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: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@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: 2024-02-12T14:06:03Z
Lists: pgsql-hackers 
Hi,

On Mon, Feb 12, 2024 at 04:19:33PM +0530, Amit Kapila wrote:
> On Mon, Feb 12, 2024 at 3:33 PM Bertrand Drouvot
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > A few random comments:
> >
> >
> > 003 ===
> >
> > +          If, after executing the function,
> > +          <link linkend="guc-hot-standby-feedback">
> > +          <varname>hot_standby_feedback</varname></link> is disabled on
> > +          the standby or the physical slot configured in
> > +          <link linkend="guc-primary-slot-name">
> > +          <varname>primary_slot_name</varname></link> is
> > +          removed,
> >
> > I think another option that could lead to slot invalidation is if primary_slot_name
> > is NULL or miss-configured.
> >
> 
> If the primary_slot_name is NULL then the function will error out.

Yeah right, it had to be non NULL initially so we know there is a physical slot (if 
not dropped) that should prevent conflicts at the first place (should hsf be on).
Please forget about comment 003 then.

> >
> > 005 ===
> >
> > +     To resume logical replication after failover from the synced logical
> > +     slots, the subscription's 'conninfo' must be altered
> >
> > Only in a pub/sub context but not for other ways of using the logical replication
> > slot(s).
> >
> 
> Right, but what additional information do you want here? I thought we
> were speaking about the in-build logical replication here so this is
> okay.

The "Logical Decoding Concepts" sub-chapter also mentions "Logical decoding clients"
so I was not sure the part added in the patch was for in-build logical replication
only.

Or maybe just reword that way "In case of in-build logical replication, to resume
after failover from the synced......"?

> 
> >
> > 008 ===
> >
> > +                       ereport(LOG,
> > +                                       errmsg("dropped replication slot \"%s\" of dbid %d",
> > +                                                  NameStr(local_slot->data.name),
> > +                                                  local_slot->data.database));
> >
> > We emit a message when an "invalidated" slot is dropped but not when we create
> > a slot. Shouldn't we emit a message when we create a synced slot on the standby?
> >
> > I think that could be confusing to see "a drop" message not followed by "a create"
> > one when it's expected (slot valid on the primary for example).
> >
> 
> Isn't the below message for sync-ready slot sufficient? Otherwise, in
> most cases, we will LOG multiple similar messages.
> 
> + ereport(LOG,
> + errmsg("newly created slot \"%s\" is sync-ready now",
> +    remote_slot->name));

Yes it is sufficient if we reach it. For example during some test, I was able to
go through this code path:

Breakpoint 2, update_and_persist_local_synced_slot (remote_slot=0x56450e7c49c0, remote_dbid=5) at slotsync.c:340
340             ReplicationSlot *slot = MyReplicationSlot;
(gdb) n
346             if (remote_slot->restart_lsn < slot->data.restart_lsn ||
(gdb)
347                     TransactionIdPrecedes(remote_slot->catalog_xmin,
(gdb)
346             if (remote_slot->restart_lsn < slot->data.restart_lsn ||
(gdb)
358                     return;

means exiting from update_and_persist_local_synced_slot() without reaching the
"newly created slot" message (the slot on the primary was "inactive").

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).