Re: DROP SUBSCRIPTION and ROLLBACK

On 07/02/17 13:10, Masahiko Sawada wrote:
> Hi all,
>
> While testing logical replciation I found that if the transaction
> issued DROP SUBSCRIPTION　rollbacks then the logical repliation stops
> and the subscription can never be removed later. The document says
> that the replication worker associated with the subscription will not
> stop until after the transaction that issued this command has
> committed but it doesn't work.

Ah then the docs are wrong and should be fixed. Maybe we should not
allow DROP SUBSCRIPTION inside transaction similarly to CREATE INDEX
CONCURRENTLY.

> The cause of this is that DropSubscription stops the apply worker and
> drops corresponding replication slot on publisher side without waiting
> for commit or rollback. The launcher process launches the apply worker
> again but the launched worker will fail to start logical replication
> because corresponding replication slot is already removed. And the
> orphan subscription can not be removed later.
>
> I think the logical replication should not stop and the corresponding
> replication slot and replication origin should not be removed until
> the transaction commits.
>
> The solution for this I came up with is that the launcher process
> stops the apply worker after DROP SUBSCRIPTION is committed rather
> than DropSubscription does. And the apply worker drops replication
> slot and replication origin before exits. Attached draft patch fixes
> this issue.
>

I don't think we can allow the slot drop to be postponed. There is too
many failure scenarios where we would leave the remote slot in the
database and that's not acceptable IMHO.

For example what happens if apply crashes during the DROP
SUBSCRIPTION/COMMIT and is not started because the delete from catalog
is now visible so the subscription is no longer there?

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services