Horton

Re: Synchronizing slots from primary to standby

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Masahiko Sawada <sawada.mshk@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: 2024-01-31T05:10:16Z
Lists: pgsql-hackers 
On Wed, Jan 31, 2024 at 2:18 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Jan 31, 2024 at 7:27 AM Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > I saw that v73-0001 was pushed, but it included some last-minute
> > changes that I did not get a chance to check yesterday.
> >
> > Here are some review comments for the new parts of that patch.
> >
> > ======
> > doc/src/sgml/ref/create_subscription.sgml
> >
> > 1.
> > connect (boolean)
> >
> >     Specifies whether the CREATE SUBSCRIPTION command should connect
> > to the publisher at all. The default is true. Setting this to false
> > will force the values of create_slot, enabled, copy_data, and failover
> > to false. (You cannot combine setting connect to false with setting
> > create_slot, enabled, copy_data, or failover to true.)
> >
> > ~
> >
> > I don't think the first part "Setting this to false will force the
> > values ... failover to false." is strictly correct.
> >
> > I think is correct to say all those *other* properties (create_slot,
> > enabled, copy_data) are forced to false because those otherwise have
> > default true values.
> >
>
> So, won't when connect=false, the user has to explicitly provide such
> values (create_slot, enabled, etc.) as false? If so, is using 'force'
> strictly correct?

Perhaps the original docs text could be worded differently; I think
the word "force" here just meant setting connection=false
forces/causes/makes those other options behave "as if" they had been
set to false without the user explicitly doing anything to them. The
point is they are made to behave *differently* to their normal
defaults.

So,
connect=false ==> this actually sets enabled=false (you can see this
with \dRs+), which is different to the default setting of 'enabled'
connect=false ==> will not create a slot (because there is no
connection), which is different to the default behaviour for
'create-slot'
connect=false ==> will not copy tables (because there is no
connection), which is different to the default behaviour for
'copy_data;'

OTOH, failover is different
connect=false ==> failover is not possible (because there is no
connection), but the 'failover' default is false anyway, so no change
to the default behaviour for this one.

>
> > ~~~
> >
> > 2.
> >           <para>
> >            Since no connection is made when this option is
> >            <literal>false</literal>, no tables are subscribed. To initiate
> >            replication, you must manually create the replication slot, enable
> > -          the subscription, and refresh the subscription. See
> > +          the failover if required, enable the subscription, and refresh the
> > +          subscription. See
> >            <xref
> > linkend="logical-replication-subscription-examples-deferred-slot"/>
> >            for examples.
> >           </para>
> >
> > IMO "see the failover if required" is very vague. See what failover?
> >
>
> AFAICS, the committed docs says: "To initiate replication, you must
> manually create the replication slot, enable the failover if required,
> ...". I am not sure what you are referring to.
>

My mistake. I was misreading the patch code without applying the
patch. Sorry for the noise.

> >
> > ======
> > src/bin/pg_upgrade/t/004_subscription.pl
> >
> > 5.
> > -# The subscription's running status should be preserved. Old subscription
> > -# regress_sub1 should be enabled and old subscription regress_sub2 should be
> > -# disabled.
> > +# The subscription's running status and failover option should be preserved.
> > +# Old subscription regress_sub1 should have enabled and failover as true while
> > +# old subscription regress_sub2 should have enabled and failover as false.
> >  $result =
> >    $new_sub->safe_psql('postgres',
> > - "SELECT subname, subenabled FROM pg_subscription ORDER BY subname");
> > -is( $result, qq(regress_sub1|t
> > -regress_sub2|f),
> > + "SELECT subname, subenabled, subfailover FROM pg_subscription ORDER
> > BY subname");
> > +is( $result, qq(regress_sub1|t|t
> > +regress_sub2|f|f),
> >   "check that the subscription's running status are preserved");
> >
> > ~
> >
> > Calling those "old subscriptions" seems misleading. Aren't these the
> > new/upgraded subscriptions being checked here?
> >
>
> Again the quoted wording is not introduced by this patch. But, I see
> your point and it is better if you can start a separate thread for it.
>

OK. I created a separate thread for this [1]

======
[1] https://www.postgresql.org/message-id/CAHut+Pu1usLPHRySPTacY1K_Q-ddSRXNFhmj_2u1NfqBC1ytng@mail.gmail.com

Kind Regards,
Peter Smith.
Fujitsu Australia.

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).