Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Nisha Moond <nisha.moond412@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-12-05T08:48:34Z
Lists: pgsql-hackers 
Hi,

On 12/5/23 6:08 AM, shveta malik wrote:
> On Mon, Dec 4, 2023 at 10:07 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>> Maybe another option could be to have the walreceiver a way to let the slot sync
>> worker knows that it (the walreceiver) was not able to start due to non existing
>> replication slot on the primary? (that way we'd avoid the slot sync worker having
>> to talk to the primary).
> 
> Few points:
> 1) I think if we do it, we should do it in generic way i.e. slotsync
> worker should go to no-op if walreceiver is not able to start due to
> any reason and not only due to invalid primary_slot_name.

Agree.

> 2) Secondly, slotsync worker needs to make sure it has synced the
> slots so far i.e. worker should not go to no-op immediately on seeing
> missing WalRcv process if there are pending slots to be synced.

Agree.

> So the generic way I see to have this optimization is:
> 1) Slotsync worker can use 'WalRcv->pid' to figure out if WalReceiver
> is running or not.

Not sure that would work because the walreceiver keeps try re-starting
and so get a pid before reaching the "could not start WAL streaming: ERROR:  replication slot "XXXX" does not exist"
error.

We may want to add an extra check on walrcv->walRcvState (or should/could be enough by its own).
But walrcv->walRcvState is set to WALRCV_STREAMING way before walrcv_startstreaming().

Wouldn't that make sense to move it once we are sure that
walrcv_startstreaming() returns true and first_stream is true, here?

"
                         if (first_stream)
+                       {
                                 ereport(LOG,
                                                 (errmsg("started streaming WAL from primary at %X/%X on timeline %u",
                                                                 LSN_FORMAT_ARGS(startpoint), startpointTLI)));
+                               SpinLockAcquire(&walrcv->mutex);
+                               walrcv->walRcvState = WALRCV_STREAMING;
+                               SpinLockRelease(&walrcv->mutex);
+                       }
"

> 2) Slotsync worker should check null 'WalRcv->pid' only when
> no-activity is observed for threshold time i.e. it can do it during
> existing logic of increasing naptime.
> 3) On finding null  'WalRcv->pid', worker can mark a flag to go to
> no-op unless WalRcv->pid becomes valid again. Marking this flag during
> increasing naptime will guarantee that the worker has taken all the
> changes so far i.e. standby is not lagging in terms of slots.
> 

2) and 3) looks good to me but with a check on walrcv->walRcvState
looking for WALRCV_STREAMING state instead of looking for a non null
WalRcv->pid.

And only if it makes sense to move the walrcv->walRcvState = WALRCV_STREAMING as
mentioned above (I think it does).

Thoughts?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).