A future exploring the nature of recursive flatMap calls on a Future.

example-output.txt

4 > 2 start
4 > 2 end
3 > 10 start
3 > 9 start
3 > 9 end
2 > 10 start
2 > 9 start
3 > 10 end
2 > 10 start
2 > 9 start
2 > 9 end
1 > 10 start
1 > 9 start
2 > 9 end
1 > 10 start
1 > 9 start
2 > 10 end
1 > 10 start
1 > 9 start
2 > 10 end
1 > 10 start
1 > 9 start
1 > 9 end
0 > 10 start
1 > 9 end
0 > 9 start
1 > 10 end
0 > 10 start
1 > 9 end
0 > 9 start
1 > 10 end
1 > 9 end
0 > 10 start
0 > 9 start
1 > 10 end
0 > 10 start
1 > 10 end
0 > 9 start
0 > 9 end
0 > 10 end
0 > 10 start
0 > 9 start
0 > 9 end
0 > 10 start
0 > 9 end
0 > 9 start
0 > 10 end
0 > 10 start
0 > 10 end
0 > 9 end
0 > 9 start
0 > 10 start
0 > 10 end
0 > 9 start
0 > 9 end
0 > 9 end
0 > 10 end
0 > 10 end
0 > 9 end
0 > 10 end
0 > 9 end
0 > 10 end

FutureTest.scala

object FutureTest extends App {

  import scala.concurrent.{ExecutionContext, Future}
  import java.util.concurrent.{ConcurrentLinkedQueue, Executors}

  implicit val executionContext = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(8))

  val fs = List(10, 9)

  val log = new ConcurrentLinkedQueue[String]()

  def go(countdown: Int, n: Int): Future[Unit] =

    Future {
      log.add(s"$countdown > $n start")
      Thread.sleep(n * 1000)
      log.add(s"$countdown > $n end")
    } flatMap { _ =>
      if (countdown > 0 ) {
        Future.traverse(fs)(f => go(countdown - 1, f)).map(_ => ())
      } else {
        Future.unit
      }
    }

  go(4, 2).onComplete {
    case _ => log.forEach(println)
  }
}