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: Peter Smith <smithpb2250@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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>
Date: 2024-02-09T05:30:30Z
Lists: pgsql-hackers 
On Fri, Feb 9, 2024 at 9:57 AM Peter Smith <smithpb2250@gmail.com> wrote:
>
> Here are some review comments for patch v81-0001.
>
> ======
>
> 1. GENERAL - ReplicationSlotInvalidationCause enum.
>
> I was thinking that the ReplicationSlotInvalidationCause should
> explicitly set RS_INVAL_NONE = 0 (it's zero anyway, but making it
> explicit with a comment /* Must be zero. */. will stop it from being
> changed in the future).
>
> ------
> /*
>  * Slots can be invalidated, e.g. due to max_slot_wal_keep_size. If so, the
>  * 'invalidated' field is set to a value other than _NONE.
>  */
> typedef enum ReplicationSlotInvalidationCause
> {
>   RS_INVAL_NONE = 0, /* Must be zero. */
>   ...
> } ReplicationSlotInvalidationCause;
> ------
>
> The reason to do this is because many places in the patch check for
> RS_INVAL_NONE, but if RS_INVAL_NONE == 0 is assured, all those code
> fragments can be simplified and IMO also become more readable.
>
> e.g. update_local_synced_slot()
>
> BEFORE
> Assert(slot->data.invalidated == RS_INVAL_NONE);
>
> AFTER
> Assert(!slot->data.invalidated);
>

I find the current code style more intuitive.

>
> 5. synchronize_slots
>
> + /*
> + * The primary_slot_name is not set yet or WALs not received yet.
> + * Synchronization is not possible if the walreceiver is not started.
> + */
> + latestWalEnd = GetWalRcvLatestWalEnd();
> + SpinLockAcquire(&WalRcv->mutex);
> + if ((WalRcv->slotname[0] == '\0') ||
> + XLogRecPtrIsInvalid(latestWalEnd))
> + {
> + SpinLockRelease(&WalRcv->mutex);
> + return;
> + }
> + SpinLockRelease(&WalRcv->mutex);
>
> The comment talks about the GUC "primary_slot_name", but the code is
> checking the WalRcv's slotname. It may be the same, but the difference
> is confusing.
>

Yeah, in this case, it would be the same because we don't allow slot
sync worker unless primary_slot_name is configured in which case
WalRcv->slotname refers to primary_slot_name. However, I think it is
better to explain here why slot synchronization is not possible or
doesn't make sense till walreceiver starts streaming and in which
case, won't it be sufficient to just check latestWalEnd?

-- 
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).