如何测试 Akka 集群中 DistributedPubSub 的发布者？

Question

我有一个参与者，其唯一职责是将从外部界面（命令行、用户等）接收到的消息转发到适当的主题。我想测试它是否正确发布这些消息。

我需要创建一些虚拟订阅者，这些订阅者会期望消息发布到某个主题并对接收到的消息做出断言。

这是我试图实现的代码：

Messages.scala

case class Foo(foo: String)

InterfaceForwardingActor.scala

import akka.actor.{Actor, ActorLogging}
import akka.cluster.pubsub.{DistributedPubSub, DistributedPubSubMediator}
import com.typesafe.config.Config

/** Actor responsible for forwarding stimuli external to the system.
  * For instance, messages from the command-line interface or from a UI.
  *
  */
class InterfaceForwardingActor extends Actor with ActorLogging {
  import DistributedPubSubMediator.Publish

  protected val mediator = DistributedPubSub(context.system).mediator

  log.info(s"Hello from interface forwarder.")

  final val topic = "info"

  def receive = {
    case foo: Foo => {
      log.info("Forwarding a Foo message")
      mediator ! Publish(topic, foo)
    }
  }  
}

和测试代码

InterfaceForwardingActorTest.scala

import akka.actor.{ActorSystem, Props}
import akka.cluster.client.ClusterClient.Publish
import akka.cluster.pubsub.DistributedPubSub
import akka.testkit.{ImplicitSender, TestKit, TestProbe}
import com.typesafe.config.ConfigFactory
import org.scalatest.{BeforeAndAfterAll, Matchers, WordSpecLike}

class InterfaceForwardingActorTest extends 
    TestKit(ActorSystem("InterfaceForwardingActorSpec")) with ImplicitSender with 
    WordSpecLike with Matchers with BeforeAndAfterAll {

  override def afterAll {
    TestKit.shutdownActorSystem(system)
  }

  "An InterfaceForwardingActor" must {
    val interfaceForwardingActor = system.actorOf(Props[InterfaceForwardingActor])

    val probe = TestProbe()
    val mediator = DistributedPubSub(system).mediator

    // subscribe the test probe to the "info" topic
    mediator ! Publish("info", probe.ref)

    "publish a Foo message" in {
      val msg = Foo("test")
      interfaceForwardingActor ! msg
      probe.expectMsg(msg)
    }
  }
}

我发现订阅info主题的probe在默认的3秒超时时间内没有收到消息，断言失败。然而，有趣的是，我确实看到日志消息表明接口转发 actor 确实在转发 Foo 消息。

我在测试中做错了什么？

Answer 1

TestProbe应该订阅测试代码中的主题：

mediator ! Subscribe("info", probe.ref)

而不是

mediator ! Publish("info", probe.ref)

分布式 pub-sub 的文档页面是 here，供参考。