Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-11-13T13:57:28Z
Lists: pgsql-hackers

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On Friday, November 10, 2023 4:16 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:
> 
> 
> >>> Also, the current coding doesn't ensure we will always give WARNING.
> >>> If we see the below code that deals with this WARNING,
> >>>
> >>> +  /* User created slot with the same name exists, emit WARNING. */
> >>> +  else if (found && s->data.sync_state == SYNCSLOT_STATE_NONE)
> >>> +  {
> >>> +    ereport(WARNING,
> >>> +        errmsg("not synchronizing slot %s; it is a user created slot",
> >>> +             remote_slot->name));
> >>> +  }
> >>> +  /* Otherwise create the slot first. */
> >>> +  else
> >>> +  {
> >>> +    TransactionId xmin_horizon = InvalidTransactionId;
> >>> +    ReplicationSlot *slot;
> >>> +
> >>> +    ReplicationSlotCreate(remote_slot->name, true, RS_EPHEMERAL,
> >>> +                remote_slot->two_phase, false);
> >>>
> >>> I think this is not a solid check to ensure that the slot existed
> >>> before. Because it could be created as soon as the slot sync worker
> >>> invokes ReplicationSlotCreate() here.
> >>
> >> Agree.
> >>
> >
> > So, having a concrete check to give WARNING would require some more
> > logic which I don't think is a good idea to handle this boundary case.
> >
> 
> Yeah good point, agree to just error out in all the case then (if we discard the
> sync_ reserved wording proposal, which seems to be the case as probably not
> worth the extra work).

Thanks for the discussion!

Here is the V33 patch set which includes the following changes:

1) Drop slots with state 'i' in promotion flow after we shut down WalReceiver.

2) Raise error if user slot with same name already exists on standby.

3) Hold spinlock when updating the porperty of the replication slot or when
   reading the slot's info without acuqiring it.

4) Fixed the bug:
- if slot is invalidated on standby but standby is restarted immediately after
  that, then it fails to recreate that slot and instead end up syncing it
  again. It is fixed by checking the invalidated flag after acquiring the slot
  and skip syncing for invalidated slots.

5) Fixed the bugs reported by Ajin[1][2].

6) Removed some unused variables.

Thanks Shveta for working on 1) 2) 4) and 5).
Thanks Ajin for testing 5).

---
TODO
There are few pending comments and bugs have not been addressed, I will work on
them in next version:

1) Comments posted by me[3]:
2) Shutdown the slotsync worker on promotion and probably let slotsync do necessary cleanups[4]
3) Consider documenting the hazard that create slot on standby may cause ERRORs
   in the log and consider making it visible in the view.
4) One bug found internally: If we give non-existing dbname in primary_conninfo
   on standby, it should be handled gracefully, launcher should skip
   slots-synchronization. 
5) One bug found internally: when wait_for_primary_slot_catchup is doing
   WaitLatch and I stop the standby using: ./pg_ctl -D ../../standbydb/ -l
   standby.log stop it does not come out of wait and shutdown hangs.

[1] https://www.postgresql.org/message-id/CAFPTHDZNV_HFAXULkaJOv_MMtLukCzDEgTaixxBwjEO_0Jg-kg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAFPTHDa5C_vHQbeqemToyucWySB0kEFbdS2WOA0PB%2BGSei2v7A%40mail.gmail.com
[3] https://www.postgresql.org/message-id/OS0PR01MB571652CCD42F1D08D5BD69D494B3A%40OS0PR01MB5716.jpnprd01.prod.outlook.com
[4] https://www.postgresql.org/message-id/64056e35-1916-461c-a816-26e40ffde3a0%40gmail.com

Best Regards,
Hou zj

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).