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

> On 16 Apr 2025, at 09:33, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> wrote:
>
> In my experience, the load of managing hundreds of replicas which all
> participate in RAFT protocol becomes more than regular transaction
> load. So making every replica a RAFT participant will affect the
> ability to deploy hundreds of replica.

No need to make all standbys voting. And no need to make plain topology. pg_consul is using 2/3 or 3/5 HA groups, and
cascadesall others from HA group. 
Existing tools already solve the original problem, Konstantin is just proposing to solve it in some standard “official”
way.

> We may build an extension which
> has a similar role in PostgreSQL world as zookeeper in Hadoop.

Patroni, pg_consul and others already use zookeeper, etcd and similar systems for consensus.
Is it any better as extension than as etcd?

> It can
> be then used for other distributed systems as well - like shared
> nothing clusters based on FDW.

I didn’t get FDW analogy. Why other distributed systems should choose Postgres extension over Zookeeper?

> There's already a proposal to bring
> CREATE SERVER to the world of logical replication - so I see these two
> worlds uniting in future.

Again, I’m lost here. Which two worlds?

> The way I imagine it is some PostgreSQL
> instances, which have this extension installed, will act as a RAFT
> cluster (similar to Zookeeper ensemble or etcd cluster).

That’s exactly what is proposed here.

> The
> distributed system based on logical replication or FDW or both will
> use this ensemble to manage its shared state. The same ensemble can be
> shared across multiple distributed clusters if it has scaling
> capabilities.

Yes, shared DCS are common these days. AFAIK, we use one Zookeeper instance per hundred Postgres clusters to coordinate
pg_consuls.

Actually, scalability is opposite to topic of this thread. Let me explain.
Currently, Postgres automatic failover tools rely on databases with built-in automatic failover. Konstantin is
proposingto shorten this loop and make Postgres use its build-in automatic failover. 

So, existing tooling allows you to have 3 hosts for DCS, with majority of 2 hosts able to elect new leader in case of
failover.
And you can have only 2 hosts for Postgres - Primary and Standby. You can have 2 big Postgres machines with 64 CPUs.
And3 one-CPU hosts for Zookeper\etcd. 

If you use build-in failover you have to resort to 3 big Postgres machines because you need 2/3 majority. Of course,
youcan install MySQL-stype arbiter - host that had no real PGDATA, only participates in voting. But this is a solution
toproblem induced by built-in autofailover. 


Best regards, Andrey Borodin.