How does Database scaling work with horizontal scaling?

Today horizontal scaling was introduced, and it works on a test server I have. But, I don't have a database attached to it yet.

What assumptions can we make about database scaling? In particular Postgres, MySQL, and MongoDB. If I were to blind guess: there is no database horizontal scaling at all.

MongoDB could replicate and have eventual consistency... :LULW:
yep no database h-scaling, and i have not yet heard any talks of adding it