AntiPatterns.scala

case class User(id: Long, name: String)
class Work { /* ... */ }
class Result { /* --- */ }

case class WorkToBeDone(user: User, work: Work)
case class WorkIsDone(user: User, result: Result)

class Worker extends Actor { /* ... */ }

class BadWorkerCoordinator extends Actor {
  
  implicit val timeout = Timeout(5 seconds)
  
  import context.dispatcher
  
  def receive = {
    case WorkToBeDone(user, work) =>
      val worker = context.actorOf(Props[Worker])
      (worker ask work) foreach { case WorkIsDone(user, result) =>
        sender ! result
      }
  }
  
}

class GoodWorkerCoordinator extends Actor {

  // Map of requester actors by user
  var requesters = Map.empty[User, ActorRef]
  
  def receive = {
    case WorkToBeDone(user, work) =>
      context.actorOf(Props[Worker]) ! work
      requesters += user -> sender
    case WorkIsDone(user, result) if requesters.isDefinedAt(user) =>
      requesters.get(user) foreach { case requester =>
        requester ! result
        requesters -= user
      }
  }
  
}

class BetterWorkerCoordinator extends Actor {
  
  def coordinate(requesters: Map[User, ActorRef]): Receive = {
    case WorkToBeDone(user, work) =>
      context.actorOf(Props[Worker]) ! work
      context.become(coordinate(requesters + (user -> sender)))
    case WorkIsDone(user, result) if requesters.isDefinedAt(user) =>
      requesters.get(user) foreach { case requester =>
        requester ! result
        context.become(coordinate(requesters - user))
      }
  }
  
  def receive = coordinate(Map.empty[User, ActorRef])
  
}