Do I need to re-use the same Akka ActorSystem or can I just create one every time I need one?

Question

Akka 2.x requires many commands to reference an ActorSystem. So, to create an instance of an actor MyActor you might say:

val system = ActorSystem() val myActor = system.actorOf(Props[MyActor]) 

Because of the frequent need for an ActorSystem: many code examples omit the creation from the code and assume that the reader knows where a system variable has come from.

If your code produces actors in different places, you could duplicate this code, possibly creating additional ActorSystem instances, or you could try to share the same ActorSystem instance by referring to some global or by passing the ActorSystem around.

The Akka documentation provides a general overview of systems of actors under the heading 'Actor Systems', and there is documentation of the ActorSystem class. But neither of these help a great deal in explaining why a user of Akka can't just rely on Akka to manage this under-the-hood.

Question(s)

• What are the implications of sharing the same ActorSystem object or creating a new one each time?

• What are the best practices here? Passing around an ActorSystem all the time seems surprisingly heavy-handed.

• Some examples give the ActorSystem a name: ActorSystem("MySystem") others just call ActorSystem(). What difference does this make, and what if you use the same name twice?

• Does akka-testkit require that you share a common ActorSystem with the one you pass to the TestKit constructor?

Answer 1

Creating an ActorSystem is very expensive, so you want to avoid creating a new one each time you need it. Also your actors should run in the same ActorSystem, unless there is a good reason for them not to. The name of the ActorSystem is also part the the path to the actors that run in it. E.g. if you create an actor in a system named MySystem it will have a path like akka://MySystem/user/$a. If you are in an actor context, you always have a reference to the ActorSystem. In an Actor you can call context.system. I don't know what akka-testkit expects, but you could take a look at the akka tests.

So to sum it up, you should always use the same system, unless there is a good reason not to do so.