Question

I’m tinkering with Akka and need some advice how to implement something specific i have in mind. I want to have an actor which i can send a DownloadFile(URI, File) message and downloads it. Since this can be paralleled, I don’t want to download file after file but have a limit of concurrent downloads.

Whats the intended way to model something like this with Akka? Other things that come to mind are: What happens if one of the “worker” actor dies for some reason? How to retry the download? Etc. etc.

I know this is a very huge question but i hope someone takes the time to answer it! Thank you!

Answer 1

Give this a shot; it creates three – but you could configure it to create as many as you like – downloaders, so that three download requests could be processed concurrenty.

sealed trait DownloaderMessage
case class DownloadFile(uri: URI, file: File) extends DownloaderMessage

object Downloader {
  val dispatcher = Dispatchers.newExecutorBasedEventDrivenWorkStealingDispatcher("pool").build
}

class Downloader extends Actor {
  self.lifeCycle = Permanent
  self.dispatcher = Downloader.dispatcher
  def receive = {
    case DownloadFile(uri, file) =>
      // do the download
  }
}

trait CyclicLoadBalancing extends LoadBalancer { this: Actor =>
  val downloaders: List[ActorRef]
  val seq = new CyclicIterator[ActorRef](downloaders)
}

trait DownloadManager extends Actor {
  self.lifeCycle = Permanent
  self.faultHandler = OneForOneStrategy(List(classOf[Exception]), 5, 5000)
  val downloaders: List[ActorRef]
  override def preStart = downloaders foreach { self.startLink(_) }
  override def postStop = self.shutdownLinkedActors()
}

class DownloadService extends DownloadManager with CyclicLoadBalancing {
  val downloaders = List.fill(3)(Actor.actorOf[Downloader])
}