Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-02-05T11:06:26Z
Lists: pgsql-hackers 
On Mon, Feb 5, 2024 at 7:59 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>

I have pushed the first patch. Next, a few comments on 0002 are as follows:
1.
+static bool
+validate_parameters_and_get_dbname(char **dbname, int elevel)

For 0002, we don't need dbname as out parameter. Also, we can rename
the function to validate_slotsync_params() or something like that.
Also, for 0003, we don't need to get the dbname from
wait_for_valid_params_and_get_dbname(), instead there could be a
common function that can be invoked from validate_slotsync_params()
and caller of wait function that caches the value of dbname.

The other parameter elevel is also not required for 0002.

2.
+ /*
+ * Make sure that concerned WAL is received and flushed before syncing
+ * slot to target lsn received from the primary server.
+ */
+ latestFlushPtr = GetStandbyFlushRecPtr(NULL);
+ if (remote_slot->confirmed_lsn > latestFlushPtr)
+ {
+ /*
+ * Can get here only if GUC 'standby_slot_names' on the primary server
+ * was not configured correctly.
+ */
+ ereport(LOG,
+ errmsg("skipping slot synchronization as the received slot sync"
+    " LSN %X/%X for slot \"%s\" is ahead of the standby position %X/%X",
+    LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
+    remote_slot->name,
+    LSN_FORMAT_ARGS(latestFlushPtr)));
+
+ return false;

In the case of a function invocation, this should be an ERROR. We can
move the comment related to 'standby_slot_names' to a later patch
where that GUC is introduced. See, if there are other LOGs in the
patch that needs to be converted to ERROR.

3. The function pg_sync_replication_slots() should be in file
slotfuncs.c and common functionality between this function and
slotsync worker can be exposed via a function in slotsync.c.

4.
/*
+ * Using the specified primary server connection, check whether we are
+ * cascading standby and validates primary_slot_name for
+ * non-cascading-standbys.
+ */
+ check_primary_info(wrconn, &am_cascading_standby,
+    &primary_slot_invalid, ERROR);
+
+ if (am_cascading_standby)
+ ereport(ERROR,
+ errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot synchronize replication slots to a cascading standby"));

primary_slot_invalid is not used in this patch. I think we can allow
the function can be executed on cascading_standby as well because this
will be used for the planned switchover.

5. I don't see any problem with allowing concurrent processes trying
to sync the same slot at the same time as each process will acquire
the slot and only one process can acquire the slot at a time, the
other will get an ERROR.

-- 
With Regards,
Amit Kapila.

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).