Re: POC: enable logical decoding when wal_level = 'replica' without a server restart - Mailing list pgsql-hackers

On Sat, May 10, 2025 at 1:05 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Sat, May 10, 2025 at 12:00 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> >
> > Right, but to an extent, this is also similar to having a requirement
> > of a logical slot on the primary. Now, it seems to me that the point
> > you are trying to make is that to allow logical decoding on standby,
> > it is okay to ask users to use pg_activate_logical_decoding() on
> > primary, but it would be inconvenient to ask them to have a logical
> > slot on primary instead. If my understanding is correct, then why do
> > you think so? We recommend that users have a physical slot on primary
> > and use it via primary_slot_name on standby to control resource
> > removal, so why can't we ask them to have a logical slot on primary to
> > allow logical decoding on standby?
>
> I was thinking of a simple use case where users do logical decoding
> from the physical standby. That is, the primary has a physical slot
> and the standby uses it via primary_slot_name, and the subscriber
> connects the standby server for logical replication with a logical
> slot on the standby. In this case, IIUC we need to require users to
> create a logical slot on the primary in order just to increase WAL
> level to 'logical', but it doesn't make sense to me. No one is going
> to use this logical slot and the primary ends up accumulating WALs.
>

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.

--
With Regards,
Amit Kapila.