Akka Concepts: Testing Actors

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Akka Concepts: Testing Actors

In Akka, actors interact via messages. You can use this to your advantage by setting up tests through that same messaging.

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With Akka, remember that actors interact only via message passing. In order to check actors' behaviors, you can do it through the messages sent and received to and 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, get a reply back, and check it.

Akka has a TestProbe

val p = TestProbe(); // record incoming messages in queue so you can assert and verify them.

And here we create an 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 a database. If that happens, how 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 used def props and we refer to it when creating the actor.

How to Probe Messages Sent to Parents

The parent is the Guardian actor if the system created it for us.

So we need a StepParent that forwards all messages to the 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 a 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 the framework. It's the probe.
expectMsg(IDidMyWorkMsg) // we expect a message IDidMyWorkMsg

There are two things to remember at the core of all this: Remember you test your actors by sending them messages. You use expectMsg to check the messages they send back.  You can also use dependency injection to be able to change behavior in tests, and you can fake the parent in order to detect the messages' traffic.

scala ,akka actors ,java ,testing ,tutorial

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