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: shveta malik <shveta.malik@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Nisha Moond <nisha.moond412@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: 2023-12-10T11:03:35Z
Lists: pgsql-hackers 
On Wed, Dec 6, 2023 at 4:53 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> v43-002:
>

Review comments on v43-0002:
=========================
1.
synchronize_one_slot()
{
...
+ /*
+ * With hot_standby_feedback enabled and invalidations handled
+ * apropriately as above, this should never happen.
+ */
+ if (remote_slot->restart_lsn < MyReplicationSlot->data.restart_lsn)
+ {
+ ereport(ERROR,
+ errmsg("not synchronizing local slot \"%s\" LSN(%X/%X)"
+    " to remote slot's LSN(%X/%X) as synchronization "
+    " would move it backwards", remote_slot->name,
+    LSN_FORMAT_ARGS(MyReplicationSlot->data.restart_lsn),
+    LSN_FORMAT_ARGS(remote_slot->restart_lsn)));
+
+ goto cleanup;
...
}

After the error, the control won't return, so the above goto doesn't
make any sense.

2.
synchronize_one_slot()
{
...
+ /* Search for the named slot */
+ if ((s = SearchNamedReplicationSlot(remote_slot->name, true)))
+ {
+ SpinLockAcquire(&s->mutex);
+ sync_state = s->data.sync_state;
+ SpinLockRelease(&s->mutex);
+ }
...
...
+ ReplicationSlotAcquire(remote_slot->name, true);
+
+ /*
+ * Copy the invalidation cause from remote only if local slot is not
+ * invalidated locally, we don't want to overwrite existing one.
+ */
+ if (MyReplicationSlot->data.invalidated == RS_INVAL_NONE)
+ {
+ SpinLockAcquire(&MyReplicationSlot->mutex);
+ MyReplicationSlot->data.invalidated = remote_slot->invalidated;
+ SpinLockRelease(&MyReplicationSlot->mutex);
+ }
+
+ /* Skip the sync if slot has been invalidated locally. */
+ if (MyReplicationSlot->data.invalidated != RS_INVAL_NONE)
+ goto cleanup;
...

It seems useless to acquire the slot if it is locally invalidated in
the first place. Won't it be better if after the search we first check
whether the slot is locally invalidated and take appropriate action?

3. After doing the above two, I think it doesn't make sense to have
goto at the remaining places in synchronize_one_slot(). We can simply
release the slot and commit the transaction at other places.

4.
+ * Returns nap time for the next sync-cycle.
+ */
+static long
+synchronize_slots(WalReceiverConn *wrconn)

Returning nap time from here appears a bit awkward. I think it is
better if this function returns any_slot_updated and then the caller
decides the adjustment of naptime.

5.
+synchronize_slots(WalReceiverConn *wrconn)
{
...
...
+ /* The syscache access needs a transaction env. */
+ StartTransactionCommand();
+
+ /*
+ * Make result tuples live outside TopTransactionContext to make them
+ * accessible even after transaction is committed.
+ */
+ MemoryContextSwitchTo(oldctx);
+
+ /* Construct query to get slots info from the primary server */
+ initStringInfo(&s);
+ construct_slot_query(&s);
+
+ elog(DEBUG2, "slot sync worker's query:%s \n", s.data);
+
+ /* Execute the query */
+ res = walrcv_exec(wrconn, s.data, SLOTSYNC_COLUMN_COUNT, slotRow);
+ pfree(s.data);
+
+ if (res->status != WALRCV_OK_TUPLES)
+ ereport(ERROR,
+ (errmsg("could not fetch failover logical slots info "
+ "from the primary server: %s", res->err)));
+
+ CommitTransactionCommand();
...
...
}

Where exactly in the above code, there is a syscache access as
mentioned above StartTransactionCommand()?

6.
-          <filename>~/.pgpass</filename> file on the standby server (use
+          <filename>~/.pgpass</filename> file on the standby server. (use
           <literal>replication</literal> as the database name).

Why do we need this change?

7.
+     standby. Additionally, similar to creating a logical replication slot
+     on the hot standby, <varname>hot_standby_feedback</varname> should be
+     set on the standby and a physical slot between the primary and the standby
+     should be used.

In this, I don't understand the relation between the first part of the
line: "Additionally, similar to creating a logical replication slot on
the hot standby ..." with the rest.

8.
However,
+     the slots which were in initiated sync_state ('i) and were not

A single quote after 'i' is missing.

9.
the slots with state 'r' and 'i' can neither be used for logical
+      decoded nor dropped by the user.

/decoded/decoding

10.
+/*
+ * Allocate and initialize slow sync worker shared memory
+ */

/slow/slot

-- 
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).