Re: Multi-Master Logical Replication

On Thu, Apr 28, 2022 at 4:24 PM Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote:
>
> В Чт, 28/04/2022 в 09:49 +1000, Peter Smith пишет:
>
> > 1.1 ADVANTAGES OF MMLR
> >
> > - Increases write scalability (e.g., all nodes can write arbitrary data).
>
> I've never heard how transactional-aware multimaster increases
> write scalability. More over, usually even non-transactional
> multimaster doesn't increase write scalability. At the best it
> doesn't decrease.
>
> That is because all hosts have to write all changes anyway. But
> side cost increases due to increased network interchange and
> interlocking (for transaction-aware MM) and increased latency.

I agree it won't increase in all cases, but it will be better in a few
cases when the user works on different geographical regions operating
on independent schemas in asynchronous mode. Since the write node is
closer to the geographical zone, the performance will be better in a
few cases.

> В Чт, 28/04/2022 в 08:34 +0000, kuroda(dot)hayato(at)fujitsu(dot)com пишет:
> > Dear Laurenz,
> >
> > Thank you for your interest in our works!
> >
> > > I am missing a discussion how replication conflicts are handled to
> > > prevent replication from breaking
> >
> > Actually we don't have plans for developing the feature that avoids conflict.
> > We think that it should be done as core PUB/SUB feature, and
> > this module will just use that.
>
> If you really want to have some proper isolation levels (
> Read Committed? Repeatable Read?) and/or want to have
> same data on each "master", there is no easy way. If you
> think it will be "easy", you are already wrong.

The synchronous_commit and synchronous_standby_names configuration
parameters will help in getting the same data across the nodes. Can
you give an example for the scenario where it will be difficult?

Regards,
Vignesh