两个参与者之间的请求（使用ActorContext）是非阻塞的。
由于对给定目标的请求的高水位线是参与者协议状态的重要组成部分，因此我只将其存储在发出请求的参与者的状态中（例如，在Scala中为Map[ActorRef[Request], Int]）。当接收到适应的响应时，第一件事是将响应中的ID与目标的高水印进行比较。
例如，在Scala中：

sealed trait RequestA
case class QueryB(target: ActorRef[RequestB]) extends RequestA
case class ResponseFromB(target: ActorRef[RequestB], id: Int, resp: ResponseB) extends RequestA
case class BTimedOut(target: ActorRef[RequestB], id: Int) extends RequestA

sealed trait RequestB

def buildRequestB(id: Int)(replyTo: ResponseB): RequestB = ???

sealed trait ResponseB

def aBehavior(highWater: Map[ActorRef[RequestB], Int]): Behavior[RequestA] =
  Behaviors.receive { (context, msg) =>
    case QueryB(target) =>
      implicit val timeout: Timeout = 10.seconds

      val nextHighwater = highWater.get(target).map(_ + 1).getOrElse(0)

      // request is sent and received "in the background"
      context.ask(target, buildRequestB(nextHighwater)) {
        case Success(resp) => ResponseFromB(target, nextHighwater, resp)
        case Failure(_) => BTimedOut(target, nextHighwater)
      }

      aBehavior(highWater + (target -> nextHighwater))

    case ResponseFromB(target, id, resp) =>
      if (highWater.get(target).contains(id)) {
        context.log.info("Accepting response: {}", resp)
        Behaviors.same
      } else {
        context.log.info("Ignoring response: {}", resp)
        Behaviors.same
      }

    case BTimedOut(target, id) =>
      context.log.warning("Ask of {} (sequence ID {}) timed out", target, id)
      Behaviors.same
  }