How can one verify messages sent to self are delivered when testing Akka actors?

Question

I have an Actor that is similar to the following Actor in function.

case class SupervisingActor() extends Actor {

    protected val processRouter = //round robin router to remote workers
    override def receive = {
        case StartProcessing => { //sent from main or someplace else
            for (some specified number of process actions ){
                processRouter ! WorkInstructions
            }
        }
        case ProcessResults(resultDetails) => {  //sent from the remote workers when they complete their work
            //do something with the results
            if(all of the results have been received){
                //*********************
                self ! EndProcess  //This is the line in question
                //*********************
            }
        }
        case EndProcess {
            //do some reporting
            //shutdown the ActorSystem
        }
    }
}

}

How can I verify the EndProcess message is sent to self in tests?

I'm using scalatest 2.0.M4, Akka 2.0.3 and Scala 1.9.2.

Answer 1

An actor sending to itself is very much an intimiate detail of how that actor performs a certain function, hence I would rather test the effect of that message than whether or not that message has been delivered. I’d argue that sending to self is the same as having a private helper method on an object in classical OOP: you also do not test whether that one is invoked, you test whether the right thing happened in the end.

As a side note: you could implement your own message queue type (see https://doc.akka.io/docs/akka/snapshot/mailboxes.html#creating-your-own-mailbox-type) and have that allow the inspection or tracing of message sends. The beauty of this approach is that it can be inserted purely by configuration into the actor under test.