Re: Feature Request - DDL deployment with logical replication

On 1 April 2018 at 00:57, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2018-03-31 22:13:42 +0800, Craig Ringer wrote:
> > We'll still need a mechanism to transport them to downstreams (like WAL
> > messages) and to send responses upstream. For responses I think we will
> > finally want to add a backchannel to the logical replication protocol as
> > I've wanted for a long while: downstream can send a COPY message on COPY
> > BOTH proto back to upstream, which passes it to a callback on the output
> > plugin for the output plugin to act on.
>
> Not necessarily? You can just send out the prepare, wait for all
> clients to ack it, and then commit/rollback prepared.
>

We then lack any mechanism by which you can NACK, saying "I can't apply
this".

So upstream will wait indefinitely. I guess we just expect the user to
intervene and ROLLBACK if they decide a replica isn't going to get the job
done, or have checked the replica's logs and found it can't apply it for
some hopefully-sane reason.

It's not like we'd auto-ROLLBACK PREPARED in response to a nack from a
downstream anyway, so all we're missing is probably info in the upstream
logs about which replica(s) cannot apply it and why.

OK. So it'd be a nice-to-have, but not vital.

--
Craig Ringer http://www.2ndQuadrant.com/
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