Testing timer events in akka - scala

Question

I have a function which uses scheduleOnce to schedule an event to happen some time in the future, and I want to write a test that checks that:
the event was indeed scheduled
it was scheduled at the correct time
the system behaves as expected when that event eventually triggers
but I don't want the test to actually wait several minutes doing nothing.
How should I best test code that uses akka's Scheduler?

Answer 1

Here is an example of mocking out the scheduler as described by #lmm. In this example, we really test the full scheduling and handling of the action as two separate scenarios. The first testing that given some condition (a message of a certain type being received in my example) we will schedule a callback, and the second being the handling of the message that gets fired back to self when that timer goes off. The code is as follows:
object TimerExampleActor{
case object ClearState
case class ScheduleStateClearing(duration:FiniteDuration)
}
class TimerExampleActor extends Actor{
import TimerExampleActor._
var state:List[Int] = Nil
def receive = {
case ScheduleStateClearing(d) =>
scheduler.scheduleOnce(d, self, ClearState)(context.dispatcher)
case ClearState =>
state = Nil
}
def scheduler = context.system.scheduler
}
Then, using specs2 and mockito, my test case is as follows:
class TimerExampleActorTest extends Specification with Mockito with NoTimeConversions{
import TimerExampleActor._
implicit val system = ActorSystem("test")
trait testscope extends Scope{
val mockScheduler = mock[Scheduler]
val actor = TestActorRef(new TimerExampleActor{
override def scheduler = mockScheduler
})
}
"A request to schedule state clearing" should{
"schedule a callback for the message ClearState to self with the supplied duration" in new testscope{
val dur = 1.minute
actor ! ScheduleStateClearing(dur)
there was one(mockScheduler).scheduleOnce(dur, actor, ClearState)(actor.underlyingActor.context.dispatcher)
}
}
"A ClearState message received by the actor" should{
"clear the interval 'state' List" in new testscope{
actor.underlyingActor.state = List(1,2,3)
actor ! ClearState
actor.underlyingActor.state mustEqual Nil
}
}
}
You can see that when I create the actor instance under test I override the method I created to get me the instance of the scheduler, allowing me to return a mock. This is not the only way to go about testing something like this, but it certainly can be one option for you to consider.

Answer 2

Make the scheduler take a time parameter. In your test use a shorter time than in your regular code.
Or... when testing you can mix in a special trait that modifies your class as needed (shortens the wait time.)