Scala waiting for sequence of futures

Question

I was hoping code like follows would wait for both futures, but it does not.

object Fiddle {
  val f1 = Future {
    throw new Throwable("baaa") // emulating a future that bumped into an exception
  }

  val f2 = Future {
    Thread.sleep(3000L) // emulating a future that takes a bit longer to complete
    2
  }

  val lf = List(f1, f2) // in the general case, this would be a dynamically sized list

  val seq = Future.sequence(lf) 

  seq.onComplete {
    _ => lf.foreach(f => println(f.isCompleted))
  }
}

val a = FuturesSequence

I assumed seq.onComplete would wait for them all to complete before completing itself, but not so; it results in:

true
false

.sequence was a bit hard to follow in the source of scala.concurrent.Future, I wonder how I would implement a parallel that waits for all original futures of a (dynamically sized) sequence, or what might be the problem here.

Edit: A related question: https://worldbuilding.stackexchange.com/questions/12348/how-do-you-prove-youre-from-the-future :)

Answer 1

A Future produced by Future.sequence completes when either:

• all the futures have completed successfully, or
• one of the futures has failed

The second point is what's happening in your case, and it makes sense to complete as soon as one of the wrapped Future has failed, because the wrapping Future can only hold a single Throwable in the failure case. There's no point in waiting for the other futures because the result will be the same failure.

Answer 2

This is an example that supports the previous answer. There is an easy way to do this using just the standard Scala APIs.

In the example, I am creating 3 futures. These will complete at 5, 7, and 9 seconds respectively. The call to Await.result will block until all futures have resolved. Once all 3 futures have completed, a will be set to List(5,7,9) and execution will continue.

Additionally, if an exception is thrown in any of the futures, Await.result will immediately unblock and throw the exception. Uncomment the Exception(...) line to see this in action.

  try {
    val a = Await.result(Future.sequence(Seq(
      Future({
        blocking {
          Thread.sleep(5000)
        }
        System.err.println("A")
        5
      }),
      Future({
        blocking {
          Thread.sleep(7000)
        }
        System.err.println("B")
        7
        //throw new Exception("Ha!")
      }),
      Future({
        blocking {
          Thread.sleep(9000)
        }
        System.err.println("C")
        9
      }))),
      Duration("100 sec"))

    System.err.println(a)
  } catch {
    case e: Exception ⇒
      e.printStackTrace()
  }

Answer 3

We can enrich Seq[Future[T]] with its own onComplete method through an implicit class:

  def lift[T](f: Future[T])(implicit ec: ExecutionContext): Future[Try[T]] =
    f map { Success(_) } recover { case e => Failure(e) }

  def lift[T](fs: Seq[Future[T]])(implicit ec: ExecutionContext): Seq[Future[Try[T]]] =
    fs map { lift(_) }

  implicit class RichSeqFuture[+T](val fs: Seq[Future[T]]) extends AnyVal {
    def onComplete[U](f: Seq[Try[T]] => U)(implicit ec: ExecutionContext) = {
      Future.sequence(lift(fs)) onComplete {
        case Success(s) => f(s)
        case Failure(e) => throw e // will never happen, because of the Try lifting
      }
    }
  }

Then, in your particular MWE, you can do:

  val f1 = Future {
    throw new Throwable("baaa") // emulating a future that bumped into an exception
  }

  val f2 = Future {
    Thread.sleep(3000L) // emulating a future that takes a bit longer to complete
    2
  }

  val lf = List(f1, f2)

  lf onComplete { _ map {
    case Success(v) => ???
    case Failure(e) => ???
  }}

This solution has the advantage of allowing you to call an onComplete on a sequence of futures as you would on a single future.

Answer 4

One common approach to waiting for all results (failed or not) is to "lift" failures into a new representation inside the future, so that all futures complete with some result (although they may complete with a result that represents failure). One natural way to get that is lifting to a Try.

Twitter's implementation of futures provides a liftToTry method that makes this trivial, but you can do something similar with the standard library's implementation:

import scala.util.{ Failure, Success, Try }

val lifted: List[Future[Try[Int]]] = List(f1, f2).map(
  _.map(Success(_)).recover { case t => Failure(t) }
)

Now Future.sequence(lifted) will be completed when every future is completed, and will represent successes and failures using Try.

And so, a generic solution for waiting on all original futures of a sequence of futures may look as follows, assuming an execution context is of course implicitly available.

  import scala.util.{ Failure, Success, Try }

  private def lift[T](futures: Seq[Future[T]]) = 
    futures.map(_.map { Success(_) }.recover { case t => Failure(t) })

  def waitAll[T](futures: Seq[Future[T]]) =
    Future.sequence(lift(futures)) // having neutralized exception completions through the lifting, .sequence can now be used

  waitAll(SeqOfFutures).map { 
    // do whatever with the completed futures
  }