Re: Identify missing publications from publisher while create/alter subscription.

On Mon, Jan 25, 2021 at 3:07 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > So basically, the create subscription will throw an error if the
> > > publication does not exist. So will you throw an error if we try to
> > > drop the publication which is subscribed by some subscription? I mean
> > > basically, you are creating a dependency that if you are creating a
> > > subscription then there must be a publication that is not completely
> > > insane but then we will have to disallow dropping the publication as
> > > well. Am I missing something?
> >
> > Do you mean DROP PUBLICATION non_existent_publication;?
> >
> > Or
> >
> > Do you mean when we drop publications from a subscription?
>
> I mean it doesn’t seem right to disallow to create the subscription if
> the publisher doesn't exist, and my reasoning was even though the
> publisher exists while creating the subscription you might drop it
> later right?. So basically, now also we can create the same scenario
> that a subscription may exist for the publication which does not
> exist.

Yes, the above scenario can be created even now. If a publication is
dropped in the publisher system, then it will not replicate/publish
the changes for that publication (publication_invalidation_cb,
rel_sync_cache_publication_cb, LoadPublications in
get_rel_sync_entry), so subscriber doesn't receive them. But the
subscription can still contain that dropped publication in it's list
of publications.

The patch proposed in this thread, just checks while creation/altering
of the subscription on the subscriber system whether or not the
publication exists on the publisher system. This is one way
dependency. But given the above scenario, there can exist another
dependency i.e. publisher dropping the publisher at any time. So to
make it a complete solution i.e. not allowing non-existent
publications from the list of publications in the subscription, we
need to detect when the publications are dropped in the publisher and
we should, may be on a next connection to the subscriber, also look at
the subscription for that dropped publication, if exists remove it.
But that's an overkill and impractical I feel. Thoughts?

I also feel the best way to remove the confusion is to document why we
allow creating subscriptions even when the specified publications
don't exist on the publisher system? Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com