Horton

RE: Synchronizing slots from primary to standby

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

View thread View on postgr.es
From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: 'Ajin Cherian' <itsajin@gmail.com>, shveta malik <shveta.malik@gmail.com>
Cc: 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>
Date: 2023-09-25T14:15:59Z
Lists: pgsql-hackers

Attachments

Dear Ajin, Shveta,

Thank you for rebasing the patch set! Here are new comments for v19_2-0001.

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()

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.

03. WalSndWaitForStandbyConfirmation()

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

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.

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.

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.

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
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED

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).