Re: Conflict detection for update_deleted in logical replication

On Wed, Jan 8, 2025 at 2:15 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Tue, Jan 7, 2025 at 2:49 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > We thought of another approach, which is to create/drop this slot first as
> > > soon as one enables/disables detect_update_deleted (E.g. create/drop slot
> > > during DDL). But it seems complicate to control the concurrent slot
> > > create/drop. For example, if one backend A enables detect_update_deteled, it
> > > will create a slot. But if another backend B is disabling the
> > > detect_update_deteled at the same time, then the newly created slot may be
> > > dropped by backend B. I thought about checking the number of subscriptions that
> > > enables detect_update_deteled before dropping the slot in backend B, but the
> > > subscription changes caused by backend A may not visable yet (e.g. not
> > > committed yet).
> > >
> >
> > This means that for the transaction whose changes are not yet visible,
> > we may have already created the slot and the backend B would end up
> > dropping it. Is it possible that during the change of this new option
> > via DDL, we take AccessExclusiveLock on pg_subscription as we do in
> > DropSubscription() to ensure that concurrent transactions can't drop
> > the slot? Will that help in solving the above scenario?
>
> If we create/stop the slot during DDL, how do we support rollback DDLs?
>

We will prevent changing this setting in a transaction block as we
already do for slot related case. See use of
PreventInTransactionBlock() in subscriptioncmds.c.

>
> Thank you for considering some ideas. As I mentioned above, we might
> need to consider a case like where 'CREATE SUBSCRIPTION ..
> (retain_conflict_info = true)' is rolled back. Having said that, this
> comment is just for simplifying the logic. If using TransactionId
> instead makes other parts complex, it would not make sense. I'm okay
> with leaving this part and improving the comment for
> oldest_nonremovable_xid, say, by mentioning that there is a window for
> XID wraparound happening between workers computing their
> oldst_nonremovable_xid and pg_conflict_detection slot being created.
>

Fair enough. Let us see what you think about my above response first.

--
With Regards,
Amit Kapila.