Not all akka stream Sinks receive the emitted data - scala

Question

When running the following akka streaming FlowGraph not all the emitted Chars are received by all Sinks.
package sample.stream
import java.io.{ FileOutputStream, PrintWriter }
import akka.actor.ActorSystem
import akka.stream.ActorFlowMaterializer
import akka.stream.scaladsl.{ Broadcast, FlowGraph, Sink, Source }
import scala.concurrent.forkjoin.ThreadLocalRandom
import scala.util.{ Failure, Success, Try }
object Sample {
def main(args: Array[String]): Unit = {
println("start")
implicit val system = ActorSystem("Sys")
import system.dispatcher
implicit val materializer = ActorFlowMaterializer()
var counter = -1
val countSource: Source[Char, Unit] = Source(() => Iterator.continually { counter += 1; (counter + 'A').toChar }.take(11))
var counter1 = 0
val consoleSink1 = Sink.foreach[Char] { counter =>
println("sink1:" + counter1 + ":" + counter)
counter1 += 1
Thread.sleep(100)
//Thread.sleep(300)
}
var counter2 = 0
val consoleSink2 = Sink.foreach[Char] { counter =>
println("sink2:" + counter2 + ":" + counter)
counter2 += 1
Thread.sleep(200)
}
val materialized = FlowGraph.closed(consoleSink1, consoleSink2)((x1, x2) => x1) { implicit builder =>
(console1, console2) =>
import FlowGraph.Implicits._
val broadcast = builder.add(Broadcast[Char](2))
countSource ~> broadcast ~> console1
broadcast ~> console2
}.run()
// ensure the output file is closed and the system shutdown upon completion
materialized.onComplete {
case Success(_) =>
system.shutdown()
case Failure(e) =>
println(s"Failure: ${e.getMessage}")
system.shutdown()
}
println("waiting the remaining ones")
//scala.concurrent.Await.ready(materialized, scala.concurrent.duration.DurationInt(100).seconds)
//system.shutdown()
println("end")
}
}
After running the following output is generated
[info] Running sample.stream.Sample
[info] start
[info] waiting the remaining ones
[info] end
[info] sink2:0:A
[info] sink1:0:A
[info] sink1:1:B
[info] sink1:2:C
[info] sink2:1:B
[info] sink1:3:D
[info] sink2:2:C
[info] sink1:4:E
[info] sink1:5:F
[info] sink2:3:D
[info] sink1:6:G
[info] sink1:7:H
[info] sink2:4:E
[info] sink2:5:F
[info] sink1:8:I
[info] sink1:9:J
[info] sink2:6:G
[info] sink2:7:H
[info] sink1:10:K
The second sink doesn't receive the 8th, 9th and 10th values: IJK but still the entire flow is ended.
What should I do to wait for both Sinks to consume all the data?
I discovered that if I change the (x1,x2)=>x1 to (x1,x2)=>x2 this will wait. That is the same with sleeping 300ms in the first sink.

Answer 1

The function that you pass to a second parameter list of FlowGraph.closed determines what materialized value is returned when you run the flow. So when you pass in (x1,x2)=>x1 you return a future which is completed when the first sink gets all elements and then the callback on that future shuts down the actor system without the second sink having a chance receiving all of the elements.
Instead, you should get both futures out and shutdown the system only when both futures are completed.
You can actually see how this approach is used in some of the akka-stream tests here.