Delay function execution

Question

What is the simplest way to delay function execution in Scala, something like JavaScript's setTimeout? Ideally without spawning thread per delayed execution, i.e. sequential execution. The closest that I was able to find was Akka's Scheduler, but that's an overkill.

For my testing purposes I'm opening thousands of connections, then they get responses in 10 seconds. In node.js it looks like:

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  setTimeout(function() {res.end('Hello World\n');}, 10000 );
}).listen(8080, '127.0.0.1');

But what would be the closest Scala version of doing the same? I don't care if res.end is going to be executed in multiple threads or queued in a single one.

Answer 1

Tired of getting flak for answering the question for simplest too simply, here are the standard JVM idioms:

$ scala
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions for evaluation. Or try :help.

scala> import java.util.{Timer,TimerTask}
import java.util.{Timer, TimerTask}

scala> val timer = new Timer
timer: java.util.Timer = java.util.Timer@2d9ffd6f

scala> def delay(f: () => Unit, n: Long) = timer.schedule(new TimerTask() { def run = f() }, n)
delay: (f: () => Unit, n: Long)Unit

scala> delay(() => println("Done"), 1000L)

scala> Done


scala> import java.util.concurrent._
import java.util.concurrent._

scala> val x = Executors.newScheduledThreadPool(2)
x: java.util.concurrent.ScheduledExecutorService = java.util.concurrent.ScheduledThreadPoolExecutor@2c5d529e

scala> x.schedule(new Callable[Int]() { def call = { println("Ran"); 42 }}, 1L, TimeUnit.SECONDS)
res3: java.util.concurrent.ScheduledFuture[Int] = java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@3ab0f534

scala> Ran

There is no API for scheduling a delayed task in the standard library, but you can make an ExecutionContext with a fixed delay, in order to use Scala Future.

scala> import scala.concurrent._
import scala.concurrent._

scala> implicit val xx = new ExecutionContext() {
     | def reportFailure(t: Throwable) = t.printStackTrace()
     | def execute(r: Runnable) = x.schedule(new Callable[Unit]() { def call = r.run() }, 1L, TimeUnit.SECONDS)
     | }
xx: scala.concurrent.ExecutionContext = $anon$1@40d3ab8b

scala> Future(println("hello"))
res4: scala.concurrent.Future[Unit] = List()

scala> hello

scala> Future(42)
res5: scala.concurrent.Future[Int] = List()                

scala> .value
res6: Option[scala.util.Try[Int]] = Some(Success(42))

Or you can use Akka's scheduler, which is the canonical answer at Scheduled Executor in Scala

The old one-liner:

Simplest is still just future { blocking(Thread.sleep(10000L)); "done" }

but I wanted to place an ad for this guy, which I just came across, which gives you a progress indicator or intermediate value. I kind of wish it had a different name, is all.

scala> import concurrent._
import concurrent._

scala> import ExecutionContext.Implicits._
import ExecutionContext.Implicits._

scala> import duration._
import duration._

scala> val deadline = 60.seconds.fromNow
deadline: scala.concurrent.duration.Deadline = Deadline(38794983852399 nanoseconds)

scala> new DelayedLazyVal(() => deadline.timeLeft.max(Duration.Zero), blocking {
     | Thread.sleep(deadline.timeLeft.toMillis)
     | Console println "Working!"
     | })
res9: scala.concurrent.DelayedLazyVal[scala.concurrent.duration.FiniteDuration] = scala.concurrent.DelayedLazyVal@50b56ef3

scala> res9()
res10: scala.concurrent.duration.FiniteDuration = 23137149130 nanoseconds

scala> res9.isDone
res11: Boolean = false

scala> res9()
res12: scala.concurrent.duration.FiniteDuration = 12499910694 nanoseconds

scala> res9()
res13: scala.concurrent.duration.FiniteDuration = 5232807506 nanoseconds

scala> Working!


scala> res9.isDone
res14: Boolean = true

scala> res9()
res15: scala.concurrent.duration.FiniteDuration = 0 days

Here's an alternative formulation with Either, to calculate a value after a delay. Using Left of course when there is still time Left.

scala> new DelayedLazyVal(()=> if (deadline.hasTimeLeft) Left(deadline.timeLeft) else
     | Right("Working!"), blocking(Thread.sleep(deadline.timeLeft.toMillis)))
res21: scala.concurrent.DelayedLazyVal[Product with Serializable with scala.util.Either[scala.concurrent.duration.FiniteDuration,String]] = scala.concurrent.DelayedLazyVal@78f9c6f2

scala> res21()
res22: Product with Serializable with scala.util.Either[scala.concurrent.duration.FiniteDuration,String] = Left(28553649064 nanoseconds)

scala> res21()
res23: Product with Serializable with scala.util.Either[scala.concurrent.duration.FiniteDuration,String] = Left(9378334087 nanoseconds)

scala> res21.isDone
res24: Boolean = false

scala> res21()
res25: Product with Serializable with scala.util.Either[scala.concurrent.duration.FiniteDuration,String] = Right(Working!)

scala> res21.isDone
res26: Boolean = true