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: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Cc: Ajin Cherian <itsajin@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Peter Smith <smithpb2250@gmail.com>, 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: 2023-09-27T12:03:19Z
Lists: pgsql-hackers

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On Mon, Sep 25, 2023 at 7:46 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> Dear Ajin, Shveta,
>
> Thank you for rebasing the patch set! Here are new comments for v19_2-0001.
>

Thank You Kuroda-san for the feedback. Most of these are addressed in
v20. Please find my response inline.

> 01. WalSndWaitForStandbyNeeded()
>
> ```
>         if (SlotIsPhysical(MyReplicationSlot))
>                 return false;
> ```
>
> Is there a possibility that physical walsenders call this function?
> IIUC following is a stacktrace for the function, so the only logical walsenders use it.
> If so, it should be Assert() instead of an if statement.
>
> logical_read_xlog_page()
> WalSndWaitForWal()
> WalSndWaitForStandbyNeeded()

It will only be called from logical-walsenders. Modified as you suggested.

>
> 02. WalSndWaitForStandbyNeeded()
>
> Can we set shouldwait in SlotSyncInitConfig()? synchronize_slot_names_list is
> searched whenever the function is called, but it is not changed automatically.
> If the slotname is compared with the list in the SlotSyncInitConfig(), the
> liner search can be reduced.

standby_slot_names and synchronize_slot_names will be removed in the
next version as per discussion in [1] and thus SlotSyncInitConfig()
will not be needed. It will be replaced by new functionality. So I am
currently leaving it as is.

>
> 03. WalSndWaitForStandbyConfirmation()
>
> We should add ProcessRepliesIfAny() during the loop, otherwise the walsender
> overlooks the death of an apply worker.
>

Done.

> 04. WalSndWaitForStandbyConfirmation()
>
> Not sure, but do we have to return early if walsenders got PROCSIG_WALSND_INIT_STOPPING
> signal? I thought that if physical walsenders get stuck, logical walsenders wait
> forever. At that time we cannot stop the primary server even if "pg_ctl stop"
> is executed.
>

yes, right.  I have added CHECK_FOR_INTERRUPTS() and 'got_STOPPING'
handling now which I think should suffice to process
PROCSIG_WALSND_INIT_STOPPING.

> 05. SlotSyncInitConfig()
>
> Why don't we free the memory for rawname, old standby_slot_names_list, and synchronize_slot_names_list?
> They seem to be overwritten.
>

Skipped for the time being due to reasons stated in pt 2.

> 06. SlotSyncInitConfig()
>
> Both physical and logical walsenders call the func, but physical one do not use
> lists, right? If so, can we add a quick exit for physical walsenders?
> Or, we should carefully remove where physical calls it.
>
> 07. StartReplication()
>
> I think we do not have to call SlotSyncInitConfig().
> Alternative approach is written in above.
>

I have removed it  from StartReplication()

> 08. the other
>
> Also, I found the unexpected behavior after both 0001 and 0002 were applied.
> Was it normal or not?
>
> 1. constructed below setup
> (ensured that logical slot existed on secondary)
> 2. stopped the primary
> 3. promoted the secondary server
> 4. disabled a subscription once
> 5. changed the connection string for subscriber
> 6. Inserted data to new primary
> 7. enabled the subscription again
> 8. got an ERROR: replication slot "sub" does not exist
>
> I expected that the logical replication would be restarted, but it could not.
> Was it real issue or my fault? The error would appear in secondary.log.
>
> ```
> Setup:
> primary--->secondary
>    |
>    |
> subscriber
> ```

I have attached the new test-script (v2), can you please try that on
the v20 set of patches. We should let the slot creation complete first
on standby and then try promotion. I have added a few extra lines in
v2 of your script for the same. In the test-case, primary's
restart-lsn was lagging behind
new-slot's restart_lsn on standby and thus standby was waiting for
primary to catch-up. Meanwhile standby got promoted and thus slot
creation got aborted. That is the reason you were not able to get the
logical replication working on the new primary. v20 has improved
handling and better logging for such a case. Please try the attached
test-script on v20.


[1]: https://www.postgresql.org/message-id/CAJpy0uA%2Bt3XP2M0qtEmrOG1gSwHghjHPno5AtwTXM-94-%2Bc6JQ%40mail.gmail.com

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).