Re: Issue with logical replication slot during switchover

On Wed, Nov 19, 2025 at 6:26 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Nov 18, 2025 at 1:30 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Sat, Nov 15, 2025 at 4:02 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Nov 14, 2025 at 2:39 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > >
> > > > The point is quite fundamental, do you think we can sync to a
> > > > pre-existing slot with the same name and failover marked as true after
> > > > the first time the node joins a new primary?
> > >
> > > Given the current behavior that we cannot create a logical slot with
> > > failover=true on the standby, it makes sense to me that we overwrite
> > > the pre-existing slot (with synced=false and failover=true) on the old
> > > primary by the slot (with synced=true and failover=true) on the new
> > > primary if their names, plugin and other properties matches and the
> > > pre-existing slot has lesser LSNs and XIDs than the one on the new
> > > primary. But at the same time, we need to consider the possible future
> > > changes that allow users to create a slot with failover=true also on
> > > the standby.
> > >
> > > Alexander pointed out[1] that allowing to create a slot with
> > > failover=true on the standby won't work with the current
> > > implementation. I agree with his analysis, and I guess we would need
> > > more changes than simply allowing it, regardless of accepting the
> > > proposed change. We might need to introduce a replication slot origin
> > > or a generation.
> > >
> >
> > AFAICS, the email you pointed out wrote about use cases, not the
> > actual code implementation. We can discuss use cases if we want to
> > pursue that implementation, but the reason why we decided not to allow
> > it was for the cases where users try to configure cascaded standbys to
> > also try to sync slots from the first standby that are already being
> > synced from the primary. There are quite a few technical challenges in
> > supporting that, like how to make sure primary waits even for cascaded
> > standbys before sending the changes to logical subscribers.
>
> Right. My point is that these are two independent issues. The fact
> that creating a slot with failover=true directly on a standby is
> difficult (due to the cascaded-standby cases you mentioned) does not,
> by itself, justify allowing us to overwrite an existing slot with
> failover=true and synced=false during slot synchronization.
>
> > OTOH, for the cases where there is a totally different logical slot on
> > standby (not present on primary) with failover=true, we can allow it
> > to be synced from standby-1 to a cascaded standby, though we need some
> > way to distinguish those cases. For example, during sync on cascaded
> > standby, we can ensure that the slot being synced is not a sync-slot
> > (failover=true and sync=true).
>
> Yes. We need some way to distinguish those slots, otherwise if users
> create a slot with the same name on the primary, the slot on standby-1
> (a cascading standby) could be overwritten. I think we would need some
> additional metadata per slot to support that safely.
>

It shouldn't be overwritten as of current HEAD because we give ERROR
if such a slot exists on standby but if we allow overwriting it as
being discussed here then we would require additional metadata per
slot to distinguish this case.

--
With Regards,
Amit Kapila.