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

On Fri, Oct 17, 2025 at 8:55 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Oct 16, 2025 at 11:10 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Thu, Oct 16, 2025 at 1:41 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> >
> > Using PMSIGNAL_BACKGROUND_WORKER_CHANGE sounds mis-using since the
> > slotsync worker is not a background worker nor logical decoding
> > activation is not related to bgworkers.
> >
> > An alternative idea is to launch the slotsync worker if wal_level
> > value on the standby is >=replica, that is, always launch it on the
> > standby if sync_replication_slots is on. Even with and without the
> > patch, we don't shutdown the slotsync worker even if logical decoding
> > gets disabled on the standby.
> >
>
> Are you talking about the case when wal_level on primary has reduced
> below logical and user will get the following message on standby:
> "logical decoding on standby requires \"wal_level\" >= \"logical\" on
> the primary"? If so, the slight difference in this case is that
> standby still has wal_level logical.
>

I believe what Sawada-san meant is that even when effective_wal_level
= replica on a standby, we should still allow the slot-sync worker to
start if 'sync_replication_slots' is enabled. This is because we
currently do not stop the worker when effective_wal_level on the
standby changes from logical to replica, so allowing it to start in
this case maintains consistent behavior. That said, my preference is
to not start the slot-sync worker if effective_wal_level is less than
logical. As I understand, this is already the behavior implemented in
the current patch.

Regarding the scenario where effective_wal_level changes from logical
to replica on a standby, my vote is to explicitly shut down the
slot-sync worker in such cases. I don't see any benefit in keeping it
running. But this can be handled in a separate patch as it is not
directly concerned with this patch.

Next is, when effective_wal_level changes from replica to logical,
should we wake up the postmaster to immediately start the slot-sync
worker? My vote is yes, but if implementing this introduces too much
complexity, especially considering it's a rare scenario, we could
leave it as is. In that case, the slot-sync worker would still start,
but possibly with a delay of up to 1-2 minutes when the postmaster is
sleeping.

thanks
Shveta