Re: Synchronizing slots from primary to standby

On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 6/19/23 12:03 PM, Amit Kapila wrote:
> > On Mon, Jun 19, 2023 at 11:34 AM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> Also I think we need to handle the case of invalidated replication slot(s): should
> >> we drop/recreate it/them? (as the main goal is to have sync slot(s) on the standby).
> >>
> >
> > Do you intend to ask what happens to logical slots invalidated (due to
> > say max_slot_wal_keep_size) on publisher? I think those should be
> > invalidated on standby too.
>
> Agree that it should behave that way.
>
> > Another thought whether there is chance
> > that the slot on standby gets invalidated due to conflict (say
> > required rows removed on primary)?
>
> That's the scenario I had in mind when asking the question above.
>
> > I think in such cases the slot on
> > primary/publisher should have been dropped/invalidated by that time.
>
> I don't think so.
>
> For example, such a scenario could occur:
>
> - there is no physical slot between the standby and the primary
> - the standby is shut down
> - logical decoding on the primary is moving forward and now there is vacuum
> operations that will conflict on the standby
> - the standby starts and reports the logical slot being invalidated (while it is
> not on the primary)
>
> In such a case (slot valid on the primary but invalidated on the standby) then I think we
> could drop and recreate the invalidated slot on the standby.
>

Will it be safe? Because after recreating the slot, it will reserve
the new WAL location and build the snapshot based on that which might
miss some important information in the snapshot. For example, to
update the slot's position with new information from the primary, the
patch uses pg_logical_replication_slot_advance() which means it will
process all records and update the snapshot via
DecodeCommit->SnapBuildCommitTxn().

The other related thing is that do we somehow need to ensure that WAL
is replayed on standby before moving the slot's position to the target
location received from the primary?

> > BTW, does the patch handles drop of logical slots on standby when the
> > same slot is dropped on publisher/primary?
> >
>
> from what I've seen, yes it looks like it behaves that way (will look closer).
>

Okay, I have asked because I don't see a call to ReplicationSlotDrop()
in the patch.

--
With Regards,
Amit Kapila.