Akka Stream Graph recover issue

问题

I have created a graph to paralellise two flows with the same input. The flows produce Future[Option[Entity]]. If flowA fails I would like to return a Future[None] but the recover does not seem to be called

    val graph: Flow[Input, (Future[Option[Entity]], Future[Option[Entity]]), NotUsed] = Flow.fromGraph(GraphDSL.create() { implicit builder =>
    import GraphDSL.Implicits._

    val broadcast = builder.add(Broadcast[Input](2))
    val zip = builder.add(Zip[Future[Option[Entity]], Future[Option[Entity]]])

    val flowAwithRecovery = flowA.recover{ case t: Throwable =>
      logger.error(t, "Error retrieving output from flowA. Resuming without them.")
      Future.successful(None)
    }

    broadcast.out(0) ~> flowAwithRecovery ~> zip.in0
    broadcast.out(1) ~> flowB ~> zip.in1

    FlowShape(broadcast.in, zip.out)
  })

When I run the graph and flowA returns a Failed Future the recover does not get executed. As a workaround I am recovering the Future at the end of the processing but I would like to put this kind of logic when I design the graph.

回答1:

The recover combinator comes into play when an exception has been propagated from upstream. A Future.failed is not an exception, but a valid element. You would need something like

flowA.map(_.recover{ case t: Throwable =>
      logger.error(t, "Error retrieving output from flowA. Resuming without them.")
      None
    })

On a different note, do you really need to pass around Futures in your flows? You're probably better off using mapAsync when building flowA and flowB and have them produce just Option[Entity].

来源：https://stackoverflow.com/questions/41833742/akka-stream-graph-recover-issue