Re: Feature Request - DDL deployment with logical replication

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От Craig Ringer
Тема Re: Feature Request - DDL deployment with logical replication
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Msg-id CAMsr+YHgHZNX0JiN1xAdByzcmpiruYh4E+RRRO2UFug2ih+1sg@mail.gmail.com
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Ответ на Re: Feature Request - DDL deployment with logical replication  (Andres Freund <andres@anarazel.de>)
Ответы Re: Feature Request - DDL deployment with logical replication  (Andres Freund <andres@anarazel.de>)
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On 1 April 2018 at 00:57, Andres Freund <andres@anarazel.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/
 PostgreSQL Development, 24x7 Support, Training & Services

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