Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Nisha Moond <nisha.moond412@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: 2024-01-17T09:38:45Z
Lists: pgsql-hackers 
Hi,

On Tue, Jan 16, 2024 at 05:27:05PM +0530, shveta malik wrote:
> PFA v62. Details:

Thanks! 

> v62-003:
> It is a new patch which attempts to implement slot-sync worker as a
> special process which is neither a bgworker nor an Auxiliary process.
> Here we get the benefit of converting enable_syncslot to a PGC_SIGHUP
> Guc rather than PGC_POSTMASTER. We launch the slot-sync worker only if
> it is hot-standby and 'enable_syncslot' is ON.

The implementation looks reasonable to me (from what I can see some parts is
copy/paste from an already existing "special" process and some parts are
"sync slot" specific) which makes fully sense.

A few remarks:

1 ===
+                * Was it the slot sycn worker?

Typo: sycn

2 ===
+                * ones), and no walwriter, autovac launcher or bgwriter or slot sync

Instead? "* ones), and no walwriter, autovac launcher, bgwriter or slot sync"

3 ===
+ * restarting slot slyc worker. If stopSignaled is set, the worker will

Typo: slyc

4 ===
+/* Flag to tell if we are in an slot sync worker process */

s/an/a/ ?

5 === (coming from v62-0002)
+       Assert(tuplestore_tuple_count(res->tuplestore) == 1);

Is it even possible for the related query to not return only one row? (I think the 
"count" ensures it).

6 ===
        if (conninfo_changed ||
                primary_slotname_changed ||
+               old_enable_syncslot != enable_syncslot ||
                (old_hot_standby_feedback != hot_standby_feedback))
        {
                ereport(LOG,
                                errmsg("slot sync worker will restart because of"
                                           " a parameter change"));

I don't think "slot sync worker will restart" is true if one change enable_syncslot
from on to off.

IMHO, v62-003 is in good shape and could be merged in v62-002 (that would ease
the review). But let's wait to see if others think differently.

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).