTER SUBSCRIPTION mysubnew1_1 REFRESH PUBLICATION WITH
(copy_data=true);
ERROR:  ALTER SUBSCRIPTION ... REFRESH with copy_data is not allowed
when failover is enabled
HINT:  Use ALTER SUBSCRIPTION ... REFRESH with copy_data = false, or
use DROP/CREATE SUBSCRIPTION.

Thoughts on above queries?

thanks
Shveta

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).