Re: Logical Replication of sequences - Mailing list pgsql-hackers

On Tue, Aug 6, 2024 at 9:54 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Aug 6, 2024 at 8:49 AM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > > > > I was reviewing the design of patch003, and I have a query. Do we need
> > > > > to even start an apply worker and create replication slot when
> > > > > subscription created is for 'sequences only'? IIUC, currently logical
> > > > > replication apply worker is the one launching sequence-sync worker
> > > > > whenever needed. I think it should be the launcher doing this job and
> > > > > thus apply worker may even not be needed for current functionality of
> > > > > sequence sync?
> > > >
> > >
> > > But that would lead to maintaining all sequence-sync of each
> > > subscription by launcher. Say there are 100 sequences per subscription
> > > and some of them from each subscription are failing due to some
> > > reasons then the launcher will be responsible for ensuring all the
> > > sequences are synced. I think it would be better to handle
> > > per-subscription work by the apply worker.
> >
> > I thought we can give that task to sequence-sync worker. Once sequence
> > sync worker is started by launcher, it keeps on syncing until all the
> > sequences are synced (even failed ones) and then exits only after all
> > are synced; instead of apply worker starting it multiple times for
> > failed sequences. Launcher to start sequence sync worker when signaled
> > by 'alter-sub refresh seq'.
> > But after going through details given by Vignesh in [1], I also see
> > the benefits of using apply worker for this task. Since apply worker
> > is already looping and doing that for table-sync, we can reuse the
> > same code for sequence sync and maintenance will be easy. So looks
> > okay if we go with existing apply worker design.
> >
>
> Fair enough. However, I was wondering whether apply_worker should exit
> after syncing all sequences for a sequence-only subscription

If apply worker exits, then on next sequence-refresh, we need a way to
wake-up launcher to start apply worker which then will start
table-sync worker. Instead, won't it be better if the launcher starts
table-sync worker directly without the need of apply worker being
present (which I stated earlier).

>  or should
> it be there for future commands that can refresh the subscription and
> add additional tables or sequences?

If we stick with apply worker starting table sync worker when needed
by continuously checking seq-sync states ('i'/'r'), then IMO, it is
better that apply-worker stays. But if we want  apply-worker to exit
and start only when needed, then why not to start sequence-sync worker
directly for seq-only subscriptions?

thanks
Shveta