Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

On Sun, May 18, 2025 at 1:09 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Sat, May 10, 2025 at 7:08 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> >
> > Can we have a parameter like immediately_reserve in
> > create_logical_slot API, similar to what we have for physical slots?
> > We need to work out the details, but that should address the kind of
> > use case you are worried about, unless I am missing something.
>
> Interesting idea. One concern in my mind is that in the use case I
> mentioned above, users would need to carefully manage the extra
> logical slot to keep the logical decoding active. The logical decoding
> is deactivated on the standby as soon as users drop all logical slots
> on the primary.
>

Yes, but the same is true for a physical slot in the case of physical
replication used via primary_slot_name parameter.

> Also, with this idea of automatically increasing WAL level, do we want
> to keep the 'logical' WAL level? If so, it requires an extra step of
> creating a non-reserved logical slot on the primary in order for the
> standby to activate the logical decoding. On the other hand, we can
> also keep the 'logical' WAL level for the compatibility and for making
> the logical decoding enabled without the coordination of WAL level
> transition.

Right, I also feel we should retain both ways to enable logical
replication at least initially. Once we get some feedback, we may
think of removing 'logical' as wal_level.

> But wal_level GUC parameter would no longer tell the
> actual WAL level to users when 'replica' + logical slots.
>

Right.

> Is it
> sufficient to provide a read-only GUC parameter, say
> effective_wal_level showing the actual WAL level being used?
>

I am not so sure about how we want to communicate this to the user,
but I guess to start with, this is a good idea.

--
With Regards,
Amit Kapila.