Re: logical replication syntax (was DROP SUBSCRIPTION, query cancellations and slot handling)

Petr Jelinek wrote:
> On 02/05/17 18:00, Robert Haas wrote:
> > On Tue, May 2, 2017 at 11:49 AM, Alvaro Herrera
> > <alvherre(at)2ndquadrant(dot)com> wrote:
> >> Petr Jelinek wrote:
> >>> So the only way to fulfill the requirement you stated is to just not try
> >>> to drop the slot, ever, on DROP SUBSCRIPTION. That makes the default
> >>> behavior leave resources on upstream that will eventually cause that
> >>> server to stop unless user notices before. I think we better invent
> >>> something that limits how much inactive slots can hold back WAL and
> >>> catalog_xmin in this release as well then.
> >>
> >> I don't understand why isn't the default behavior to unconditionally
> >> drop the slot. Why do we ever want the slot to be kept?
> >
> > What if the remote server doesn't exist any more?
>
> Or what if the slot is used by other subscription (because you restored
> dump containing subscriptions to another server for example), or you
> have server that does not have outside network access anymore, or many
> other reasons.

So there are two different scenarios: one is that you expect the slot
drop to fail for whatever reason; the other is that you want the slot to
be kept because it's needed for something else. Maybe those two should
be considered separately.

1) keep the slot because it's needed for something else.
I see two options:
a) change the something else so that it uses another slot with the
same location. Do we have some sort of "clone slot" operation?
b) have an option to dissociate the slot from the subscription prior
to the drop.

2) don't drop because we know it won't work. I see two options:
c) ignore a drop slot failure, i.e. don't cause a transaction abort.
An easy way to implement this is just add a PG_TRY block, but we
dislike adding those and not re-throwing the error.
d) rethink drop slot completely; maybe instead of doing it
immediately, it should be a separate task, so we first close the
current transaction (which dropped the subscription) and then we open
a second one to drop the slot, so that if the drop slot fails, the
subscription does not come back to life.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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