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: shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@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>, Peter Smith <smithpb2250@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.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: 2024-03-01T02:17:26Z
Lists: pgsql-hackers

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On Thursday, February 29, 2024 7:17 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Thu, Feb 29, 2024 at 3:23 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> >
> > Few additional comments on the latest patch:
> > =================================
> > 1.
> >  static XLogRecPtr
> >  WalSndWaitForWal(XLogRecPtr loc)
> > {
> > ...
> > + if (!XLogRecPtrIsInvalid(RecentFlushPtr) && loc <= RecentFlushPtr &&
> > + (!replication_active || StandbyConfirmedFlush(loc, WARNING))) {
> > + /*
> > + * Fast path to avoid acquiring the spinlock in case we already know
> > + * we have enough WAL available and all the standby servers have
> > + * confirmed receipt of WAL up to RecentFlushPtr. This is
> > + particularly
> > + * interesting if we're far behind.
> > + */
> >   return RecentFlushPtr;
> > + }
> > ...
> > ...
> > + * Wait for WALs to be flushed to disk only if the postmaster has not
> > + * asked us to stop.
> > + */
> > + if (loc > RecentFlushPtr && !got_STOPPING) wait_event =
> > + WAIT_EVENT_WAL_SENDER_WAIT_FOR_WAL;
> > +
> > + /*
> > + * Check if the standby slots have 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. Note that 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.
> > + */
> > + else if (replication_active &&
> > + !StandbyConfirmedFlush(RecentFlushPtr, WARNING)) { wait_event =
> > + WAIT_EVENT_WAIT_FOR_STANDBY_CONFIRMATION;
> > + wait_for_standby = true;
> > + }
> > + else
> > + {
> > + /* Already caught up and doesn't need to wait for standby_slots. */
> >   break;
> > + }
> > ...
> > }
> >
> > Can we try to move these checks into a separate function that returns
> > a boolean and has an out parameter as wait_event?
> >
> > 2. How about naming StandbyConfirmedFlush() as
> StandbySlotsAreCaughtup?
> >
> 
> Some more comments:
> ==================
> 1.
> + foreach_ptr(char, name, elemlist)
> + {
> + ReplicationSlot *slot;
> +
> + slot = SearchNamedReplicationSlot(name, true);
> +
> + if (!slot)
> + {
> + GUC_check_errdetail("replication slot \"%s\" does not exist", name);
> + ok = false; break; }
> +
> + if (!SlotIsPhysical(slot))
> + {
> + GUC_check_errdetail("\"%s\" is not a physical replication slot",
> + name); ok = false; break; } }
> 
> Won't the second check need protection via ReplicationSlotControlLock?

Yes, added.

> 
> 2. In WaitForStandbyConfirmation(), we are anyway calling
> StandbyConfirmedFlush() before the actual sleep in the loop, so does calling it
> at the beginning of the function will serve any purpose? If so, it is better to add
> some comments explaining the same.

It is used as a fast-path to avoid calling condition variable stuff, I think we can directly
put failover check and list check in the beginning instead of calling that function.

> 
> 3. Also do we need to perform the validation checks done in
> StandbyConfirmedFlush() repeatedly when it is invoked in a loop? We can
> probably separate those checks and perform them just once.

I have removed slot.failover check from the StandbyConfirmedFlush
function, so that we can do it when necessary. I didn’t change the check
for standby_slot_names_list because the list could be changed in the loop
when reloading config.

> 
> 4.
> +   *
> +   * XXX: If needed, this can be attempted in future.
> 
> Remove this part of the comment.

Removed.

Attach the V102 patch set which addressed Amit and Shveta's comments.
Thanks Shveta for helping addressing the comments off-list.

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).