Re: [17] CREATE SUBSCRIPTION ... SERVER

On Sat, Sep 2, 2023 at 12:24 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>
> On Fri, 2023-09-01 at 12:28 +0530, Ashutosh Bapat wrote:
> > Thinking larger, how about we allow any FDW to be used here.
>
> That's a possibility, but I think that means the subscription would
> need to constantly re-check the parameters rather than relying on the
> FDW's validator.
>
> Otherwise it might be the wrong kind of FDW, and the user might be able
> to circumvent the password_required protection. It might not even be a
> postgres-related FDW at all, which would be a bit strange.
>
> If it's constantly re-checking the parameters then it raises the
> possibility that some "ALTER SERVER" or "ALTER USER MAPPING" succeeds
> but then subscriptions to that foreign server start failing, which
> would not be ideal. But I could be fine with that.

Why do we need to re-check parameters constantly? We will need to
restart subscriptions which are using the user mapping of FDW when
user mapping or server options change. If that mechanism isn't there,
we will need to build it. But that's doable.

I didn't understand your worry about circumventing password_required protection.

>
> > But I think there's some value in bringing
> > together these two subsystems which deal with foreign data logically
> > (as in logical vs physical view of data).
>
> I still don't understand how a core dependency on an extension would
> work.

We don't need to if we allow any FDW (even if non-postgreSQL) to be
specified there. For non-postgresql FDW the receiver will need to
construct the appropriate command and use appropriate protocol to get
the changes and apply locally. The server at the other end may not
even have logical replication capability. The extension or "input
plugin" (as against output plugin) would decide whether it can deal
with the foreign server specific logical replication protocol. We add
another callback to FDW which can return whether the given foreign
server supports logical replication or not. If the users
misconfigured, their subscriptions will throw errors.

But with this change we open a built-in way to "replicate in" as we
have today to "replicate out".

--
Best Wishes,
Ashutosh Bapat