scala - Why Future.sequence executes my futures in parallel rather than in series?

Question

The word "sequence" means a series of actions one after the other.

object Test {

  def main(args: Array[String]) {

    def producer() = {
      val list = Seq(
          future { println("startFirst"); Thread.sleep(3000); println("stopFirst") }, 
          future { println("startSecond"); Thread.sleep(1000); println("stopSecond") }
      )
      Future.sequence(list)
    }

   Await.result(producer, Duration.Inf)
  }
}

Therefore I expect this program to print: startFirst stopFirst startSecond stopSecond

or even: startSecond stopSecond startFirst stopFirst

but not (as it happens): startFirst startSecond stopSecond stopFirst

Why this method is not called Future.parallel()? And what should I use to guarantee that all futures in a Seq of futures are triggered serially (as opposed to in parallel) ?

Answer 1

The futures are running concurrently because they have been started concurrently :). To run them sequentially you need to use flatMap:

Future { println("startFirst"); 
         Thread.sleep(3000); 
         println("stopFirst") 
        }.flatMap{
         _ =>  Future { 
                       println("startSecond"); 
                       Thread.sleep(1000); 
                       println("stopSecond") 
               }
        }

Future.sequence just turns Seq[Future[T]] => Future[Seq[T]] which means gather results of all already started futures and put it in future .