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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: shveta malik <shveta.malik@gmail.com>, "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: 2023-11-06T08:27:42Z
Lists: pgsql-hackers 
On Mon, Nov 6, 2023 at 7:01 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Friday, November 3, 2023 7:32 PM Amit Kapila <amit.kapila16@gmail.com>
> >
> > 5.
> > @@ -228,6 +230,28 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo
> > fcinfo, bool confirm, bool bin
> >   NameStr(MyReplicationSlot->data.plugin),
> >   format_procedure(fcinfo->flinfo->fn_oid))));
> > ..
> > + if (XLogRecPtrIsInvalid(upto_lsn))
> > + wal_to_wait = end_of_wal;
> > + else
> > + wal_to_wait = Min(upto_lsn, end_of_wal);
> > +
> > + /* Initialize standby_slot_names_list */ SlotSyncInitConfig();
> > +
> > + /*
> > + * Wait for specified streaming replication standby servers (if any)
> > + * to confirm receipt of WAL upto wal_to_wait.
> > + */
> > + WalSndWaitForStandbyConfirmation(wal_to_wait);
> > +
> > + /*
> > + * The memory context used to allocate standby_slot_names_list will be
> > + * freed at the end of this call. So free and nullify the list in
> > + * order to avoid usage of freed list in the next call to this
> > + * function.
> > + */
> > + SlotSyncFreeConfig();
> >
> > What if there is an error in WalSndWaitForStandbyConfirmation() before calling
> > SlotSyncFreeConfig()? I think the problem you are trying to avoid by freeing it
> > here can occur. I think it is better to do this in a logical decoding context and
> > free the list along with it as we are doing in commit c7256e6564(see PG15).
>
> I will analyze more about this case and update in next version.
>

Okay, thanks for considering it.

> > Also,
> > it is better to allocate this list somewhere in
> > WalSndWaitForStandbyConfirmation(), probably in WalSndGetStandbySlots,
> > that will make the code look neat and also avoid allocating this list when
> > failover is not enabled for the slot.
>
> Changed as suggested.
>

After doing this, do we need to call SlotSyncInitConfig() from other
places as below?

+ SlotSyncInitConfig();
+ WalSndGetStandbySlots(&standby_slot_cpy, false);

Can we entirely get rid of calling SlotSyncInitConfig() from all
places except WalSndGetStandbySlots()? Also, after that or otherwise,
the comments atop also need modification.

-- 
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).