DAGFuture.scala

import scala.concurrent.{Await, Future}
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
import scala.util.Try

case class Node[T](value: T, children: List[Node[T]])

object DagFuture extends App {

  def run[A, B](node: Node[A], result: B)(nodeEval: (Node[A], B) => B)(aggregator: Traversable[B] => B): Future[B] = {
    val nodeResult = Future(nodeEval(node, result))
    val allResults = nodeResult flatMap { r => Future.sequence(nodeResult :: node.children.map { x => run(x, r)(nodeEval)(aggregator) }) }
    allResults map aggregator
  }

  val debugSum = (l: Traversable[Int]) => {
    println(s"aggregating: $l")
    l sum
  }

  def debugNodeEval(f: (Node[Int], Int) => Int)(n: Node[Int], r: Int) = {
    val eval = Try {
      f(n, r)
    }
    println(s"node: $n, result: $r, eval: $eval")
    eval get
  }

  val debugNodeEvalDefault = debugNodeEval((n, r) => n.value + r) _

  val singleNodeDag = Node(1, Nil)
  val multiNodeDag = Node(1, List(Node(20, Nil), Node(300, Nil)))

  println("\nSINGLE NODE DAG EXAMPLE:")
  val singleNodeFuture = run(singleNodeDag, 0)(debugNodeEvalDefault)(debugSum)
  val singleNodeResult = Await.result(singleNodeFuture, 5 seconds)
  println(s"Single node result: $singleNodeResult")

  println("\nDAG PATH LENGTH EXAMPLE:")
  val pathLengthFuture = run(multiNodeDag, 0)(debugNodeEvalDefault)(debugSum)
  val pathLengthResult = Await.result(pathLengthFuture, 5 seconds)
  println(s"Path length: $pathLengthResult")

  println("\nFAILED DAG ROOT NODE EXAMPLE:")
  val failedRootNodeFuture = run(multiNodeDag, 0)(debugNodeEval((n, r) => throw new Exception))(debugSum)
  val failedRootNodePromise = Await.ready(failedRootNodeFuture, 5 seconds)
  println(s"Failed root node: ${failedRootNodePromise.value}")

  println("\nFAILED DAG CHILD NODE EXAMPLE:")
  val failedChildNodeFuture = run(multiNodeDag, 0)(debugNodeEval((n, r) => if (n.value == 300) throw new Exception else n.value + r))(debugSum)
  val failedChildNodePromise = Await.ready(failedChildNodeFuture, 5 seconds)
  println(s"Failed child node: ${failedChildNodePromise.value}")
}

Prints:

SINGLE NODE DAG EXAMPLE:
node: Node(1,List()), result: 0, eval: Success(1)
aggregating: List(1)
Single node result: 1

DAG PATH LENGTH EXAMPLE:
node: Node(1,List(Node(20,List()), Node(300,List()))), result: 0, eval: Success(1)
node: Node(20,List()), result: 1, eval: Success(21)
aggregating: List(21)
node: Node(300,List()), result: 1, eval: Success(301)
aggregating: List(301)
aggregating: List(1, 21, 301)
Path length: 323

FAILED DAG ROOT NODE EXAMPLE:
node: Node(1,List(Node(20,List()), Node(300,List()))), result: 0, eval: Failure(java.lang.Exception)
Failed root node: Some(Failure(java.lang.Exception))

FAILED DAG CHILD NODE EXAMPLE:
node: Node(1,List(Node(20,List()), Node(300,List()))), result: 0, eval: Success(1)
node: Node(20,List()), result: 1, eval: Success(21)
node: Node(300,List()), result: 1, eval: Failure(java.lang.Exception)
aggregating: List(21)
Failed child node: Some(Failure(java.lang.Exception))

See question/answer: http://stackoverflow.com/questions/36349747/execute-dag-like-operations-in-scala-future/36381078#36381078