Asynchronous IO in Scala with futures

Let's say I'm getting a (potentially big) list of images to download from some URLs. I'm using Scala, so what I would do is :

import scala.actors.Futures._

// Retrieve URLs from somewhere
val urls: List[String] = ...

// Download image (blocking operation)
val fimages: List[Future[...]] = urls.map (url => future { download url })

// Do something (display) when complete
fimages.foreach (_.foreach (display _))

I'm a bit new to Scala, so this still looks a little like magic to me :

• Is this the right way to do it? Any alternatives if it is not?
• If I have 100 images to download, will this create 100 threads at once, or will it use a thread pool?
• Will the last instruction (display _) be executed on the main thread, and if not, how can I make sure it is?

Thanks for your advice!

Use Futures in Scala 2.10. They were joint work between the Scala team, the Akka team, and Twitter to reach a more standardized future API and implementation for use across frameworks. We just published a guide at: http://docs.scala-lang.org/overviews/core/futures.html

Beyond being completely non-blocking (by default, though we provide the ability to do managed blocking operations) and composable, Scala's 2.10 futures come with an implicit thread pool to execute your tasks on, as well as some utilities to manage time outs.

import scala.concurrent.{future, blocking, Future, Await, ExecutionContext.Implicits.global}
import scala.concurrent.duration._

// Retrieve URLs from somewhere
val urls: List[String] = ...

// Download image (blocking operation)
val imagesFuts: List[Future[...]] = urls.map {
  url => future { blocking { download url } }
}

// Do something (display) when complete
val futImages: Future[List[...]] = Future.sequence(imagesFuts)
Await.result(futImages, 10 seconds).foreach(display)

Above, we first import a number of things:

• future: API for creating a future.
• blocking: API for managed blocking.
• Future: Future companion object which contains a number of useful methods for collections of futures.
• Await: singleton object used for blocking on a future (transferring its result to the current thread).
• ExecutionContext.Implicits.global: the default global thread pool, a ForkJoin pool.
• duration._: utilities for managing durations for time outs.

imagesFuts remains largely the same as what you originally did- the only difference here is that we use managed blocking- blocking. It notifies the thread pool that the block of code you pass to it contains long-running or blocking operations. This allows the pool to temporarily spawn new workers to make sure that it never happens that all of the workers are blocked. This is done to prevent starvation (locking up the thread pool) in blocking applications. Note that the thread pool also knows when the code in a managed blocking block is complete- so it will remove the spare worker thread at that point, which means that the pool will shrink back down to its expected size.

(If you want to absolutely prevent additional threads from ever being created, then you ought to use an AsyncIO library, such as Java's NIO library.)

Then we use the collection methods of the Future companion object to convert imagesFuts from List[Future[...]] to a Future[List[...]].

The Await object is how we can ensure that display is executed on the calling thread-- Await.result simply forces the current thread to wait until the future that it is passed is completed. (This uses managed blocking internally.)

val all = Future.traverse(urls){ url =>
  val f = future(download url) /*(downloadContext)*/
  f.onComplete(display)(displayContext)
  f
}
Await.result(all, ...)

1. Use scala.concurrent.Future in 2.10, which is RC now.
2. which uses an implicit ExecutionContext
3. The new Future doc is explicit that onComplete (and foreach) may evaluate immediately if the value is available. The old actors Future does the same thing. Depending on what your requirement is for display, you can supply a suitable ExecutionContext (for instance, a single thread executor). If you just want the main thread to wait for loading to complete, traverse gives you a future to await on.

1. Yes, seems fine to me, but you may want to investigate more powerful twitter-util or Akka Future APIs (Scala 2.10 will have a new Future library in this style).

2. It uses a thread pool.

3. No, it won't. You need to use the standard mechanism of your GUI toolkit for this (SwingUtilities.invokeLater for Swing or Display.asyncExec for SWT). E.g.

    fimages.foreach (_.foreach(im => SwingUtilities.invokeLater(new Runnable { display im })))