Unit vs Integration Akka Testing

This is the sixth post in the series about integrating sync clients with async systems (1, 2, 3, 4, 5). Here we’ll see how to test Akka actors with different testing styles.

Unit vs Integration Testing

Nowadays everybody agrees with the famous testing pyramid:

Harder to achieve is an agreement on what integration, unit, functional or acceptance means. It is reasonable as applications are structured differently depending on language, architecture and domain. I’ll try to distill some essences though:

  • Unit: the key here is isolation. Some people talk about isolated production code, e.g. a single function or a bunch of methods within a class (one public and the rest private). Some other people talks about isolated or independent tests, i.e. tests that may be executed in parallel as they don’t access to any shared resource. A third view is that a unit test execution should be synchronous without any concurrency concern. That’s Akka’s view:

Testing isolated pieces of code without involving the actor model, meaning without multiple threads; this implies completely deterministic behavior concerning the ordering of events and no concurrency concerns and will be called Unit Testing in the following.

  • Integration: this kind of tests often involves that several classes, modules or services are exercised. With Akka we’ll test several actors but the key concept is that we’ll use multi-threaded scheduling:

Testing (multiple) encapsulated actors including multi-threaded scheduling; this implies non-deterministic order of events but shielding from concurrency concerns by the actor model and will be called Integration Testing in the following.

Akka Unit Testing

When we unit test objects we look for:

  • Check the returned value.
  • Verify calls to collaborators.
  • Inspect the internal state. This might be an smell in some cases.

The concept of returned value is slightly different in Akka. Akka focuses on messages and not on method invocations. Checking the returned value involves two actors and two messages. If the collaborators of the Actor Under Test are also Actors, verifying calls will involve two actors and two messages too. If we use a multi-threaded dispatcher for scheduling this, this scenario will be out of our unit testing definition. Let’s focus then on testing internal state.

Akka actors are completely encapsulated and the only communication channel is the mailbox. TestActorRef is provided by Akka so we can gain access into the internals of an actor and unit test it. One of its specialised forms is TestFSMRef allowing us to test Finite State Machines. Let’s see an example from our platform:

"Item FSM" should {
  "move into active state when it receives an item" in {
    val fsm = TestFSMRef(new ItemFSM(itemReportedProducer, itemDeletedBus))
    fsm.stateName shouldBe Idle
    fsm.stateData shouldBe Uninitialized

    fsm ! ItemReported(itemId)

    within(200 millis){
      fsm.stateName shouldBe Active
      fsm.stateData.asInstanceOf[ItemsToBeDeleted].items shouldBe items

As you can see TestFSMRef wraps the actor that we want to test and exposes its internal state. That wrapper has other useful methods like setting state programatically or manipulating FSM timers.

I want to share something that confused me a little the first time, but it’s important to understand. We need to recall that unit testing in Akka means using one single thread in order to achieve a deterministic order of events. TestActorRef uses CallingThreadDispatcher by default. This dispatcher runs invocations on the current thread only so we could do an unit test that checks the returned value of an actor with this style.

class EchoActor extends Actor {
 override def receive = {
   case message ⇒ sender() ! message

"send back messages unchanged" in {
  import akka.pattern.ask
  import scala.concurrent.duration._
  implicit val timeout = Timeout(5 seconds)

  val actorRef = TestActorRef(new EchoActor)

  val future = actorRef ? "hello world"
  val Success(result: String) = future.value.get
  result should be("hello world")

Let’s see how we can test this, and other scenarios, with an integration testing style in Akka.

Akka Integration testing

Akka provides the TestKit class for integration testing. Let’s see one of our tests written using that class:

class ItemFSMSpec() extends TestKit(ActorSystem("ItemFSMSpec")) with ImplicitSender

"send a complete message to the original sender when one deleted item is received and there are no more messages pending" in {
    val worker = TestFSMRef(new ItemFSM(itemReportedProducer, itemDeletedBus))
    worker ! ItemReported(itemId)

    itemDeletedBus.publish(MsgEnvelope(item.partitionKey, ItemDeleted(item)))

    within(200 millis) {

In this particular test we’re interested in the message that the FSM is dispatching to the sender. Let’s take another look at this line:

worker ! ItemReported(itemId)

Here we’re saying: send a message of ItemReported type to the actor assigned in the val worker. But who is sending that message? Extending and mixing TestKit and ImplicitSender creates a testActor that will be the messages sender. TestKit exposes some methods like expectMsg to allow inspecting the mailbox of that testActor. within acts like Scalatest eventually but with more power. For instance, as the documentation says:

It should be noted that if the last message-receiving assertion of the block is expectNoMsg or receiveWhile, the final check of the within is skipped in order to avoid false positives due to wake-up latencies. This means that while individual contained assertions still use the maximum time bound, the overall block may take arbitrarily longer in this case.

Another interesting class is TestProbe. If we have several actors in our integration test and we want to verify different messages sent between the different actors, using a single testActor might be confusing. Even with a single actor, using a TestProbe improves readability in some cases:

"flush a FSM when it receives a Failed message" in {
  val fsmProbe = TestProbe()
  val actorFactory: (ActorContext, ActorRef) => ActorRef = (context, self) => fsmProbe.ref
  val coordinator = TestActorRef(ItemReportedCoordinator.props(actorFactory))

  fsmProbe.send(coordinator, Result(Left(Exception("Some exception message"))))


In the previous post we introduced the actor factory in order to create an actor pool. Here TestProbe helps understand with better clarity as to who is sending and expecting the messages.


Testability is one of the main assets of Akka. The biggest challenge is understanding what we want to test: internal business logic of the actor or the async exchange of messages between different actors.

Part 1 | Part 2 | Part 3 | Part 4 | Part 5

Reference: Unit vs Integration Akka Testing from our JCG partner Felipe Fernandez at the Crafted Software blog.

Felipe Fernandez

Felipe is a developer interested on creating well-crafted software, not only for users, but for future maintainers too. He believes that simple, clean and expressive code should be an ethical standard for the software industry and will mentor that concern. Felipe is a strong believer in the power of social communities and thinks that one of the pillars of professionalism lies in sharing knowledge and caring about the health of the industry.
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