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: startFirststopFirststartSecondstopSecond
or even:startSecondstopSecondstartFirststopFirst
but not (as it happens): startFirststartSecondstopSecondstopFirst
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) ?
Best Answer
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 .
a little change to the original Future.sequence will make the future execution serialized:
def seq[A, M[X] <: TraversableOnce[X]](in: M[() => Future[A]])(implicit cbf: CanBuildFrom[M[()=>Future[A]], A, M[A]], executor: ExecutionContext): Future[M[A]] = {in.foldLeft(Future.successful(cbf(in))) {(fr, ffa) => for (r <- fr; a <- ffa()) yield (r += a)} map (_.result())}and your code will look like this:
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") }})FutureExt.seq(list)}Await.result(producer, Duration.Inf)}}which is very like your original code and with same result collection with the original Future.sequence()
Linearize:
import scala.concurrent._import scala.collection.mutable.Builderimport scala.collection.generic.CanBuildFromimport language.higherKinds/*** Linearize asynchronously applies a given function in-order to a sequence of values, producing a Future with the result of the function applications.* Execution of subsequent entries will be aborted if an exception is thrown in the application of the function.*/def linearize[T, U, C[T] <: Traversable[T]](s: C[T])(f: T => U)(implicit cbf: CanBuildFrom[C[T], U, C[U]], e: ExecutionContext): Future[C[U]] = {def next(i: Iterator[T], b: Builder[U, C[U]]): Future[C[U]] = if(!i.hasNext) Future successful b.result else Future { b += f(i.next()) } flatMap { b => next(i, b) }next(s.toIterator, cbf(s))}scala> linearize(1 to 100)(_.toString) foreach printlnscala> Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100)From: https://gist.github.com/viktorklang/3347939
If you have a sequence of some values and you want to map them to Future's and execute them in series:
import scala.concurrent.ExecutionContext.Implicits.globalimplicit class SeqExtension[A](s: Seq[A]) {def foldLeftToFuture[B](initial: B)(f: (B, A) => Future[B])(implicit ec: ExecutionContext): Future[B] =s.foldLeft(Future(initial))((future, item) => future.flatMap(f(_, item)))def mapInSeries[B](f: A => Future[B])(implicit ec: ExecutionContext): Future[Seq[B]] =s.foldLeftToFuture(Seq[B]())((seq, item) => f(item).map(seq :+ _))}val stringsFuture: Future[Seq[String]] = Seq(1, 2, 3).mapInSeries[String](i => Future(i.toString))val strings = Await.result(stringsFuture, Duration.Inf) // List("1", "2", "3")or you can use for {..} yield block to get sequence of Future. Future.sequence convert Seq[Future] to Future[Seq]