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

On Fri, Jul 18, 2025 at 3:03 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jul 18, 2025 at 2:27 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Thu, Jul 17, 2025 at 3:06 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Jun 18, 2025 at 3:23 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Wed, Jun 18, 2025 at 2:39 PM Bertrand Drouvot
> > > > <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > On Tue, Jun 10, 2025 at 02:00:55PM -0700, Masahiko Sawada wrote:
> > > > > > > > > > > 0001 patch allows us to create a logical slot without WAL reservation.
> > > > >
> > > > > Thanks for the patch and sorry to be late in this conversation.
> > > > >
> > > > > The thing that worry me a bit with this is that that could be easy to attempt
> > > > > to use the slot "by mistake" and then (as a consequence) trigger WAL reservation
> > > > > by mistake on the primary. I think that this mistake is more likely to happen
> > > > > with a logical slot as compared to a physical slot.
> > > > >
> > > >
> > > > Yes, agreed. Another concern is the possibility of someone
> > > > intentionally using it and associating it with a subscription. If the
> > > > subscription is later dropped, it could also cause the slot to be
> > > > removed.
> > > >
> > >
> > > IIUC, your main concern here is that the last slot on
> > > primary/publisher could be dropped unintentionally by the user leading
> > > to invalidating the logical slots on any physical-standby connected
> > > with publisher, right?
> > >
> >
> > Right.
> >
> > > What if we make DROP SUBSCRIPTION fail if it can lead to removal of
> > > the last slot on publisher and allow DROP to succeed when the
> > > subscription's drop_slot_force (a new subscription option) is set?
> > > Now, users can still be allowed to Drop the subscription, if it
> > > disassociates the subscription from the slot by using method explained
> > > in docs [1] (See Notes section). Similarly when a user is trying to
> > > drop the last logical slot via pg_drop_replication_slot, we will allow
> > > it only with the force option. This should ensure that the user is
> > > aware of the consequences of dropping the last slot.
> > >
> >
> > One concern I have is regarding the default setting of
> > 'force_slot_drop' . I assume the default value of this new DROP-SUB
> > argument will be 'false' to prevent customers from inadvertently
> > dropping the last slot on the publisher. But, would this be
> > acceptable, considering that users may have DROP-SUBSCRIPTION commands
> > in their scripts which would suddenly stop dropping slot now?
> >
>
> That would only happen when users use this new idea of enabling
> wal_level to 'logical' on the fly. I think the users having existing
> setups with pub-sub would have kept the default wal_level to 'logical'
> on publisher.
>

Okay, but then we will have to avoid doing the enhancement of getting
rid of wal_level='logical' as suggested in [1].

Even if we do so, I am not very much convinced for this argument and its value.
--The value of ''force_slot_drop" will hold its meaning only in a
conditional scenario. Assuming default is false, then it will still
drop the slots until it is last slot and wal_level < logical on
primary. This behavior can seem a bit unintuitive or confusing from
the user's perspective.
--If the user is trying to actually retain the slot by giving
force_slot_drop=false , then how are we going to track that i.e.
distinguish from its default.

Bertrand has proposed a similar design in [2]. We can revisit that as well once.

I'm continuing to think it through and will share any further thoughts
if something comes to mind.

[1]: https://www.postgresql.org/message-id/CAD21AoD5aONyxZHGG5-gQhQnAMuF9dByLn0%2BtreF8cRT06bqkA%40mail.gmail.com
[2]: https://www.postgresql.org/message-id/aFPRqXR41xOhE597%40ip-10-97-1-34.eu-west-3.compute.internal

thanks
Shveta