Maybe I'm reading it wrong, but that looks like yet another multi-master high-availability system, which is also an important property for a system to have but more or less unrelated tow hat I was talking about.

As far as open-source add-ons for true horizontal scaling, I think [Citus](https://github.com/citusdata/citus) is the most well-known and sophisticated, but I don't have enough experience with it in production to have a particularly strong opinion yet. It might work quite well, but I still fundamentally doubt that it will ever be as good as a system that was designed to support horizontal sharding from the ground up.