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: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>
Date: 2023-07-01T01:42:48Z
Lists: pgsql-hackers 
On Thu, Jun 29, 2023 at 3:52 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> 2. I want to confirm the reason why new replication command is added.
>

Are you referring LIST_SLOTS command? If so, I don't think that is
required and instead, we can use a query to fetch the required
information.

> IIUC the
>    launcher connects to primary by using primary_conninfo connection string, but
>    it establishes the physical replication connection so that any SQL cannot be executed.
>    Is it right? Another approach not to use is to specify the target database via
>    GUC, whereas not smart. How do you think?
> 3. You chose the per-db worker approach, however, it is difficult to extend the
>    feature to support physical slots. This may be problematic. Was there any
>    reasons for that? I doubted ReplicationSlotCreate() or advance functions might
>    not be used from other databases and these may be reasons, but not sure.
>    If these operations can do without connecting to specific database, I think
>    the architecture can be changed.
>

I think this point needs some investigation but do we want just one
worker that syncs all the slots? That may lead to lag in keeping the
slots up-to-date. We probably need some tests.

> 4. Currently the launcher establishes the connection every time. Isn't it better
>    to reuse the same one instead?
>

I feel it is not the launcher but a separate sync slot worker that
establishes the connection. It is not clear to me what exactly you
have in mind. Can you please explain a bit more?

> Following comments are assumed the configuration, maybe the straightfoward:
>
> primary->standby
>    |->subscriber
>
> 5. After constructing the system, I dropped the subscription on the subscriber.
>    In this case the logical slot on primary was removed, but that was not replicated
>    to standby server. Did you support the workload or not?
>

This should work.

>
> 6. Current approach may delay the startpoint of sync.
>
> Assuming that physical replication system is created first, and then the
> subscriber connects to the publisher node. In this case the launcher connects to
> primary earlier than the apply worker, and reads the slot. At that time there are
> no slots on primary, so launcher disconnects from primary and waits a time period (up to 3min).
> Even if the apply worker creates the slot on publisher, but the launcher on standby
> cannot notice that. The synchronization may start 3 min later.
>

I feel this should be based on some GUC like
'wal_retrieve_retry_interval' which we are already using in the
launcher or probably a new one if that doesn't seem to match.

-- 
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).