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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: Nisha Moond <nisha.moond412@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-12-04T05:15:18Z
Lists: pgsql-hackers 
On Mon, Dec 4, 2023 at 10:40 AM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Fri, Dec 1, 2023 at 5:40 PM Nisha Moond <nisha.moond412@gmail.com> wrote:
> >
> > Review for v41 patch.
>
> Thanks for the feedback.
>
> >
> > 1.
> > ======
> > src/backend/utils/misc/postgresql.conf.sample
> >
> > +#enable_syncslot = on # enables slot synchronization on the physical
> > standby from the primary
> >
> > enable_syncslot is disabled by default, so, it should be 'off' here.
> >
>
> Sure, I will change it.
>
> > ~~~
> > 2.
> > IIUC, the slotsyncworker's connection to the primary is to execute a
> > query. Its aim is not walsender type connection, but at primary when
> > queried, the 'backend_type' is set to 'walsender'.
> > Snippet from primary db-
> >
> > datname  |   usename   | application_name | wait_event_type | backend_type
> > ---------+-------------+------------------+-----------------+--------------
> > postgres | replication | slotsyncworker   | Client          | walsender
> >
> > Is it okay?
> >
>
> Slot sync worker uses 'libpqrcv_connect' for connection which sends
> 'replication'-'database' key-value pair as one of the connection
> options. And on the primary side, 'ProcessStartupPacket' on the basis
> of this key-value pair sets the process as walsender one (am_walsender
> = true).
> And thus this reflects as backend_type='walsender' in
> pg_stat_activity. I do not see any harm in this backend_type for
> slot-sync worker currently. This is on a similar line of connections
> used for logical-replications. And since a slot-sync worker also deals
> with wals-positions (lsns), it is okay to maintain backend_type as
> walsender unless you (or others) see any potential issue in doing
> that. So let me know.
>
> > ~~~
> > 3.
> > As per current logic, If there are slots on primary with disabled
> > subscriptions, then, when standby is created it replicates these slots
> > but can't make them sync-ready until any activity happens on the
> > slots.
> > So, such slots stay in 'i' sync-state and get dropped when failover
> > happens. Now, if the subscriber tries to enable their existing
> > subscription after failover, it gives an error that the slot does not
> > exist.
> >
>
> yes, this is expected as Amit explained in [1]. But let me review if
> we need to document this case for disabled subscriptions. i.e.
> disabled subscription if enabled after promotion might not work.

Sorry, missed to mention the link earlier:
[1]: https://www.postgresql.org/message-id/CAA4eK1J5Hxp%2BzhvptyyjqQ4JSQzwnkFRXtQn8v9opxtZmmY_Ug%40mail.gmail.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).