Re: Allow logical failover slots to wait on synchronous replication

On Fri, Jul 19, 2024 at 2:52 AM John H <johnhyvr(at)gmail(dot)com> wrote:
>
> Hi Shveta,
>
> Thanks for taking a look at the patch.
>
> > > will leave user no option to unlink failover-enabled logical
> > > subscribers from the wait on synchronous standbys.
>
> That's a good point. I'm a bit biased in that I don't think there's a
> great reason why someone would
> want to replicate logical changes out of the synchronous cluster
> without it having been synchronously replicated
> but yes this would be different behavior compared to strictly the slot one.
>
> > ...
> > So when 'synchronized_standby_slots' is comma separated list, we pick
> > those slots; if it is empty, then no wait on standbys, and if its
> > value is 'DEFAULT' as configured by user, then go with
> > 'synchronous_standby_names'. Thoughts?
>
> I think I'd prefer having a separate GUC if the alternative is to reserve
> special keywords in 'synchronized_standby_slots' but I'm not sure if I
> feel strongly about that.

My only concern is, earlier we provided a way to set the failover
property of slots even without mandatorily wait on physical standbys.
But now we will be changing this behaviour. Okay, we can see what
other comments. If we plan to go this way, we can change docs to
clearly mention this.

> > > 2)
> > > When 'synchronized_standby_slots' is configured but standby named in
> > > it is down blocking logical replication, then we get a WARNING in
> > > subscriber's log file:
> > >
> > > WARNING: replication slot "standby_2" specified in parameter
> > > synchronized_standby_slots does not have active_pid.
> > > DETAIL: Logical replication is waiting on the standby associated with
> > > "standby_2".
> > > HINT: Consider starting standby associated with "standby_2" or amend
> > > parameter synchronized_standby_slots.
> > >
> > > But OTOH, when 'synchronous_standby_names' is configured instead of
> > > 'synchronized_standby_slots' and any of the standbys listed is down
> > > blocking logical replication, we do not get any sort of warning. It is
> > > inconsistent behavior. Also user might be left clueless on why
> > > subscribers are not getting changes.
>
> Ah that's a gap. Let me add some logging/warning in a similar fashion.
> Although I think I'd have the warning be relatively generic (e.g.
> changes are blocked because
> they're not synchronously committed)
>

okay, sounds good.

thanks
Shveta