Worker.scala

override def receive: Receive = {
  case Crawl(url) =>
    urlsPending = urlsPending :+ url
    startHttpGetIfPossible()
    
  // ...
}

private def startHttpGetIfPossible(): Unit = {
  urlsPending match {
    case url +: tail if !getInProgress =>
      getInProgress = true
      urlsPending = tail

      import context.dispatcher
      http.get(url).onComplete(r => self ! HttpGetResult(url, r))

    case _ =>
  }
}