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>, Nisha Moond <nisha.moond412@gmail.com>
Cc: 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: 2023-12-04T16:36:56Z
Lists: pgsql-hackers 
Hi,

On 12/4/23 6:10 AM, shveta malik wrote:
> On Fri, Dec 1, 2023 at 5:40 PM Nisha Moond <nisha.moond412@gmail.com> wrote:
>>
>> Review for v41 patch.
> 
> Thanks for the feedback.
> 
>> ~~~
>> 2.
>> IIUC, the slotsyncworker's connection to the primary is to execute a
>> query. Its aim is not walsender type connection, but at primary when
>> queried, the 'backend_type' is set to 'walsender'.
>> Snippet from primary db-
>>
>> datname  |   usename   | application_name | wait_event_type | backend_type
>> ---------+-------------+------------------+-----------------+--------------
>> postgres | replication | slotsyncworker   | Client          | walsender
>>
>> Is it okay?
>>
> 
> Slot sync worker uses 'libpqrcv_connect' for connection which sends
> 'replication'-'database' key-value pair as one of the connection
> options. And on the primary side, 'ProcessStartupPacket' on the basis
> of this key-value pair sets the process as walsender one (am_walsender
> = true).
> And thus this reflects as backend_type='walsender' in
> pg_stat_activity. I do not see any harm in this backend_type for
> slot-sync worker currently. This is on a similar line of connections
> used for logical-replications. And since a slot-sync worker also deals
> with wals-positions (lsns), it is okay to maintain backend_type as
> walsender unless you (or others) see any potential issue in doing
> that. So let me know.

I don't see any issue as well (though I understand it might
seems weird to see a walsender process being spawned doing non
replication stuff)

> 
>> ~~~
>> 4. primary_slot_name GUC value test:
>>
>> When standby is started with a non-existing primary_slot_name, the
>> wal-receiver gives an error but the slot-sync worker does not raise
>> any error/warning. It is no-op though as it has a check 'if
>> (XLogRecPtrIsInvalid(WalRcv->latestWalEnd)) do nothing'.   Is this
>> okay or shall the slot-sync worker too raise an error and exit?
>>
>> In another case, when standby is started with valid primary_slot_name,
>> but it is changed to some invalid value in runtime, then walreceiver
>> starts giving error but the slot-sync worker keeps on running. In this
>> case, unlike the previous case, it even did not go to no-op mode (as
>> it sees valid WalRcv->latestWalEnd from the earlier run) and keep
>> pinging primary repeatedly for slots.  Shall here it should error out
>> or at least be no-op until we give a valid primary_slot_name?
>>
> 

Nice catch, thanks!

> I reviewed it. There is no way to test the existence/validity of
> 'primary_slot_name' on standby without making a connection to primary.
> If primary_slot_name is invalid from the start, slot-sync worker will
> be no-op (as you tested) as WalRecv->latestWalENd will be invalid, and
> if 'primary_slot_name' is changed to invalid on runtime, slot-sync
> worker will still keep on pinging primary. But that should be okay (in
> fact needed) as it needs to sync at-least the previous slot's
> positions (in case it is delayed in doing so for some reason earlier).
> And once the slots are up-to-date on standby, even if worker pings
> primary, it will not see any change in slots lsns and thus go for
> longer nap. I think, it is not worth the effort to introduce the
> complexity of checking validity of 'primary_slot_name' on primary from
> standby for this rare scenario.
> 

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).

Not sure about the extra effort to make it works though.

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).