Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, shveta malik <shvetamalik@gmail.com>
Date: 2023-12-12T09:14:28Z
Lists: pgsql-hackers 
On Mon, Dec 11, 2023 at 7:12 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Dec 11, 2023 at 2:41 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > >
> > > 5.
> > > +synchronize_slots(WalReceiverConn *wrconn)
> > > {
> > > ...
> > > ...
> > > + /* The syscache access needs a transaction env. */
> > > + StartTransactionCommand();
> > > +
> > > + /*
> > > + * Make result tuples live outside TopTransactionContext to make them
> > > + * accessible even after transaction is committed.
> > > + */
> > > + MemoryContextSwitchTo(oldctx);
> > > +
> > > + /* Construct query to get slots info from the primary server */
> > > + initStringInfo(&s);
> > > + construct_slot_query(&s);
> > > +
> > > + elog(DEBUG2, "slot sync worker's query:%s \n", s.data);
> > > +
> > > + /* Execute the query */
> > > + res = walrcv_exec(wrconn, s.data, SLOTSYNC_COLUMN_COUNT, slotRow);
> > > + pfree(s.data);
> > > +
> > > + if (res->status != WALRCV_OK_TUPLES)
> > > + ereport(ERROR,
> > > + (errmsg("could not fetch failover logical slots info "
> > > + "from the primary server: %s", res->err)));
> > > +
> > > + CommitTransactionCommand();
> > > ...
> > > ...
> > > }
> > >
> > > Where exactly in the above code, there is a syscache access as
> > > mentioned above StartTransactionCommand()?
> > >
> >
> > It is in walrcv_exec (libpqrcv_processTuples). I have changed the
> > comments to add this info.
> >
>
> Okay, I see that the patch switches context twice once after starting
> the transaction and the second time after committing the transaction,
> why is that required? Also, can't we extend the duration of the
> transaction till the remote_slot information is constructed?

If we extend duration, we have to extend till remote_slot information
is consumed and not only till it is constructed.

> I am
> asking this because the context used is TopMemoryContext which should
> be used only if we need something specific to be retained at the
> process level which doesn't seem to be the case here.
>

Okay, I understand your concern. But this needs more thoughts on shall
we have all the slots synchronized in one txn or is it better to have
it existing way i.e. each slot being synchronized in its own txn
started in synchronize_one_slot. If we go by the former, can it have
any implications? I need to review this bit more before concluding.
.
> I have noticed a few other minor things:
> 1.
> postgres=# select * from pg_logical_slot_get_changes('log_slot_2', NULL, NULL);
> ERROR:  cannot use replication slot "log_slot_2" for logical decoding
> DETAIL:  This slot is being synced from the primary server.
> ...
> ...
> postgres=# select * from pg_drop_replication_slot('log_slot_2');
> ERROR:  cannot drop replication slot "log_slot_2"
> DETAIL:  This slot is being synced from the primary.
>
> I think the DETAIL message should be the same in the above two cases.
>
> 2.
> +void
> +WalSndWaitForStandbyConfirmation(XLogRecPtr wait_for_lsn)
> +{
> + List    *standby_slots;
> +
> + Assert(!am_walsender);
> +
> + if (!MyReplicationSlot->data.failover)
> + return;
> +
> + standby_slots = GetStandbySlotList(true);
> +
> + ConditionVariablePrepareToSleep(&WalSndCtl->wal_confirm_rcv_cv);
> ...
> ...
>
> Shouldn't we return if the standby slot names list is NIL unless there
> is a reason to do ConditionVariablePrepareToSleep() or any of the code
> following it?
>
> --
> 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).