I am trying to write a function that returns a future. Within this future 3 other future functions are called, the third depends on the results of the first two. My instinct is to use a for comphrension, but when I try to match on the tuple I get this error:
value map is not a member of Object
Which refers to the match of (result1, result2) below
def future1 = Future { ... }
def future2 = Future { ... }
def future3(a, b): Future[T] = Future { .... }
def future4: Future[T] = {
for {
result1 <- future1
result2 <- future2
result3 <- (result1, result2) match {
case (x, y) => future3(result1, result2)
case (_, _) => ???
}
} yield result3
}
It can be solved by making whatever is under ??? return Future[T]
The method1 below is an attempt to return the results of two Future methods someFutureMethod1 and someFutureMethod2 combined. Any ideas how to make this work? Ideally the method that ends first should wait until the second ends, and then return the future values combined.
def method1 ( s: Seq[Int]): Future [(Int,Int)] = {
s.map { sx =>
val future = someFutureMethod1 (sx)
future.map {
result => result
}
val future2 = someFutureMethod2 (sx)
future2.map {
result2 => result2
}
(result,result2) // <-- the method should return the two results in the future
}
}
def someFutureMethod1 (i: Int) = Future {
i + 1
}
def someFutureMethod2 (i: Int) = Future {
i + 2
}
You combine Future's in a map, so it looks like you have to return a Seq of such Future's:
def method1 ( s: Seq[Int]): Seq[Future [(Int,Int)]] =
s.map { sx =>
val future = someFutureMethod1 (sx)
val future2 = someFutureMethod2 (sx)
future.zip(future2)
}
In the code below I have to return the result of a future that is invoked after another future. I'm getting the following error in the future2.map line:
type mismatch; found : scala.concurrent.Future[play.api.mvc.Result]
required: play.api.mvc.Result
How to make this work?
def method1 = Action.async { request =>
val future1 = f1
future1.map { result1 =>
val future2 = f2
future2.map { result2 =>
Ok(result1+result2+"")
}
}
}
def f1 = Future { 1 }
def f2 = Future { 2 }
You could do this in many ways. But first, you need to understand how map and flatMap work with Future:
def map[S](f: (T) ⇒ S): Future[S]
def map[S](f: (T) ⇒ Future[S]): Future[Future[S]]
def flatMap[S](f: (T) ⇒ Future[S]): Future[S]
Note that,in above signatures, you are calling map and flatMap with a value that already is a future i.e Future[<some-value>].map(...) or Future[<some-value>].flatMap(...).
Approach 1:
def method1 = Action.async { request =>
val future1 = f1
future1.flatMap { result1 => //replaced map with flatMap
val future2 = f2
future2.map { result2 =>
Ok(result1+result2+"")
}
}
}
def f1 = Future { 1 }
def f2 = Future { 2 }
Approach 2:
def method1 = Action.async { request =>
val future1 = f1
future1.flatMap { result1 => //replaced map with flatMap
val future2 = f2
future2.flatMap { result2 => //replaced map with flatMap
Future.successful{Ok(result1+result2+"")} // used Future.successful{} to generate a Future of Result
}
}
}
def f1 = Future { 1 }
def f2 = Future { 2 }
Changing future1.map to future1.flatMap should do the trick. Mapping over a Future returns another Future and changes the value inside. In this case, you're returning a Future that contains another Future that contains a Result. By using flatMap, it essentially flattens the nested Future[Future[Result]] into a Future[Result].
I would like something like runProgram2 but currently that part does not compile. Is there a way to write it somewhat like runProgram2 but so it compiles..
package transformer
import scala.concurrent.{ExecutionContext, Promise, Future}
import ExecutionContext.Implicits.global
import java.util.concurrent.TimeUnit
import scala.concurrent.duration.Duration
object TestingForComprehensions2 {
def main(args: Array[String]) = {
val future1: Future[String] = runMyProgram()
future1.onSuccess {
case r:String => System.out.println("result="+r)
}
val future2: Future[String] = runMyProgram2()
future2.onSuccess {
case r:String => System.out.println("result="+r)
}
System.out.println("waiting")
Thread.sleep(600000)
}
def runMyProgram() : Future[String] = {
val future = serviceCall()
val middle = serviceCallWrap(future)
val future2 = middle.flatMap(serviceCall2)
val future3 = future2.map(processAllReturnCodes)
future3
}
def runMyProgram2() : Future[String] = {
for {
result1 <- serviceCall()
middle = serviceCallWrap(result1)
result2 <- serviceCall2(middle)
} yield processAllReturnCodes(result2)
}
def processAllReturnCodes(theMsg: String) : String = {
"dean"+theMsg
}
def serviceCall() : Future[Int] = {
val promise = Promise.successful(5)
promise.future
}
def serviceCallWrap(f:Future[Int]) : Future[Int] = {
f
}
def serviceCall2(count:Int) : Future[String] = {
val promise = Promise.successful("hithere"+count)
promise.future
}
}
serviceCallWrap expects a future, but you are passing an Int, first step would be to remove the result1 <- serviceCall() part and call directly middle = serviceCallWrap(serviceCall()).
Now middle is a future but serviceCall2 takes an Int, in this case you can extract the value from the future using middle <- serviceCallWrap(serviceCall()), all together:
def runMyProgram2() : Future[String] = {
for {
middle <- serviceCallWrap(serviceCall())
result2 <- serviceCall2(middle)
} yield processAllReturnCodes(result2)
}
If you want to keep the comprehension structure:
def runMyProgram2() : Future[String] = {
for {
result1 <- serviceCall()
middle <- serviceCallWrap(Future(result1))
result2 <- serviceCall2(middle)
} yield processAllReturnCodes(result2)
}
The only thing you can't keep (AFAIK) is the assignment expression for middle, if you want to keep that you need to change serviceCall2 signature. This approach though has exactly the same result as the first approach, it's only more verbose.
I think I have this figured out now. or at least this compiles so I can do it on a different line.....
def runMyProgram2() : Future[String] = {
for {
result1 <- serviceCall()
middle <- serviceCallWrap(Promise.successful(result1).future)
result2 <- serviceCall2(middle)
} yield processAllReturnCodes(result2)
}
Suppose I have several futures and need to wait until either any of them fails or all of them succeed.
For example: Let there are 3 futures: f1, f2, f3.
If f1 succeeds and f2 fails I do not wait for f3 (and return failure to the client).
If f2 fails while f1 and f3 are still running I do not wait for them (and return failure)
If f1 succeeds and then f2 succeeds I continue waiting for f3.
How would you implement it?
You could use a for-comprehension as follows instead:
val fut1 = Future{...}
val fut2 = Future{...}
val fut3 = Future{...}
val aggFut = for{
f1Result <- fut1
f2Result <- fut2
f3Result <- fut3
} yield (f1Result, f2Result, f3Result)
In this example, futures 1, 2 and 3 are kicked off in parallel. Then, in the for comprehension, we wait until the results 1 and then 2 and then 3 are available. If either 1 or 2 fails, we will not wait for 3 anymore. If all 3 succeed, then the aggFut val will hold a tuple with 3 slots, corresponding to the results of the 3 futures.
Now if you need the behavior where you want to stop waiting if say fut2 fails first, things get a little trickier. In the above example, you would have to wait for fut1 to complete before realizing fut2 failed. To solve that, you could try something like this:
val fut1 = Future{Thread.sleep(3000);1}
val fut2 = Promise.failed(new RuntimeException("boo")).future
val fut3 = Future{Thread.sleep(1000);3}
def processFutures(futures:Map[Int,Future[Int]], values:List[Any], prom:Promise[List[Any]]):Future[List[Any]] = {
val fut = if (futures.size == 1) futures.head._2
else Future.firstCompletedOf(futures.values)
fut onComplete{
case Success(value) if (futures.size == 1)=>
prom.success(value :: values)
case Success(value) =>
processFutures(futures - value, value :: values, prom)
case Failure(ex) => prom.failure(ex)
}
prom.future
}
val aggFut = processFutures(Map(1 -> fut1, 2 -> fut2, 3 -> fut3), List(), Promise[List[Any]]())
aggFut onComplete{
case value => println(value)
}
Now this works correctly, but the issue comes from knowing which Future to remove from the Map when one has been successfully completed. As long as you have some way to properly correlate a result with the Future that spawned that result, then something like this works. It just recursively keeps removing completed Futures from the Map and then calling Future.firstCompletedOf on the remaining Futures until there are none left, collecting the results along the way. It's not pretty, but if you really need the behavior you are talking about, then this, or something similar could work.
You can use a promise, and send to it either the first failure, or the final completed aggregated success:
def sequenceOrBailOut[A, M[_] <: TraversableOnce[_]](in: M[Future[A]] with TraversableOnce[Future[A]])(implicit cbf: CanBuildFrom[M[Future[A]], A, M[A]], executor: ExecutionContext): Future[M[A]] = {
val p = Promise[M[A]]()
// the first Future to fail completes the promise
in.foreach(_.onFailure{case i => p.tryFailure(i)})
// if the whole sequence succeeds (i.e. no failures)
// then the promise is completed with the aggregated success
Future.sequence(in).foreach(p trySuccess _)
p.future
}
Then you can Await on that resulting Future if you want to block, or just map it into something else.
The difference with for comprehension is that here you get the error of the first to fail, whereas with for comprehension you get the first error in traversal order of the input collection (even if another one failed first). For example:
val f1 = Future { Thread.sleep(1000) ; 5 / 0 }
val f2 = Future { 5 }
val f3 = Future { None.get }
Future.sequence(List(f1,f2,f3)).onFailure{case i => println(i)}
// this waits one second, then prints "java.lang.ArithmeticException: / by zero"
// the first to fail in traversal order
And:
val f1 = Future { Thread.sleep(1000) ; 5 / 0 }
val f2 = Future { 5 }
val f3 = Future { None.get }
sequenceOrBailOut(List(f1,f2,f3)).onFailure{case i => println(i)}
// this immediately prints "java.util.NoSuchElementException: None.get"
// the 'actual' first to fail (usually...)
// and it returns early (it does not wait 1 sec)
Here is a solution without using actors.
import scala.util._
import scala.concurrent._
import java.util.concurrent.atomic.AtomicInteger
// Nondeterministic.
// If any failure, return it immediately, else return the final success.
def allSucceed[T](fs: Future[T]*): Future[T] = {
val remaining = new AtomicInteger(fs.length)
val p = promise[T]
fs foreach {
_ onComplete {
case s # Success(_) => {
if (remaining.decrementAndGet() == 0) {
// Arbitrarily return the final success
p tryComplete s
}
}
case f # Failure(_) => {
p tryComplete f
}
}
}
p.future
}
You can do this with futures alone. Here's one implementation. Note that it won't terminate execution early! In that case you need to do something more sophisticated (and probably implement the interruption yourself). But if you just don't want to keep waiting for something that isn't going to work, the key is to keep waiting for the first thing to finish, and stop when either nothing is left or you hit an exception:
import scala.annotation.tailrec
import scala.util.{Try, Success, Failure}
import scala.concurrent._
import scala.concurrent.duration.Duration
import ExecutionContext.Implicits.global
#tailrec def awaitSuccess[A](fs: Seq[Future[A]], done: Seq[A] = Seq()):
Either[Throwable, Seq[A]] = {
val first = Future.firstCompletedOf(fs)
Await.ready(first, Duration.Inf).value match {
case None => awaitSuccess(fs, done) // Shouldn't happen!
case Some(Failure(e)) => Left(e)
case Some(Success(_)) =>
val (complete, running) = fs.partition(_.isCompleted)
val answers = complete.flatMap(_.value)
answers.find(_.isFailure) match {
case Some(Failure(e)) => Left(e)
case _ =>
if (running.length > 0) awaitSuccess(running, answers.map(_.get) ++: done)
else Right( answers.map(_.get) ++: done )
}
}
}
Here's an example of it in action when everything works okay:
scala> awaitSuccess(Seq(Future{ println("Hi!") },
Future{ Thread.sleep(1000); println("Fancy meeting you here!") },
Future{ Thread.sleep(2000); println("Bye!") }
))
Hi!
Fancy meeting you here!
Bye!
res1: Either[Throwable,Seq[Unit]] = Right(List((), (), ()))
But when something goes wrong:
scala> awaitSuccess(Seq(Future{ println("Hi!") },
Future{ Thread.sleep(1000); throw new Exception("boo"); () },
Future{ Thread.sleep(2000); println("Bye!") }
))
Hi!
res2: Either[Throwable,Seq[Unit]] = Left(java.lang.Exception: boo)
scala> Bye!
For this purpose I would use an Akka actor. Unlike the for-comprehension, it fails as soon as any of the futures fail, so it's a bit more efficient in that sense.
class ResultCombiner(futs: Future[_]*) extends Actor {
var origSender: ActorRef = null
var futsRemaining: Set[Future[_]] = futs.toSet
override def receive = {
case () =>
origSender = sender
for(f <- futs)
f.onComplete(result => self ! if(result.isSuccess) f else false)
case false =>
origSender ! SomethingFailed
case f: Future[_] =>
futsRemaining -= f
if(futsRemaining.isEmpty) origSender ! EverythingSucceeded
}
}
sealed trait Result
case object SomethingFailed extends Result
case object EverythingSucceeded extends Result
Then, create the actor, send a message to it (so that it will know where to send its reply to) and wait for a reply.
val actor = actorSystem.actorOf(Props(new ResultCombiner(f1, f2, f3)))
try {
val f4: Future[Result] = actor ? ()
implicit val timeout = new Timeout(30 seconds) // or whatever
Await.result(f4, timeout.duration).asInstanceOf[Result] match {
case SomethingFailed => println("Oh noes!")
case EverythingSucceeded => println("It all worked!")
}
} finally {
// Avoid memory leaks: destroy the actor
actor ! PoisonPill
}
This question has been answered but I am posting my value class solution (value classes were added in 2.10) since there isn't one here. Please feel free to criticize.
implicit class Sugar_PimpMyFuture[T](val self: Future[T]) extends AnyVal {
def concurrently = ConcurrentFuture(self)
}
case class ConcurrentFuture[A](future: Future[A]) extends AnyVal {
def map[B](f: Future[A] => Future[B]) : ConcurrentFuture[B] = ConcurrentFuture(f(future))
def flatMap[B](f: Future[A] => ConcurrentFuture[B]) : ConcurrentFuture[B] = concurrentFutureFlatMap(this, f) // work around no nested class in value class
}
def concurrentFutureFlatMap[A,B](outer: ConcurrentFuture[A], f: Future[A] => ConcurrentFuture[B]) : ConcurrentFuture[B] = {
val p = Promise[B]()
val inner = f(outer.future)
inner.future onFailure { case t => p.tryFailure(t) }
outer.future onFailure { case t => p.tryFailure(t) }
inner.future onSuccess { case b => p.trySuccess(b) }
ConcurrentFuture(p.future)
}
ConcurrentFuture is a no overhead Future wrapper that changes the default Future map/flatMap from do-this-then-that to combine-all-and-fail-if-any-fail. Usage:
def func1 : Future[Int] = Future { println("f1!");throw new RuntimeException; 1 }
def func2 : Future[String] = Future { Thread.sleep(2000);println("f2!");"f2" }
def func3 : Future[Double] = Future { Thread.sleep(2000);println("f3!");42.0 }
val f : Future[(Int,String,Double)] = {
for {
f1 <- func1.concurrently
f2 <- func2.concurrently
f3 <- func3.concurrently
} yield for {
v1 <- f1
v2 <- f2
v3 <- f3
} yield (v1,v2,v3)
}.future
f.onFailure { case t => println("future failed $t") }
In the example above, f1,f2 and f3 will run concurrently and if any fail in any order the future of the tuple will fail immediately.
You might want to checkout Twitter's Future API. Notably the Future.collect method. It does exactly what you want: https://twitter.github.io/scala_school/finagle.html
The source code Future.scala is available here:
https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Future.scala
You can use this:
val l = List(1, 6, 8)
val f = l.map{
i => future {
println("future " +i)
Thread.sleep(i* 1000)
if (i == 12)
throw new Exception("6 is not legal.")
i
}
}
val f1 = Future.sequence(f)
f1 onSuccess{
case l => {
logInfo("onSuccess")
l.foreach(i => {
logInfo("h : " + i)
})
}
}
f1 onFailure{
case l => {
logInfo("onFailure")
}