Re: Allow logical failover slots to wait on synchronous replication

On Thu, Sep 19, 2024 at 12:02 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Sep 17, 2024 at 9:08 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Mon, Sep 16, 2024 at 4:04 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Sep 16, 2024 at 2:55 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Mon, Sep 16, 2024 at 11:13 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > >
> > > >
> > > > > Another question aside from the above point, what if someone has
> > > > > specified logical subscribers in synchronous_standby_names? In the
> > > > > case of synchronized_standby_slots, we won't proceed with such slots.
> > > > >
> > > >
> > > > Yes, it is a possibility. I have missed this point earlier. Now I
> > > > tried a case where I give a mix of logical subscriber and physical
> > > > standby in 'synchronous_standby_names' on pgHead, it even takes that
> > > > 'mix' configuration and starts waiting accordingly.
> > > >
> > > > synchronous_standby_names = 'FIRST 2(logicalsub_1, phy_standby_1,
> > > > phy_standby_2)';
> > > >
> > >
> > > This should not happen as we don't support syncing failover slots on
> > > logical subscribers.
> >
> > +1
> >
> > > The other point to consider here is that the user
> > > may not have set 'sync_replication_slots' on all the physical standbys
> > > mentioned in 'synchronous_standby_names' and in that case, it doesn't
> > > make sense to wait for WAL to get flushed on those standbys. What do
> > > you think?
> > >
> >
> > Yes, it is a possibility. But then it is a possibility in case of
> > 'synchronized_standby_slots' as well. User may always configure one of
> > the standbys in 'synchronized_standby_slots' while may not configure
> > slot-sync GUCs on that standby (hot_standby_feedback,
> > sync_replication_slots etc). In such a case, logical replication is
> > dependent upon the concerned physical standby even though latter is
> > not syncing failover slots.
> >
>
> The difference is that the purpose of 'synchronized_standby_slots' is
> to mention slot names for which the user expects logical walsenders to
> wait before sending the logical changes to subscribers. OTOH,
> 'synchronous_standby_names' has a different purpose as well. It is not
> clear to me if the users would be interested in syncing failover slots
> to all the standbys mentioned in 'synchronous_standby_names'.
>

Okay, I see your point. But not sure what could be the solution here
except documenting. But let me think more.

thanks
Shveta