Re: Allow logical failover slots to wait on synchronous replication

Hi Shveta,

On Sun, Jul 21, 2024 at 8:42 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:

> > 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

I took a look at having similar warnings the existing
'synchronized_standby_slots' feature has
and I don't think it's particularly feasible.

The checks/warnings for 'synchronized_standby_slots' are intended to
protect against misconfiguration.
They consist of slot validation (valid slot_name, not logical slot,
slot has not been invalidated), and
whether or not the slot is active.

I don't think there's a "misconfiguration" equivalent for waiting on
synchronous_commit.
With the current proposal, once you have (synchronous_commit enabled
&& failover_slots), logical
decoding is dependent on whether or not the writes have been
replicated to a synchronous replica.
If there is no data being replicated out of the logical slot, it is
because from the perspective of the
database no writes have been committed yet. I don't think it would
make sense to add logging/warning as to
why a transaction is still not committed (e.g. which potential replica
is the one lagging). There isn't a
nice way to determine why synchronous commit is waiting without being
particularly invasive, and even then
it could be particularly noisy (e.g. provide all the application_names
that we are potentially waiting on).

Thanks,

--
John Hsu - Amazon Web Services