Testing Akka Streams divertTo with streams testkit

I have a graph composed of several flows, each of which returns an Either of some error or the actual result. The code is using divertTo in order to send any Lefts to a different sink than the happy-path downstream sink.

The problem I am having is that when using the Akka Streams testkit, I can't find a way to probe the sink used in the divertTo call. I can probe the happy-path sink fine but I really need to find a way to test the not-so-happy paths to prove that my flow is working.

Has anybody done this type of thing before using the streams testkit?

Solution

The problem I am having is that...I can't find a way to probe the sink used in the divertTo call....Has anybody done this type of thing before using the streams testkit?

Yes: https://github.com/akka/akka/blob/release-2.5/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphPartitionSpec.scala

From the above spec:

"divertTo must send matching elements to the sink" in assertAllStagesStopped {
  val odd = TestSubscriber.probe[Int]()
  val even = TestSubscriber.probe[Int]()
  Source(1 to 2).divertTo(Sink.fromSubscriber(odd), _ % 2 != 0).to(Sink.fromSubscriber(even)).run()
  even.request(1)
  even.expectNoMessage(1.second)
  odd.request(1)
  odd.expectNext(1)
  even.expectNext(2)
  odd.expectComplete()
  even.expectComplete()
}