使用akka流时的事件顺序

Question

阅读akka的文档，我不太清楚消息的顺序，以及是否可以强制执行。让我用我为聊天服务器编写的一小部分代码来设置我问题的上下文。

def flowShape(user: User) = GraphDSL
  .create(Source.actorRef[ChatMessage](bufferSize = 5, OverflowStrategy.fail)) {
    implicit builder =>
      implicit chatSource =>

      import GraphDSL.Implicits._

      val messageFromOutside = builder.add(Flow[String].map {
        case msg: String => UserTextMessage(user, msg)
        case _ => InvalidMessage
      })

      val merge = builder.add(Merge[ChatMessage](2))
      // UPDATE --> this is where the change comes
      // val merge = builder.add(Concat[ChatMessage](2))

      // val channelActorSink = Sink.actorRefWithAck(channelActor, ActorInitMessage, AckMessage, UserLeft(user))
      val channelActorSink = Sink.actorRef(channelActor, UserLeft(user))

      val actorAsSource = builder.materializedValue.map { actor => UserJoined(user, actor) }

      actorAsSource ~> merge.in(0)
      messageFromOutside.out ~> merge.in(1)
      merge ~> channelActorSink

      FlowShape(messageFromOutside.in, chatSource.out)
}

为了让事情变得简单，我用这个流形状和一个非常简单的源和水槽。就像这样--

val source = Source(List[String]("hi", "hello", "what are you upto", "this is nice"))
val sink = Sink.foreach[ChatMessage] {
  case tm: UserTextMessage => println(s"${tm.user.username} :: ${tm.content}")
  case ul: UserLeft => println(s"${ul.user.username} just left the channel")
  case uj: UserJoined => println(s"${uj.user.username} just joined the channel")
  case _ => println(s"do not know what I just received")
}

val mychatchannel = new Channel(420, myactorsystem)

source.via(mychatchannel.chatFlow(User("sushruta"))).runWith(sink)

现在，我的担心来了。在终端中打印的事件顺序是完全不确定的。我也不知道该怎么解决。这是我得到的输出--

[INFO] [11/10/2017 17:42:20.431] [akka-streams-akka.actor.default-dispatcher-5] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/10/2017 17:42:20.441] [akka-streams-akka.actor.default-dispatcher-5] [akka://akka-streams/user/channel-actor-420] received a user joined message
[INFO] [11/10/2017 17:42:20.443] [akka-streams-akka.actor.default-dispatcher-5] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/10/2017 17:42:20.444] [akka-streams-akka.actor.default-dispatcher-5] [akka://akka-streams/user/channel-actor-420] sushruta sent a message

输出中缺少第一个消息hi。hi消息似乎是在打印UserJoin message之前发送的。

我尝试通过使用actorRefWithAck (我在上面的代码中注释掉)来修复它(并在消息传递方面添加一些安全性)。它给出了类似的输出。

[INFO] [11/11/2017 06:33:03.731] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] channel initialized and ready to take events
[INFO] [11/11/2017 06:33:03.735] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/11/2017 06:33:03.736] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] received a user joined message
[INFO] [11/11/2017 06:33:03.737] [akka-streams-akka.actor.default-dispatcher-4] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/11/2017 06:33:03.737] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/11/2017 06:33:03.738] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] sushruta sent a message
[INFO] [11/11/2017 06:33:03.738] [akka-streams-akka.actor.default-dispatcher-3] [akka://akka-streams/user/channel-actor-420] received a UserLeft message

显然，似乎正在发生的是，在发送UserJoin消息之前，源正在发送消息。我怎么才能解决这个问题？从概念上讲，我想我希望在源出现之前，但在它实际发送第一条消息之前，立即发送UserJoin message。这有可能吗？

谢谢

Answer 1

把溪流想象成水管:当有水时，它就会流动。合并运算符不关心来自哪个侧元素。如果你想订购这些输入，你需要告诉Akka，而不是使用Concat。