Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-04-01T00:56:29Z
Lists: pgsql-hackers

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On Friday, March 29, 2024 2:50 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Fri, Mar 29, 2024 at 6:36 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> wrote:
> >
> >
> > Attach a new version patch which fixed an un-initialized variable
> > issue and added some comments.
> >
> 
> The other approach to fix this issue could be that the slotsync worker get the
> serialized snapshot using pg_read_binary_file() corresponding to restart_lsn
> and writes those at standby. But there are cases when we won't have such a file
> like (a) when we initially create the slot and reach the consistent_point, or (b)
> also by the time the slotsync worker starts to read the remote snapshot file, the
> snapshot file could have been removed by the checkpointer on the primary (if
> the restart_lsn of the remote has been advanced in this window). So, in such
> cases, we anyway need to advance the slot. I think these could be optimizations
> that we could do in the future.
> 
> Few comments:

Thanks for the comments.

> =============
> 1.
> - if (slot->data.database != MyDatabaseId)
> + if (slot->data.database != MyDatabaseId && !fast_forward)
>   ereport(ERROR,
>   (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
>   errmsg("replication slot \"%s\" was not created in this database", @@ -526,7
> +527,7 @@ CreateDecodingContext(XLogRecPtr start_lsn,
>   * Do not allow consumption of a "synchronized" slot until the standby
>   * gets promoted.
>   */
> - if (RecoveryInProgress() && slot->data.synced)
> + if (RecoveryInProgress() && slot->data.synced &&
> + !IsSyncingReplicationSlots())
> 
> 
> Add comments at both of the above places.

Added.

> 
> 
> 2.
> +extern XLogRecPtr pg_logical_replication_slot_advance(XLogRecPtr moveto,
> +   bool *found_consistent_point);
> +
> 
> This API looks a bit awkward as the functionality doesn't match the name. How
> about having a function with name
> LogicalSlotAdvanceAndCheckReadynessForDecoding(moveto,
> ready_for_decoding) with the same functionality as your patch has for
> pg_logical_replication_slot_advance() and then invoke it both from
> pg_logical_replication_slot_advance and slotsync.c. The function name is too
> big, we can think of a shorter name. Any ideas?

How about LogicalSlotAdvanceAndCheckDecodingState() Or just
LogicalSlotAdvanceAndCheckDecoding()? (I used the suggested
LogicalSlotAdvanceAndCheckReadynessForDecoding in this version, It can be renamed in
next version if we agree).

Attach the V3 patch which addressed above comments and Kuroda-san's
comments[1]. I also adjusted the tap-test to only check the confirmed_flush_lsn
after syncing, as the restart_lsn could be different from the remote one due to
the new slot_advance() call. I am also testing some optimization idea locally
and will share if ready.

[1] https://www.postgresql.org/message-id/TYCPR01MB1207757BB2A32B6815CE1CCE7F53A2%40TYCPR01MB12077.jpnprd01.prod.outlook.com

Best Regards,
Hou zj

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).