Re: How to shorten a chain of logically replicated servers

On Tue, 2019-12-31 at 15:51 -0800, Mike Lissner wrote:
> Hi, I'm trying to figure out how to shorten a chain of logically
> replicating servers. Right now we have three servers replicating like
> so:
>
> A --> B --> C
>
> And I'd like to remove B from the chain of replication so that I only have:
>
> A --> C
>
> Of course, doing this without losing data is the goal. If the
> replication to C breaks temporarily, that's fine, so long as all the
> changes on A make it to C eventually.
>
> I'm not sure how to proceed with this. My best theory is:
>
> 1. In a transaction, DISABLE the replication from A to B and start a
> new PUBLICATION on A that C will subscribe to in step ③ below. The
> hope is that this will simultaneously stop sending changes to B while
> starting a log of new changes that can later be sent to C.
>
> 2. Let any changes queued on B flush to C. (How to know when they're
> all flushed?)
>
> 3. Subscribe C to the new PUBLICATION created in step ①. Create the
> subscription with copy_data=False. This should send all changes to C
> that hadn't been sent to B, without sending the complete tables.
>
> 4. DROP all replication to/from B (this is just cleanup; the incoming
> changes to B were disabled in step ①, and outgoing changes from B were
> flushed in step ②).
>
> Does this sound even close to the right approach? Logical replication
> can be a bit finicky, so I'd love to have some validation of the
> general approach before I go down this road.

I don't think that will work.

Any changes on A that take place between step 1 and step 3 wouldn't be
replicated to C.

You'd have to suspend all data modification on A in that interval.

Yours,
Laurenz Albe
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