Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, Nisha Moond <nisha.moond412@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, shveta malik <shveta.malik@gmail.com>
Date: 2023-12-22T10:32:21Z
Lists: pgsql-hackers

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On Fri, Dec 22, 2023 at 3:11 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Thursday, December 21, 2023 5:39 PM Bertrand Drouvot <bertranddrouvot.pg@gmail.com> wrote:
> >
> > On Thu, Dec 21, 2023 at 02:23:12AM +0000, Zhijie Hou (Fujitsu) wrote:
> > > On Wednesday, December 20, 2023 8:42 PM Zhijie Hou (Fujitsu)
> > <houzj.fnst@fujitsu.com> wrote:
> > > >
> > > > Attach the V51 patch set which addressed Kuroda-san's comments.
> > > > I also tried to improve the test in 0003 to make it stable.
> > >
> > > The patches conflict with a recent commit dc21234.
> > > Here is the rebased V51_2 version, there is no code changes in this version.
> > >
> >
> > Thanks!
> >
> > I've a few remarks regarding 0001:
>
> Thanks for the comments!
>
> >
> > 1 ===
> >
> > In the commit message what about replacing "Allow logical walsenders to wait
> > for the physical standbys" with "Force some logical walsenders to wait for the
> > physical standbys"?
>
> I feel 'Allow' is OK, as the GUC standby_slot_names is optional for user. ISTM, 'force'
> means we always wait for physical standbys regardless of the GUC.
>
> >
> > Also I think it would be better to first explain what we are trying to achieve and
> > after explain how we do it (adding a new flag in CREATE SUBSCRIPTION and so
> > on).
>
> Noted. We are about to split the patches, so will improve each commit message after that.
>
> >
> > 4 ===
> >
> > @@ -248,10 +262,13 @@ ReplicationSlotValidateName(const char *name, int
> > elevel)
> >   *     during getting changes, if the two_phase option is enabled it can skip
> >   *     prepare because by that time start decoding point has been moved. So
> > the
> >   *     user will only get commit prepared.
> > + * failover: If enabled, allows the slot to be synced to physical standbys so
> > + *     that logical replication can be resumed after failover.
> >
> > s/allows/forces ?
>
> I think whether the slot is synced also depends on the
> GUC setting on standby, so I feel 'allow' is fine here.
>
> >
> > 5 ===
> >
> > +       bool            ok;
> >
> > parse_ok maybe?
>
> The flag is also used to store the slot type check result, so I feel 'ok' is
> better here.
>
> >
> > 6 ===
> >
> > +       /* Need a modifiable copy of string. */
> > +       rawname = pstrdup(*newval);
> >
> > It seems to me that the single line comments in the neighborhood functions
> > (see
> > RestoreSlotFromDisk() for example) don't finish with ".". Worth to follow the
> > same format for all what we add in slot.c?
>
> I felt we have both styles in slot.c, but it seems Kuroda-san also
> prefer removing the ".", so will address.
>
> >
> > 7 ===
> >
> > +static void
> > +parseAlterReplSlotOptions(AlterReplicationSlotCmd *cmd, bool *failover)
> >
> > ParseAlterReplSlotOptions instead?
>
> I think it followed parseCreateReplSlotOptions, but I agree that it looks
> inconsistent with other names. Will address.
>
> > 11 ===
> >
> > +    * When the wait event is WAIT_FOR_STANDBY_CONFIRMATION, wait on
> > another
> > +    * CV that is woken up by physical walsenders when the walreceiver has
> > +    * confirmed the receipt of LSN.
> >
> > s/that is woken up by/that is broadcasted by/ ?
>
> Will reword the comment here.
>
> >
> > 12 ===
> >
> > We are mentioning in several places that the replication can be resumed after a
> > failover. Should we add a few words about possible lag? (see [1])
> >
> > [1]:
> > https://www.postgresql.org/message-id/CAA4eK1KihniOK21mEVYtSOHRQiG
> > NyToUmENWp7hPbH_PMsqzkA%40mail.gmail.com
>
> It feels like the implementation detail to me, but noted. We will think more
> about the document.
>
>
> The comments not mentioned above look good to me.
>
> Best Regards,
> Hou zj


PFA v53. Changes are:

patch001:
1) Addressed comments in [1] for v51-001. Thanks Hou-san for working on this.

patch002:
2) Addressed comments in [2] for v52-002.
3) Fixed CFBot failure. The failure was caused by an assert in
wait_for_primary_slot_catchup() for null confirmed_lsn received. In
wait_for_primary_slot_catchup(), we had an assumption that if
restart_lsn is valid and 'conflicting' is also false, then we must
have non-null confirmed_lsn. But this is not true. It is possible to
get null values for confirmed_lsn and catalog_xmin if on the primary
server the slot is just created with a valid restart_lsn and slot-sync
worker has fetched the slot before the primary server could set valid
confirmed_lsn and catalog_xmin. In
pg_create_logical_replication_slot(), there is a small window between
CreateInitDecodingContext-->ReplicationSlotReserveWal() which sets
restart_lsn and DecodingContextFindStartpoint() which sets
confirmed_lsn. If the slot-sync worker fetches the slot in this
window, confirmed_lsn received will be NULL. Corrected the code to
remove assert and added one additional condition that confirmed_lsn
should be valid before moving the slot to 'r'.

[1]: https://www.postgresql.org/message-id/ZYQHvgBpH0GgQaJK%40ip-10-97-1-34.eu-west-3.compute.internal
[2]: https://www.postgresql.org/message-id/TY3PR01MB98893274D5A4FD4F86CC04A0F595A%40TY3PR01MB9889.jpnprd01.prod.outlook.com

thanks
Shveta

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).