Re: pg_upgrade and logical replication

On Mon, Sep 25, 2023 at 10:05:41AM +0530, Amit Kapila wrote:
> I also don't think that this patch has to solve the problem of
> publishers in any way but as per my understanding, if due to some
> reason we are not able to do the upgrade of publishers, this can add
> more steps for users than they have to do now for logical replication
> set up after upgrade. This is because now after restoring the
> subscription rel's and origin, as soon as we start replication after
> creating the slots on the publisher, we will never be able to
> guarantee data consistency. So, they need to drop the entire
> subscription setup including truncating the relations, and then set it
> up from scratch which also means they need to somehow remember or take
> a dump of the current subscription setup. According to me, the key
> point is to have a mechanism to set up slots correctly to allow
> replication (or subscriptions) to work after the upgrade. Without
> that, it appears to me that we are restoring a subscription where it
> can start from some random LSN and can easily lead to data consistency
> issues where it can miss some of the updates.

Sure, that's assuming that the publisher side is upgraded. FWIW, my
take is that there's room to move forward with this patch anyway in
favor of cases like rollover upgrades to the subscriber.

> This is the primary reason why I prioritized to work on the publisher
> side before getting this patch done, otherwise, the solution for this
> patch was relatively clear. I am not sure but I guess this could be
> the reason why originally we left it in the current state, otherwise,
> restoring subscription rel's or origin doesn't seem to be too much of
> an additional effort than what we are doing now.

By "additional effort", you are referring to what the patch is doing,
with the binary dump of pg_subscription_rel, right?
--
Michael