Re: Built-in Raft replication - Mailing list pgsql-hackers

* Alastair Turner <minion@decodable.me> [25/04/16 15:58]:

> > > If you use build-in failover you have to resort to 3 big Postgres
> > machines because you need 2/3 majority. Of course, you can install
> > MySQL-stype arbiter - host that had no real PGDATA, only participates in
> > voting. But this is a solution to problem induced by built-in autofailover.
> >
> > Users find it a waste of resources to deploy 3 big PostgreSQL
> > instances just for HA where 2 suffice even if they deploy 3
> > lightweight DCS instances. Having only some of the nodes act as DCS
> > and others purely PostgreSQL nodes will reduce waste of resources.
> >
> > The experience of other projects/products with automated failover based on
> quorum shows that this is a critical issue for adoption. In the In-memory
> Data Grid space (Coherence, Geode/GemFire) the question of how to ensure
> that some nodes didn't carry any data comes up early in many architecture
> discussions. When RabbitMQ shipped their Quorum Queues feature, the first
> and hardest area of pushback was around all nodes hosting message content.
>
> It's not just about the requirement for compute resources, it's also about
> bandwidth and latency. Many large organisations have, for historical
> reasons, pairs of data centres with very good point-to-point connectivity.
> As the requirement for quorum witnesses has come up for all sorts of
> things, including storage arrays, they have built arbiter/witness sites at
> branches, colocation providers or even on the public cloud. More than not
> holding user data or processing queries, the arbiter can't even be sent the
> replication stream for the user data in the database, it just won't fit
> down the pipe.
>
> Which feels like a very difficult requirement to meet if the replication
> model for all data is being changed to a quorum model.

I agree master/replica deployment layouts are very popular and are
not going to directly benefit from raft. They'll still work, but no
automation will be available, just like today with Patroni.

However, if the storage cost is an argument, then the logical path is to
disaggregate storage/compute altogether, i.e. use projects like
neon.