Horton

Re: Synchronizing slots from primary to standby

Masahiko Sawada <sawada.mshk@gmail.com>

View thread View on postgr.es
From: Masahiko Sawada <sawada.mshk@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: 2024-02-20T12:49:08Z
Lists: pgsql-hackers 
On Tue, Feb 20, 2024 at 12:33 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Tue, Feb 20, 2024 at 8:25 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> >
> > I've reviewed the v91 patch. Here are random comments:
>
> Thanks for the comments.
>
> > ---
> >  /*
> >   * Checks the remote server info.
> >   *
> > - * We ensure that the 'primary_slot_name' exists on the remote server and the
> > - * remote server is not a standby node.
> > + * Check whether we are a cascading standby. For non-cascading standbys, it
> > + * also ensures that the 'primary_slot_name' exists on the remote server.
> >   */
> >
> > IIUC what the validate_remote_info() does doesn't not change by this
> > patch, so the previous comment seems to be clearer to me.
> >
> > ---
> >     if (remote_in_recovery)
> > +   {
> > +       /*
> > +        * If we are a cascading standby, no need to check further for
> > +        * 'primary_slot_name'.
> > +        */
> >         ereport(ERROR,
> >                 errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
> >                 errmsg("cannot synchronize replication slots from a
> > standby server"));
> > +   }
> > +   else
> > +   {
> > +       bool        primary_slot_valid;
> >
> > -   primary_slot_valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
> > -   Assert(!isnull);
> > +       primary_slot_valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
> > +       Assert(!isnull);
> >
> > -   if (!primary_slot_valid)
> > -       ereport(ERROR,
> > -               errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> > -               errmsg("bad configuration for slot synchronization"),
> > -       /* translator: second %s is a GUC variable name */
> > -               errdetail("The replication slot \"%s\" specified by
> > \"%s\" does not exist on the primary server.",
> > -                         PrimarySlotName, "primary_slot_name"));
> > +       if (!primary_slot_valid)
> > +           ereport(ERROR,
> > +                   errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> > +                   errmsg("bad configuration for slot synchronization"),
> > +           /* translator: second %s is a GUC variable name */
> > +                   errdetail("The replication slot \"%s\" specified
> > by \"%s\" does not exist on the primary server.",
> > +                             PrimarySlotName, "primary_slot_name"));
> > +   }
> >
> > I think it's a refactoring rather than changes required by the
> > slotsync worker. We can do that in a separate patch but why do we need
> > this change in the first place?
>
> In v90, this refactoring was made due to the fact that
> validate_remote_info() was supposed to behave differently for SQL
> function and slot-sync worker. SQL-function was supposed to ERROR out
> while the worker was supposed to LOG and become no-op. And thus the
> change was needed. In v91, we made this functionality same i.e. both
> sql function and worker will error out but missed to remove this
> refactoring. Thanks for catching this, I will revert it in the next
> version. To match the refactoring, I made the comment change too, will
> revert that as well.
>
> > ---
> > +        ValidateSlotSyncParams(ERROR);
> > +
> >          /* Load the libpq-specific functions */
> >          load_file("libpqwalreceiver", false);
> >
> > -        ValidateSlotSyncParams();
> > +        (void) CheckDbnameInConninfo();
> >
> > Is there any reason why we move where to check the parameters?
>
> Earlier DBname verification was done inside ValidateSlotSyncParams()
> and thus it was needed to load 'libpqwalreceiver' before we call this
> function. Now we have moved dbname verification in a separate call and
> thus the above order got changed. ValidateSlotSyncParams() is a common
> function used by SQL function and worker. Earlier slot sync worker was
> checking all the GUCs after starting up and was exiting each time any
> GUC was invalid. It was suggested that it would be better to check the
> GUCs before starting the slot sync worker in the postmaster itself,
> making the ValidateSlotSyncParams() move to postmaster (see
> SlotSyncWorkerAllowed).  But it was not a good idea to load libpq in
> postmaster and thus we moved libpq related verification out of
> ValidateSlotSyncParams(). This resulted in the above change.  I hope
> it answers your query.

Thank you for the explanation. It makes sense to me to move the check.

As for ValidateSlotSyncParams() called by SlotSyncWorkerAllowed(), I
have two comments:

1. The error messages are not very descriptive and seem not to match
other messages the postmaster says. When starting the standby server
with misconfiguration about the slotsync, I got the following messages
from the postmaster:

2024-02-20 17:01:16.356 JST [456741] LOG:  database system is ready to
accept read-only connections
2024-02-20 17:01:16.358 JST [456741] LOG:  bad configuration for slot
synchronization
2024-02-20 17:01:16.358 JST [456741] HINT:  "hot_standby_feedback"
must be enabled.

It says "bad configuration" but is still working, and does not say
further information such as whether it skipped to start the slotsync
worker etc. I think these messages could work for the slotsync worker
but we might want to have more descriptive messages for the
postmaster. For example, "skipped starting slot sync worker because
hot_standby_feedback is disabled".

2. If the wal_level is not logical, the server will need to restart
anyway to change the wal_level and have the slotsync worker work. Does
it make sense to have the postmaster exit if the wal_level is not
logical and sync_replication_slots is enabled? For instance, we have
similar checks in PostmsaterMain():

    if (summarize_wal && wal_level == WAL_LEVEL_MINIMAL)
        ereport(ERROR,
                (errmsg("WAL cannot be summarized when wal_level is
\"minimal\"")));

Regards,

--
Masahiko Sawada
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).