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

On Wed, Oct 15, 2025 at 9:50 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> 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.
>

Do you think we can use PMSIGNAL_BACKGROUND_WORKER_CHANGE for this
reason? Otherwise, I see different processes have separate reasons
like PMSIGNAL_START_AUTOVAC_LAUNCHER, PMSIGNAL_START_AUTOVAC_WORKER,
or PMSIGNAL_START_WALRECEIVER to signal postmaster for similar
purpose.

--
With Regards,
Amit Kapila.