Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@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: 2024-02-26T07:29:07Z
Lists: pgsql-hackers 
Hi,

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.

> > 
> > 7 ===
> > 
> > + * Return a copy of standby_slot_names_list if the copy flag is set to
> > + true,
> > 
> > Not sure, but would it be worth explaining why one would want to set to flag to
> > true or false? (i.e why one would not want to receive the original list).
> 
> I think the usage can be found from the caller's code, e.g we need to remove
> the slots that caught up from the list each time, so we cannot directly modify
> the global list. The GetStandbySlotList function is general function and I feel
> we can avoid adding more comments here.

Okay, yeah makes sense.

> > 
> > 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].

I see. But should'nt we add a check on restart_lsn as this is done here in 
pg_get_replication_slots()?

"
case WALAVAIL_REMOVED:


/*
* If we read the restart_lsn long enough ago, maybe that file
* has been removed by now. However, the walsender could have
* moved forward enough that it jumped to another file after
* we looked. If checkpointer signalled the process to
* termination, then it's definitely lost; but if a process is
* still alive, then "unreserved" seems more appropriate.

if (!XLogRecPtrIsInvalid(slot_contents.data.restart_lsn))

"

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).

Currently we are not waiting for this slot while it's still serving the standby
which does not seem good too, thoughts?

> Attach the V98 patch set which addressed above comments.
> I also adjusted few comments based on off-list comments from Shveta.

Thanks!

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).