Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

View thread View on postgr.es
From: Amit Kapila <amit.kapila16@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-12-19T13:19:03Z
Lists: pgsql-hackers 
On Tue, Dec 19, 2023 at 5:17 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Mon, Dec 18, 2023 at 4:22 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Fri, Dec 15, 2023 at 11:03 AM shveta malik <shveta.malik@gmail.com> wrote:
> > >
> > > Sorry, I missed attaching the patch. PFA v48.
> > >
> >
> > Few comments on v48_0002
> > ========================
>
> Thanks for reviewing. These are addressed in v50.
>

I was still reviewing the v48 version and have a few comments as
below. If some of these are already addressed or not relevant, feel
free to ignore them.
1.
+ /*
+ * Slot sync worker can be stopped at any time.
+ * Use exit status 1 so the background worker is restarted.

We don't need to start the second line of comment in a separate line.

2.
+ * The assumption is that these dropped local invalidated slots will get
+ * recreated in next sync-cycle and it is okay to drop and recreate such slots

In the above line '.. local invalidated ..' sounds redundant. Shall we
remove it?

3.
+ if (remote_slot->confirmed_lsn > WalRcv->latestWalEnd)
+ {
+ SpinLockRelease(&WalRcv->mutex);
+ elog(ERROR, "skipping sync of slot \"%s\" as the received slot sync "


This error message looks odd to me. At least, it should be exiting
instead of skipping because we won't continue after this.

4.
+ /* User created slot with the same name exists, raise ERROR. */
+ if (sync_state == SYNCSLOT_STATE_NONE)
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("skipping sync of slot \"%s\" as it is a user created"
+ " slot", remote_slot->name),
+ errdetail("This slot has failover enabled on the primary and"
+    " thus is sync candidate but user created slot with"
+    " the same name already exists on the standby")));

Same problem as above. The skipping in error message doesn't seem to
be suitable for the purpose. Additionally, errdetail message should
end with a full stop.

5.
+ /* Slot ready for sync, so sync it. */
+ if (sync_state == SYNCSLOT_STATE_READY)
+ {
+ /*
+ * Sanity check: With hot_standby_feedback enabled and
+ * invalidations handled appropriately as above, this should never
+ * happen.
+ */
+ if (remote_slot->restart_lsn < slot->data.restart_lsn)
+ elog(ERROR,
+ "not synchronizing local slot \"%s\" LSN(%X/%X)"
+ " to remote slot's LSN(%X/%X) as synchronization "

The start of the error message sounds odd. Shall we say 'cannot
synchronize ...'?

6. All except one of the callers of local_slot_update() marks the slot
dirty and the same is required as well. I think the remaining caller
should also mark it dirty and we should move
ReplicationSlotMarkDirty() in the caller space.

7.
+synchronize_one_slot(WalReceiverConn *wrconn, RemoteSlot *remote_slot)
{
...
+ /*
+ * Copy the invalidation cause from remote only if local slot is not
+ * invalidated locally, we don't want to overwrite existing one.
+ */
+ if (slot->data.invalidated == RS_INVAL_NONE)
+ {
+ SpinLockAcquire(&slot->mutex);
+ slot->data.invalidated = remote_slot->invalidated;
+ SpinLockRelease(&slot->mutex);
+ }
...

It doesn't seem that after changing the invalidated flag, we always
mark the slot dirty. Am, I missing something?

8.
+ /*
+ * Drop local slots that no longer need to be synced. Do it before
+ * synchronize_one_slot to allow dropping of slots before actual sync
+ * which are invalidated locally while still valid on the primary server.
+ */
+ drop_obsolete_slots(remote_slot_list);

The second part of the above comment seems redundant as that is obvious.

9.
+static WalReceiverConn *
+remote_connect(void)
+{
+ WalReceiverConn *wrconn = NULL;
+ char    *err;
+
+ wrconn = walrcv_connect(PrimaryConnInfo, true, false,
+ cluster_name[0] ? cluster_name : "slotsyncworker",
+ &err);
+ if (wrconn == NULL)
+ ereport(ERROR,
+ (errcode(ERRCODE_CONNECTION_FAILURE),
+ errmsg("could not connect to the primary server: %s", err)));
+ return wrconn;
+}

Do we need a function for this? It appears to be called from just one
place, so not sure if it is helpful to have a function for this.

-- 
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).