This horizontal scale-out is not a distributed system, since consensus ("source of truth") still lives in one machine. I think this is a key idea, though my ex

This horizontal scale-out is not a distributed system, since consensus ("source of truth") still lives in one machine. I think this is a key idea, though my experience with it only comes from writing game servers. Things are much simpler when there is a single "source of truth" per user. In my system, there is an "active" state maintained in an "instance" where the user is rapidly updated in memory, and a "persisted" state stored to the database when a user joins an instance. This keeps the frequency of database writes down. (And enables many users to be able to interact with each other in realtime.)