Date: Mon, 14 Apr 2025 22:00:22 +0300
From: Konstantin Osipov <kostja.osipov@gmail.com>
To: Kirill Reshke <reshkekirill@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Built-in Raft replication
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* Kirill Reshke <reshkekirill@gmail.com> [25/04/14 20:48]:
> > I am considering starting work on implementing a built-in Raft
> > replication for PostgreSQL.
> >
> 
> Just some thought on top of my mind, if you need my voice here:
> 
> I have a hard time believing the community will be positive about this
> change in-core. It has more changes as contrib extension. In fact, if
> we want a built-in consensus algorithm, Paxos is a better option,
> because you can use postgresql as local crash-safe storage for single
> decree paxos, just store your state (ballot number, last voice) in a
> heap table.

But Raft is a log replication algorithm, not a consensus
algorithm. It does use consensus, but that's for leader election.
Paxos could be used for log replication, but that would be
expensive. In fact etcd uses Raft, and etcd is used by Patroni. So
I completely lost your line of thought here.

> OTOH Raft needs to write its own log, and what's worse, it sometimes
> needs to remove already written parts of it (so, it is not appended
> only, unlike WAL). If you have a production system which maintains two
> kinds of logs with different semantics, it is a very hard system to
> maintain..

My proposal is exactly to replace (or rather, extend) the current
synchronous log replication with Raft. Entry removal is possible to
stack on top of append-only format, and production implementations
exist which do that.

So, no, it's a single log, and in fact the current WAL will do.

> There is actually a prod-ready (non open source) implementation of
> RAFT as extension, called BiHA, by pgpro.

My guess biha is an extension since a proprietary code is easier
to maintain that way. I'd rather say the fact that there is a
proprietary implementation out in the field confirms it could be a
good idea to have it in PostgreSQL trunk. 

In any case I'm interested in contributing to the trunk, not
building a proprietary module/fork.

-- 
Konstantin Osipov