Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2024-02-26T11:48:25Z
Lists: pgsql-hackers 
On Mon, Feb 26, 2024 at 12:59 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Mon, Feb 26, 2024 at 02:18:58AM +0000, Zhijie Hou (Fujitsu) wrote:
> > On Friday, February 23, 2024 6:12 PM Bertrand Drouvot <bertranddrouvot.pg@gmail.com> wrote:
> > > +       if (!ok)
> > > +               GUC_check_errdetail("List syntax is invalid.");
> > > +
> > > +       /*
> > > +        * If there is a syntax error in the name or if the replication slots'
> > > +        * data is not initialized yet (i.e., we are in the startup process), skip
> > > +        * the slot verification.
> > > +        */
> > > +       if (!ok || !ReplicationSlotCtl)
> > > +       {
> > > +               pfree(rawname);
> > > +               list_free(elemlist);
> > > +               return ok;
> > > +       }
> > >
> > > we are testing the "ok" value twice, what about using if...else if... instead and
> > > test it once? If so, it might be worth to put the:
> > >
> > > "
> > > +       pfree(rawname);
> > > +       list_free(elemlist);
> > > +       return ok;
> > > "
> > >
> > > in a "goto".
> >
> > There were comments to remove the 'goto' statement and avoid
> > duplicate free code, so I prefer the current style.
>
> The duplicate free code would come from the if...else if... rewrite but then
> the "goto" would remove it, so I'm not sure to understand your point.
>

I think Hou-San wants to say that there was previously a comment to
remove goto and now you are saying to introduce it. But, I think we
can avoid both code duplication and goto, if the first thing we check
in the function is ReplicationSlotCtl and return false if the same is
not set. Won't that be better?

>
> > >
> > > 10 ===
> > >
> > > +                       if (slot->data.invalidated != RS_INVAL_NONE)
> > > +                       {
> > > +                               /*
> > > +                                * Specified physical slot have been invalidated,
> > > so no point
> > > +                                * in waiting for it.
> > >
> > > We discovered in [1], that if the wal_status is "unreserved" then the slot is still
> > > serving the standby. I think we should handle this case differently, thoughts?
> >
> > I think the 'invalidated' slot can still be used is a separate bug.
> > Because
> > once the slot is invalidated, it can neither protect WALs or ROWs from being
> > removed even if the restart_lsn of the slot can be moved forward after being invalidated.
> >
> > If the standby can move restart_lsn forward for invalidated slots, then
> > it should also set the 'invalidated' flag back to NONE, otherwise the slot
> > cannot serve its purpose anymore. I also reported similar bug before[1].
>
...
>
> My point is that I think we should behave like it's not a bug and then adapt the
> code accordingly here (until the bug gets fixed).
>

oh, I think this doesn't sound like a good idea to me. We should fix
that bug independently rather than adding code in new features to
consider the bug as a valid behavior. It will add the burden on us to
remember and remove the additional new check(s).

-- 
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).