Horton

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: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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>, shveta malik <shveta.malik@gmail.com>
Date: 2024-02-13T09:59:45Z
Lists: pgsql-hackers 
On Fri, Feb 9, 2024 at 10:04 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> +reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
> {
> ...
> + /*
> + * Find the oldest existing WAL segment file.
> + *
> + * Normally, we can determine it by using the last removed segment
> + * number. However, if no WAL segment files have been removed by a
> + * checkpoint since startup, we need to search for the oldest segment
> + * file currently existing in XLOGDIR.
> + */
> + oldest_segno = XLogGetLastRemovedSegno() + 1;
> +
> + if (oldest_segno == 1)
> + {
> + TimeLineID cur_timeline;
> +
> + GetWalRcvFlushRecPtr(NULL, &cur_timeline);
> + oldest_segno = XLogGetOldestSegno(cur_timeline);
> ...
> ...
>
> This means that if the restart_lsn of the slot is from the prior
> timeline then the standby needs to wait for longer times to sync the
> slot. Ideally, it should be okay because I don't think even if
> restart_lsn of the slot may be from some prior timeline than the
> current flush timeline on standby, how often that case can happen?

I tested this behaviour on v85 patch, it is working as expected i.e.
if remot_slot's lsn belongs to a prior timeline then on executing
pg_sync_replication_slots() function, it creates a temporary slot and
waits for primary to catch up. And once primary catches up, the next
execution of SQL function persistes the slot and syncs it.

Setup: primary-->standby1-->standby2

Steps:
1) Insert data on primary. It gets replicated to both standbys.
2) Create logical slot on primary and execute
pg_sync_replication_slots() on standby1. The slot gets synced and
persisted on standby1.
3) Shutdown standby2.
4) Insert data on primary. It gets replicated to standby1.
5) Shutdown primary and promote standby1.
6) Insert some data on standby1/new primary directly.
7) Start standby2: It now needs to catch up old data of timeline1
(from step 4) + new data of timeline2 (from step 6) . It does that. On
reaching the end of the old timeline, walreceiver gets restarted and
starts streaming using the new timeline.
8) Execute pg_sync_replication_slots() on standby2 to sync the slot.
Now remote_slot's lsn belongs to a prior timeline on standby2. In my
test-run, remote_slot's lsn belonged to segno=4 on standby2, while the
oldest segno of current_timline(2) was 6. Thus it created the slot
locally with lsn belonging to the oldest segno 6 of cur_timeline(2)
but did not persist it as remote_slot's lsn was behind.
9) Now on standby1/new-primary, advance the logical slot by calling
pg_replication_slot_advance().
10) Execute pg_sync_replication_slots() again on standby2, now the
local temporary slot gets persisted as the restart_lsn of primary has
caught up.

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).