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>, Ajin Cherian <itsajin@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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: 2024-02-08T10:33:19Z
Lists: pgsql-hackers 
On Wed, Feb 7, 2024 at 5:32 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> Sure, made the suggested function name changes. Since there is no
> other change, I kept the version as v80_2.
>

Few comments on 0001
===================
1.
+ * the slots on the standby and synchronize them. This is done on every call
+ * to SQL function pg_sync_replication_slots.
>

I think the second sentence can be slightly changed to: "This is done
by a call to SQL function pg_sync_replication_slots." or "One can call
SQL function pg_sync_replication_slots to invoke this functionality."

2.
+update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
{
...
+ SpinLockAcquire(&slot->mutex);
+ slot->data.plugin = plugin_name;
+ slot->data.database = remote_dbid;
+ slot->data.two_phase = remote_slot->two_phase;
+ slot->data.failover = remote_slot->failover;
+ slot->data.restart_lsn = remote_slot->restart_lsn;
+ slot->data.confirmed_flush = remote_slot->confirmed_lsn;
+ slot->data.catalog_xmin = remote_slot->catalog_xmin;
+ slot->effective_catalog_xmin = remote_slot->catalog_xmin;
+ SpinLockRelease(&slot->mutex);
+
+ if (remote_slot->catalog_xmin != slot->data.catalog_xmin)
+ ReplicationSlotsComputeRequiredXmin(false);
+
+ if (remote_slot->restart_lsn != slot->data.restart_lsn)
+ ReplicationSlotsComputeRequiredLSN();
...
}

How is it possible that after assigning the values from remote_slot
they can differ from local slot values?

3.
+ /*
+ * Find the oldest existing WAL segment file.
+ *
+ * Normally, we can determine it by using the last removed segment
+ * number. However, if no WAL segment files have been removed by a
+ * checkpoint since startup, we need to search for the oldest segment
+ * file currently existing in XLOGDIR.
+ */
+ oldest_segno = XLogGetLastRemovedSegno() + 1;
+
+ if (oldest_segno == 1)
+ oldest_segno = XLogGetOldestSegno(0);

I feel this way isn't there a risk that XLogGetOldestSegno() will get
us the seg number from some previous timeline which won't make sense
to compare segno in reserve_wal_for_local_slot. Shouldn't you need to
fetch the current timeline and send as a parameter to this function as
that is the timeline on which standby is communicating with primary.

4.
+ if (remote_slot->confirmed_lsn > latestFlushPtr)
+ ereport(ERROR,
+ errmsg("skipping slot synchronization as the received slot sync"

I think the internal errors should be reported with elog as you have
done at other palces in the patch.


5.
+synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
{
...
+ /*
+ * 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);
+
+ /* Make sure the invalidated state persists across server restart */
+ ReplicationSlotMarkDirty();
+ ReplicationSlotSave();
+ slot_updated = true;
+ }
...
}

Do we need to copy the 'invalidated' from remote to local if both are
same? I think this will happen for each slot each time because
normally slots won't be invalidated ones, so there is needless writes.

6.
+ * Returns TRUE if any of the slots gets updated in this sync-cycle.
+ */
+static bool
+synchronize_slots(WalReceiverConn *wrconn)
...
...

+void
+SyncReplicationSlots(WalReceiverConn *wrconn)
+{
+ PG_TRY();
+ {
+ validate_primary_slot_name(wrconn);
+
+ (void) synchronize_slots(wrconn);

For the purpose of 0001, synchronize_slots() doesn't seems to use
return value. So, I suggest to change it accordingly and move the
return value in the required patch.

7.
+ /*
+ * The primary_slot_name is not set yet or WALs not received yet.
+ * Synchronization is not possible if the walreceiver is not started.
+ */
+ latestWalEnd = GetWalRcvLatestWalEnd();
+ SpinLockAcquire(&WalRcv->mutex);
+ if ((WalRcv->slotname[0] == '\0') ||
+ XLogRecPtrIsInvalid(latestWalEnd))
+ {
+ SpinLockRelease(&WalRcv->mutex);
+ return false;

For the purpose of 0001, we should give WARNING here.

-- 
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).