Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Peter Smith <smithpb2250@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: 2024-02-29T01:34:31Z
Lists: pgsql-hackers

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On Monday, February 26, 2024 7:52 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Mon, Feb 26, 2024 at 7:49 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> wrote:
> >
> > Attach the V98 patch set which addressed above comments.
> >
> 
> Few comments:
> =============
> 1.
>  WalSndWaitForWal(XLogRecPtr loc)
>  {
>   int wakeEvents;
> + bool wait_for_standby = false;
> + uint32 wait_event;
> + List    *standby_slots = NIL;
>   static XLogRecPtr RecentFlushPtr = InvalidXLogRecPtr;
> 
> + if (MyReplicationSlot->data.failover && replication_active)
> + standby_slots = GetStandbySlotList(true);
> +
>   /*
> - * Fast path to avoid acquiring the spinlock in case we already know we
> - * have enough WAL available. This is particularly interesting if we're
> - * far behind.
> + * Check if all the standby servers have confirmed receipt of WAL up to
> + * RecentFlushPtr even when we already know we have enough WAL available.
> + *
> + * Note that we cannot directly return without checking the status of
> + * standby servers because the standby_slot_names may have changed,
> + which
> + * means there could be new standby slots in the list that have not yet
> + * caught up to the RecentFlushPtr.
>   */
> - if (RecentFlushPtr != InvalidXLogRecPtr &&
> - loc <= RecentFlushPtr)
> - return RecentFlushPtr;
> + if (!XLogRecPtrIsInvalid(RecentFlushPtr) && loc <= RecentFlushPtr) {
> + FilterStandbySlots(RecentFlushPtr, &standby_slots);
> 
> I think even if the slot list is not changed, we will always process each slot
> mentioned in standby_slot_names once. Can't we cache the previous list of
> slots for we have already waited for? In that case, we won't even need to copy
> the list via GetStandbySlotList() unless we need to wait.
> 
> 2.
>  WalSndWaitForWal(XLogRecPtr loc)
>  {
> + /*
> + * Update the standby slots that have not yet caught up to the flushed
> + * position. It is good to wait up to RecentFlushPtr and then let it
> + * send the changes to logical subscribers one by one which are
> + * already covered in RecentFlushPtr without needing to wait on every
> + * change for standby confirmation.
> + */
> + if (wait_for_standby)
> + FilterStandbySlots(RecentFlushPtr, &standby_slots);
> +
>   /* Update our idea of the currently flushed position. */
> - if (!RecoveryInProgress())
> + else if (!RecoveryInProgress())
>   RecentFlushPtr = GetFlushRecPtr(NULL);
>   else
>   RecentFlushPtr = GetXLogReplayRecPtr(NULL); ...
> /*
> * If postmaster asked us to stop, don't wait anymore.
> *
> * It's important to do this check after the recomputation of
> * RecentFlushPtr, so we can send all remaining data before shutting
> * down.
> */
> if (got_STOPPING)
> break;
> 
> I think because 'wait_for_standby' may not be set in the first or consecutive
> cycles we may send the WAL to the logical subscriber before sending it to the
> physical subscriber during shutdown.

Here is the v101 patch set which addressed above comments.

This version will cache the oldest standby slot's LSN each time we waited for
them to catch up. The cached LSN is invalidated when we reload the GUC config.
In the WalSndWaitForWal function, instead of traversing the entire standby list
each time, we can check the cached LSN to quickly determine if the standbys
have caught up. When a shutdown signal is received, we continue to wait for the
standby slots to catch up. When waiting for the standbys to catch up after
receiving the shutdown signal, an ERROR is reported if any slots are dropped,
invalidated, or inactive. This measure is taken to prevent the walsender from
waiting indefinitely.

Best Regards,
Hou zj

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).