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: shveta malik <shveta.malik@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@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: 2023-10-31T10:45:13Z
Lists: pgsql-hackers 
On Fri, Oct 27, 2023 at 4:04 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Fri, Oct 27, 2023 at 3:26 PM shveta malik <shveta.malik@gmail.com> wrote:
> ==========
>
> 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.
>

Agreed.

> 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?
>

Yeah, I also think we can't allow refresh with copy=true when
'failover' is enabled.

I think the current implementation of this flag seems a bit clumsy
because 'failover' is a slot property and we are trying to map it to
plugin_options. It has to be considered similar to the opt_temporary
option while creating the slot.

We have create_replication_slot and drop_replication_slot in
repl_gram.y. How about if introduce alter_replication_slot and handle
the 'failover' flag with that? The idea is we will either enable
'failover' at the time create_replication_slot by providing an
optional failover option or execute a separate command
alter_replication_slot. I think we probably need to perform this
command before the start of streaming.

I think we will have the following options to allow alter of the
'failover' property: (a) we can allow altering 'failover' only for the
'disabled' subscription; to achieve that, we need to open a connection
during alter subscription and change this property of slot; (b) apply
worker detects the change in 'failover' option; run the
alter_replication_slot command; this needs more analysis as
apply_worker is already doing streaming and changing slot property in
between could be tricky.

-- 
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).