Re: pg_upgrade and logical replication

On Fri, Sep 22, 2023 at 4:36 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Thu, Sep 21, 2023 at 02:35:55PM +0530, Amit Kapila wrote:
> > It is because after upgrade of both publisher and subscriber, the
> > subscriptions won't work. Both publisher and subscriber should work,
> > otherwise, the logical replication set up won't work. I think we can
> > probably do this, if we can document clearly how the user can make
> > their logical replication set up work after upgrade.
>
> Yeah, well, this comes back to my original point that the upgrade of
> publisher nodes and subscriber nodes should be treated as two
> different problems or we're mixing apples and oranges (and a node
> could have both subscriber and publishers). While being able to
> support both is a must, it is going to be a two-step process at the
> end, with the subscribers done first and the publishers done after.
> That's also kind of the point that Julien makes in top message of this
> thread.
>
> I agree that docs are lacking in the proposed patch in terms of
> restrictions, assumptions and process flow, but taken in isolation the
> problem of the publishers is not something that this patch has to take
> care of.
>

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.

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.

--
With Regards,
Amit Kapila.