Horton

Re: Synchronizing slots from primary to standby

Nisha Moond <nisha.moond412@gmail.com>

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From: Nisha Moond <nisha.moond412@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, 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: 2023-12-01T12:10:33Z
Lists: pgsql-hackers 
Review for v41 patch.

1.
======
src/backend/utils/misc/postgresql.conf.sample

+#enable_syncslot = on # enables slot synchronization on the physical
standby from the primary

enable_syncslot is disabled by default, so, it should be 'off' here.

~~~
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?

~~~
3.
As per current logic, If there are slots on primary with disabled
subscriptions, then, when standby is created it replicates these slots
but can't make them sync-ready until any activity happens on the
slots.
So, such slots stay in 'i' sync-state and get dropped when failover
happens. Now, if the subscriber tries to enable their existing
subscription after failover, it gives an error that the slot does not
exist.

~~~
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?

--
Thanks,
Nisha

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).