在失败的情况下解决ask的Akka期货问题

Question

我在一个Spray应用程序中使用ask模式调用一个Actor，并将结果作为HTTP响应返回。我将故障从参与者映射到自定义错误代码。

val authActor = context.actorOf(Props[AuthenticationActor])

callService((authActor ? TokenAuthenticationRequest(token)).mapTo[LoggedInUser]) { user =>
  complete(StatusCodes.OK, user)
}

def callService[T](f: => Future[T])(cb: T => RequestContext => Unit) = {
 onComplete(f) {
  case Success(value: T) => cb(value)
  case Failure(ex: ServiceException) => complete(ex.statusCode, ex.errorMessage)
  case e => complete(StatusCodes.InternalServerError, "Unable to complete the request. Please try again later.")
  //In reality this returns a custom error object.
 }
}

当authActor发送一个失败时，这可以正常工作，但是如果authActor抛出一个异常，则在请求超时完成之前什么也不会发生。例如：

override def receive: Receive = {
  case _ => throw new ServiceException(ErrorCodes.AuthenticationFailed, "No valid session was found for that token")
}

override def receive: Receive = {
case default =>
  try {
    default match {
      case _ => throw new ServiceException("")//Actual code would go here
    }
  }
  catch {
    case se: ServiceException =>
      logger.error("Service error raised:", se)
      sender ! Failure(se)
    case ex: Exception =>
      sender ! Failure(ex)
      throw ex
  }
}

这样，如果它是预期的错误（即ServiceException)，则通过创建一个失败来处理它。如果它是意外的，它会立即返回一个失败，以便解决未来的问题，但随后抛出异常，以便仍然可以由SupervisorStrategy处理。

Answer 1

如果您想要一种方法，在意外异常的情况下自动向发件人发送回响应，那么类似这样的方法可以为您工作：

trait FailurePropatingActor extends Actor{
  override def preRestart(reason:Throwable, message:Option[Any]){
    super.preRestart(reason, message)
    sender() ! Status.Failure(reason)
  }
}

我们重写prerestart，并将失败作为状态传播回发送方。failure，这将导致上游future失败。另外，在这里调用super.preRestart也很重要，因为这是发生子停止的地方。在演员中使用这个类似于以下内容：

case class GetElement(list:List[Int], index:Int)
class MySimpleActor extends FailurePropatingActor {  
  def receive = {
    case GetElement(list, i) =>
      val result = list(i)
      sender() ! result
  }  
}

如果我像这样调用这个actor的实例：

import akka.pattern.ask
import concurrent.duration._

val system = ActorSystem("test")
import system.dispatcher
implicit val timeout = Timeout(2 seconds)
val ref = system.actorOf(Props[MySimpleActor])
val fut = ref ? GetElement(List(1,2,3), 6)

fut onComplete{
  case util.Success(result) => 
    println(s"success: $result")

  case util.Failure(ex) => 
    println(s"FAIL: ${ex.getMessage}")
    ex.printStackTrace()    
}     

class MyBadFutureUsingActor extends FailurePropatingActor{
  import context.dispatcher

  def receive = {
    case GetElement(list, i) => 
      val orig = sender()
      val fut = Future{
        val result = list(i)
        orig ! result
      }      
  } 
}

class MyGoodFutureUsingActor extends FailurePropatingActor{
  import context.dispatcher
  import akka.pattern.pipe

  def receive = {
    case GetElement(list, i) => 
      val fut = Future{
        list(i)
      }

      fut pipeTo sender()
  } 
}

class MyGoodFutureUsingActor extends FailurePropatingActor{
  import context.dispatcher
  import akka.pattern.pipe

  def receive = {
    case GetElement(list, i) => 
      val fut = Future{
        list(i)
      }

      fut.to(self, sender())

    case d:Double =>
      sender() ! d * 2

    case Status.Failure(ex) =>
      throw ex
  } 
}

如果该行为变得常见，您可以将其提供给任何需要它的参与者，如下所示：

trait StatusFailureHandling{ me:Actor =>
  def failureHandling:Receive = {
    case Status.Failure(ex) =>
      throw ex      
  }
}

class MyGoodFutureUsingActor extends FailurePropatingActor with StatusFailureHandling{
  import context.dispatcher
  import akka.pattern.pipe

  def receive = myReceive orElse failureHandling

  def myReceive:Receive = {
    case GetElement(list, i) => 
      val fut = Future{
        list(i)
      }

      fut.to(self, sender())

    case d:Double =>
      sender() ! d * 2        
  } 
}