Why is Akka Streams swallowing my exceptions?

Question

Why is the exception in

import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.Source

object TestExceptionHandling {
  def main(args: Array[String]): Unit = {
    implicit val actorSystem = ActorSystem()
    implicit val materializer = ActorMaterializer()(defaultActorSystem)

    Source(List(1, 2, 3)).map { i =>
      if (i == 2) {
        throw new RuntimeException("Please, don't swallow me!")
      } else {
        i
      }
    }.runForeach { i =>
      println(s"Received $i")
    }
  }
}

silently ignored? I can see that the stream gets stopped after printing Received 1, but nothing is logged. Note that the problem is not the logging configuration in general, as I see a lot of output if I set akka.log-config-on-start = on in my application.conf file.

Answer 1

I'm now using a custom Supervision.Decider that makes sure exceptions are properly logged, that can be set up like this:

val decider: Supervision.Decider = { e =>
  logger.error("Unhandled exception in stream", e)
  Supervision.Stop
}

implicit val actorSystem = ActorSystem()
val materializerSettings = ActorMaterializerSettings(actorSystem).withSupervisionStrategy(decider)
implicit val materializer = ActorMaterializer(materializerSettings)(actorSystem)

Also, as has been pointed out by Vikor Klang, in the example given above, the exception could also be "caught" via

Source(List(1, 2, 3)).map { i =>
  if (i == 2) {
    throw new RuntimeException("Please, don't swallow me!")
  } else {
    i
  }
}.runForeach { i =>
  println(s"Received $i")
}.onComplete {
  case Success(_) =>
    println("Done")
  case Failure(e) =>
    println(s"Failed with $e")
}

Note however, that this approach won't help you with

Source(List(1, 2, 3)).map { i =>
  if (i == 2) {
    throw new RuntimeException("Please, don't swallow me!")
  } else {
    i
  }
}.to(Sink.foreach { i =>
  println(s"Received $i")
}).run()

since run() returns Unit.