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Akka Concepts - Testing Actor Systems

Remember that actors interact only using message passing. In order to check actors behavior you can do it through the messages sent to them and back from them.  So how do you test actors? You send them messages :)
To test actors that communicate only with messages, you need to send it a message and get reply back and check it.
akka has a TestProbe
val p = TestProbe(); // record incoming messages in queue so you can assert and verify them.
Creating actor system for tests:
implicit val system = ActorSystem("TestSys")
val toggle = system.actorOf(Props[Toggle]) // this is the actor we are going to test.
val p = TestProbe() // this is the test client actor which will record the messages.

p.send(toggle, "How are you") // probe --> tested actor: how are you?
p.exepectMsg("happy") // assert result is happy.
To have the probe actor created for you:
new TestKit(ActorSystem("TestSys")) with ImplicitSender { // we are in probe actor.
  val toggle = system.actorOf(Props[Toggle])
  toggle ! "how are you?" // we are already in probe actor.
}
managing external dependencies
Some of your actors might need to access database what do you do it?
  1. Dependency Injection
  2. Just Override - Add overridable factory methods
class DBManager extends Actor {
  def props: Props = Props[DBWorker] // Override this method when you want a fakeDB!
  
  def receive = {
    ...
    // Do not use context.actorOf(Props[DBWorker]) instead use the factory method see below:
    val dbworker = context.actorOf(props, "dbworker") // So we can override the def!
    ...
  }
}
In our case we have used def props and we refer to it when creating the actor.
How to probe messages sent to parent?
The parent is the Guardian actor if the system created it for us.
So we need a StepParent which forwards all messages to probe.
class StepParent(child: Props, probe: ActorRef) extends Actor {
  contex.actorOf(child, "child")
  def receive = {
    case msg => probe.tell(msg, sender) // forward all messages to probe!
  }
}
In a similar way we can create FosterParent actor which listens to all messages in both directions from child to parent and vice versa.
Define your test class
class GetterSpec extends TestKit(ActorSystem("GetterSpec")) with ImplicitSender ...
A test case snippet:
system.actorOf(Props(new StepParent(fakeDBWorker, testActor), "name")) // testActor is given by framework it's the probe.
expectMsg(IDidMyWorkMsg) // we expect a message IDidMyWorkMsg
So in general you send messages to your actors and you use expectMsg to check the messages they send back.
BOOK: the best book I have found for working with akka actors, it's surprisingly much better than all others, as it drives you through of a flow of building an app, making mistakes and the continuously improving the design of your app.  The book is: "Mastering Akka"


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