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

On Wed, Feb 3, 2021, at 2:13 AM, Bharath Rupireddy wrote:
> On Mon, Jan 25, 2021 at 10:32 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> > If a publication which does not exist is specified during create subscription, then we should throw an error similar to step 2 behavior. Similarly if a publication which does not exist is specified during alter subscription, then we should throw an error similar to step 4 behavior. If publication is dropped after subscription is created, this should be removed when an alter subscription subname refresh publication is performed similar to step 4.
> > Thoughts?
Postgres implemented a replication mechanism that is decoupled which means that
both parties can perform "uncoordinated" actions. As you shown, the CREATE
SUBSCRIPTION informing a non-existent publication is one of them. I think that
being permissive in some situations can prevent you from painting yourself into
a corner. Even if you try to be strict on the subscriber side, the other side
(publisher) can impose you additional complexity.

You are arguing that in the initial phase, the CREATE SUBSCRIPTION has a strict
mechanism. It is a fair point. However, impose this strictness for the other
SUBSCRIPTION commands should be carefully forethought. If we go that route, I
suggest to have the current behavior as an option. The reasons are: (i) it is
backward-compatible, (ii) it allows us some known flexibility (non-existent
publication), and (iii) it would probably allow us to fix a scenario created by
the strict mode. This strict mode can be implemented via new parameter
validate_publication (ENOCAFFEINE to propose a better name) that checks if the
publication is available when you executed the CREATE SUBSCRIPTION. Similar
parameter can be used in ALTER SUBSCRIPTION ... SET PUBLICATION and ALTER
SUBSCRIPTION ... REFRESH PUBLICATION.

> To not mandate any new behaviour, I would suggest to have a new option
> for ALTER SUBSCRIPTION ... REFRESH PUBLICATION WITH
> (remove_dropped_publications = false). The new option
> remove_dropped_publications will have a default value false, when set
> to true it will check if the publications that are present in the
> subscription list are actually existing on the publisher or not, if
> not remove them from the list. And also in the documentation we need
> to clearly mention the consequence of this new option setting to true,
> that is, the dropped publications if created again will need to be
> added to the subscription list again.
REFRESH PUBLICATION is not the right command to remove publications. There is a
command for it: ALTER SUBSCRIPTION ... SET PUBLICATION.

The other alternative is to document that non-existent publication names can be
in the subscription catalog and it is ignored while executing SUBSCRIPTION
commands. You could possibly propose a NOTICE/WARNING that informs the user
that the SUBSCRIPTION command contains non-existent publication.

--
Euler Taveira
EDB https://www.enterprisedb.com/