Akka Streams Error Handling. How to know which row failed? - scala

Question

I read this article on akka streams error handling
http://doc.akka.io/docs/akka/2.5.4/scala/stream/stream-error.html
and wrote this code.
val decider: Supervision.Decider = {
case _: Exception => Supervision.Restart
case _ => Supervision.Stop
}
implicit val actorSystem = ActorSystem()
implicit val actorMaterializer = ActorMaterializer(ActorMaterializerSettings(actorSystem).withSupervisionStrategy(decider))
val source = Source(1 to 10)
val flow = Flow[Int].map{x => if (x != 9) 2 * x else throw new Exception("9!")}
val sink : Sink[Int, Future[Done]] = Sink.foreach[Int](x => println(x))
val graph = RunnableGraph.fromGraph(GraphDSL.create(sink){implicit builder => s =>
import GraphDSL.Implicits._
source ~> flow ~> s.in
ClosedShape
})
val future = graph.run()
future.onComplete{ _ =>
actorSystem.terminate()
}
Await.result(actorSystem.whenTerminated, Duration.Inf)
This works very well .... except that I need to scan the output to see which row did not get processed. Is there a way for me to print/log the row which failed? [Without putting explicit try/catch blocks in each and every flow that I write?]
So for example If I was using actors (as opposed to streams) I could have written a life cycle event of an actor and I could have logged when an actor restarted along with the message which was being processed at the time of restart.
but here I am not using actors explicitly (although they are used internally). Are there life cycle events for a Flow / Source / Sink?

Answer 1

Just a small modification to your code:
val decider: Supervision.Decider = {
case e: Exception =>
println("Exception handled, recovering stream:" + e.getMessage)
Supervision.Restart
case _ => Supervision.Stop
}
If you pass meaningful messages to your exceptions in the stream, the line for example, you can print them in the supervision decider.
I used println to give a quick and short answer, but strongly recommend to use
some logging libraries such as scala-logging