Cake pattern w/ akka: Providing implicit actorSystem to several layers

akka, cake-pattern, scala

Solution

Self types is not a requirement for building a caked architecture, actually i use self types only in cases when a trait is a component of a layer. So when i need to place some implicit into the scope (for example ActorRefFactory for Spray Client) i just mix a trait in :

trait ActorSystemProvider {
  implicit def actorSystem: ActorSystem
}

And on the lowest layer (so called "end of the world") i have the following code structure:

trait ServiceStack
  extends SomeModule
     with SomeModule2
     with SomeModule3 
     with ActorSystemProvider 

object ServiceLauncher extends App with ServiceStack {
  val actorSystem = ActorSystem("ServiceName")
}

It's an oversimplified example (if you want a great example of a real system build on top of a Cake Pattern then you should definitely take a look at the Precog system, example where different modules/layers connects), but not you can mix implicit ActorSystem when you need it.

Problem

I'm currently baking my first cake pattern, so please bear with me. I took my working monolithic app and I cutted it into functional layers. The cut looks clean but resulted in two of the layers that depend on an implicit ActorSystem. I tried to solve this dependency like this: ``` trait LayerA { this: ActorSystemProvider => private implicit val implicitActorSystem = actorSystem import implicitActorSystem.dispatcher // implicit execution ctx ... } ``` ... and similarly for LayerX My assembly class looks like: ``` class Assembly extends LayerA with LayerB with LayerX with ActorSystemProvider ``` where ActorSystemProvider simply instantiates the actor system. This does not work given that the `ActorSystem` does not exist when the dependencies are resolved and the val's are instantiated, resulting in a NPE. This also looks really ugly and I'm sure there has to be a nicer/easier way to deal with it. How should I deal with shared implicit dependencies among layers when using the cake pattern, like `ActorSystem` in this case? Thanks

Original source