On Tue, Jul 26, 2022 at 7:13 AM Jonathan S. Katz <jkatz@postgresql.org> wrote:
>
> On 7/25/22 4:54 AM, vignesh C wrote:
> >
> > Let's take a simple case to understand why copy_data = force is
> > required to replicate between two primaries for table t1 which has
> > data as given below:
>
> > step4 - Node-2:
> > Create Subscription sub2 Connection '<node-1 details>' Publication
> > pub1_2 with (origin = none, copy_data=on);
> > If we had allowed the create subscription to be successful with
> > copy_data = on. After this the data will be something like this:
> > Node-1:
> > 1, 2, 3, 4, 5, 6, 7, 8
> >
> > Node-2:
> > 1, 2, 3, 4, 5, 6, 7, 8, 5, 6, 7, 8
> >
> > So, you can see that data on Node-2 (5, 6, 7, 8) is duplicated. In
> > case, table t1 has a unique key, it will lead to a unique key
> > violation and replication won't proceed.
> >
> > To avoid this we will throw an error:
> > ERROR: could not replicate table "public.t1"
> > DETAIL: CREATE/ALTER SUBSCRIPTION with origin = none and copy_data =
> > on is not allowed when the publisher has subscribed same table.
> > HINT: Use CREATE/ALTER SUBSCRIPTION with copy_data = off/force.
>
> Thanks for the example. I agree that it is fairly simple to reproduce.
>
> I understand that "copy_data = force" is meant to protect a user from
> hurting themself. I'm not convinced that this is the best way to do so.
>
> For example today I can subscribe to multiple publications that write to
> the same table. If I have a primary key on that table, and two of the
> subscriptions try to write an identical ID, we conflict. We don't have
> any special flags or modes to guard against that from happening, though
> we do have documentation on conflicts and managing them.
>
> AFAICT the same issue with "copy_data" also exists in the above scenario
> too, even without the "origin" attribute.
>
That's true but there is no parameter like origin = NONE which
indicates that constraint violations or duplicate data problems won't
occur due to replication. In the current case, I think the situation
is different because a user has specifically asked not to replicate
any remote data by specifying origin = NONE, which should be dealt
differently. Note that current users or their setup won't see any
difference/change unless they specify the new parameter origin as
NONE.
--
With Regards,
Amit Kapila.