Event-sourcing with akka-persistance: growing state as list?

Question

I am designing a backend using CQRS + Event sourcing, using Akka + Scala. I am not sure about how to handle a growing state. For instance, I will have a growing list of users. To my understanding, each user will be created following a UserCreated event, such events will be replayed by the PersistentActor, and the users will be stored in a collection. Something like:

class UsersActor extends PersistentActor {

    override def persistenceId = ....

    private case class UsersState(users: List[User])

    private var state = UsersState()

    ....
}

Obviously such state would eventually grow too big to be held in memory by this actor, so I guess I'm doing something wrong.

I found this example project: the idea seems that each user should be held by a different actor, and loaded (from the event history) as needed.

What is the right way to do this? Thanks a lot.

Answer 1

The answer is: each aggregate/entity (in my example, each User) gets its own actor, which embeds the state for that particular entity and that one only.