object Messages {
  case class Process(epub: File, isbn: String, processFun: (Elem, String) => Unit)
  case object Fin
}

class FixedNumberOfChildren(maxChildren: Int) extends Actor {
  var numChildren: Int = 0
  override def receive: Receive = {
    case m:ProcessEpub =>
      if (numChildren >= maxChildren)
        self ! m
      else {
        val child = context.actorOf(Props(classOf[EpubProcessorActor]))
        context.watch(child)
        numChildren += 1
        child ! m
      }
    case Terminated(_) =>
      numChildren -= 1
    case Fin =>
      context.system.shutdown()
  }
}

class ProcessorActor extends Actor with StrictLogging {
  def toXml(stream: Stream[Update], isbn: String): Elem = {...}

  override def receive: Actor.Receive = {
    case Process(epub, isbn, f) =>
      val updates = {epub, isbn => stream[Update]}
      val xml = toXml(updates, isbn)
      f(xml, isbn)
      self ! PoisonPill
  }
}