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

On Wed, Oct 15, 2025 at 3:56 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Oct 14, 2025 at 11:40 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Tue, Oct 14, 2025 at 2:24 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > Thanks. I was testing slot-sync worker flow and noticed that even if
> > > 'sync_replication_slots' is enabled, it takes quite some time (1-2
> > > mins) for slot sync worker to start. On noticing
> > > XLOG_LOGICAL_DECODING_STATUS_CHANGE, isn't there a way to (or
> > > shouldn't we) wakeup postmaster to start slotsync worker immediately?
> > >
> > > Logs for timestamps reference:
> > >
> > > 2025-10-14 14:41:32.852 IST [208475] LOG: replication slot
> > > synchronization requires "effective_wal_level" >= "logical" on the
> > > primary
> > > 2025-10-14 14:41:32.852 IST [208475] HINT: To enable logical decoding
> > > on primary, set "wal_level" >= "logical" or create at least one
> > > logical slot when "wal_level" = "replica".
> > > 2025-10-14 14:41:35.499 IST [208481] LOG: update logical decoding status to 1
> > > 2025-10-14 14:41:35.499 IST [208481] CONTEXT: WAL redo at 0/03000060
> > > for XLOG/LOGICAL_DECODING_STATUS_CHANGE: true
> > > 2025-10-14 14:42:53.611 IST [208529] LOG: slot sync worker started
> > > ------
> >
> > I think that this particular situation happened because the postmaster
> > was sleeping and didn't have any tasks. If a new connection comes in
> > or the existing connection exits, the postmaster launches the slotsync
> > worker. I believe it would not be a serious issue in practice.
> >
>
> Right, but OTOH, do we see any harm if we wake up the postmaster on
> such an event? It shouldn't happen frequently to matter either way
> though.

It's not harmful in practice. I've considered waking up the postmaster
with a new PMSignalReason, but I thought it's overkill to introduce a
new reason just to wake up the postmaster. If there is a convenient
way, I'd like to use that.

>
> > > There was a lot of discussion regarding slotsync worker in the past,
> > > please let me know if it is concluded and I missed it somehow.
> >
> > Since we've discussed that it's out of scope of this patch that we
> > shutdown dynamically upon logical decoding status changes, even if
> > logical decoding status gets disabled on the standby, the slotsync
> > worker keeps working. If it gets enabled again, we don't need to wake
> > up the postmaster as the slotsync worker would already be running. I
> > think we can introduce more dynamic control over the slotsync worker
> > in a separate patch if we need.
> >
>
> I think it is better to stop the slotsync worker at this event because
> without a patch we won't even allow the server to restart if the
> wal_level on standby is less than logical. But, I agree it could be
> done as a separate patch as well.

Agreed. I'll implement it in a follow-up patch.

Regards,

--
Masahiko Sawada
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