Unit testing scala actors

Question

Anyone know of a good way to unit test Scala actors? In the general sense I have an actor that receives a message and will send out other messages in response. This is done on multiple threads, and an actor that is not correct may either send the wrong messages or no message at all. I need a simple way of creating a mockup actor that send and receives messages to the actor being tested. Any experiences in this area?

Answer 1

Because of the dynamic nature of actor-style message passing, mocking actors is usually no trouble at all. Just create an actor which receives the desired message and you're home free. You will of course need to ensure that this mock actor is the one to which messages are passed, but that shouldn't be a problem as long as the actor you are attempting to test is reentrant.

Answer 2

I think the complexity depends on a couple factors...

1. How stateful is the actor?

If it behaves like a idempotent function, only asynchronous, then it should be a simple matter of mocking up an actor that sends a message and then checks that it receives the expected messages back. You probably want to use a react/receiveWithin on the mock actor in case there is response within a reasonable period of time you can fail rather than hanging.

However if the messages aren't independent of one another, then you should test it with various sequences of messages and expected results.

1. How many actors will the actor being tested interact with?

If an actor is expected to interact with many others, and it is stateful, then it should be tested with several actors sending/receiving messages. Since you probably have no guarantee of the order in which the messages will arrive, you should be sure to either permute the orders in which the actors send the messages or introduce random pauses in the actors generating messages and run the test many times.

I'm not aware of any prebuilt frameworks for testing actors, but you could possibly look to Erlang for inspiration.

http://svn.process-one.net/contribs/trunk/eunit/doc/overview-summary.html

Answer 3

I have been wondering about how to test Actors myself.

Here is what I came up with, does anybody see problems with this approach?

Rather than send messages directly, what if your actor delegated message sending to a function?

Then your tests can swap out the function with one that tracks the number of times called and/or the arguments with which the method was called:

class MyActor extends Actor {

  var sendMessage:(Actor, ContactMsg) => Unit = {
    (contactActor, msg) => {
      Log.trace("real sendMessage called")
      contactActor ! msg 
    }
  }

  var reactImpl:PartialFunction(Any, Unit) = {
      case INCOMING(otherActor1, otherActor2, args) => {

        /* logic to test */
        if(args){
          sendMessage(otherActor1, OUTGOING_1("foo"))

        } else {
          sendMessage(otherActor2, OUTGOING_2("bar"))
        }
      }
  }

  final def act = loop {
    react {
      reactImpl
    }
  }

Your test case might contain code like:

// setup the test
var myActor = new MyActor
var target1 = new MyActor
var target2 = new MyActor
var sendMessageCalls:List[(Actor, String)] = Nil

/*
* Create a fake implementation of sendMessage
* that tracks the arguments it was called with
* in the sendMessageCalls list:
*/
myActor.sendMessage = (actor, message) => {
  Log.trace("fake sendMessage called")
  message match {
    case OUTGOING_1(payload) => { 
      sendMessageCalls = (actor, payload) :: sendMessageCalls
    }
    case _ => { fail("Unexpected Message sent:"+message) }
  }
}

// run the test
myActor.start
myActor.reactImpl(Incoming(target1, target2, true))

// assert the results
assertEquals(1, sendMessageCalls.size)
val(sentActor, sentPayload) = sendMessageCalls(0)
assertSame(target1, sentActor)
assertEquals("foo", sentPayload)
// .. etc.

Answer 4

My attempt at unit testing an actor (it works). I'm using Specs as a framework.

object ControllerSpec extends Specification {
  "ChatController" should{
    "add a listener and respond SendFriends" in{
        var res = false
        val a = actor{}
        val mos = {ChatController !? AddListener(a)}
        mos match{
             case SendFriends => res = true
             case _ => res = false
        }
        res must beTrue
    }

How this works is by sending a synchronous call to the singleton ChatController. ChatController responds by use of reply(). The response is sent as a return of the called function, which gets stored into mos. Then a match is applied to mos getting the case class that was sent from ChatController. If the result is what is expected (SendFriends) set res to true. The res must beTrue assertion determines the success or failure of test.

My actor singleton that I'm testing

import ldc.socialirc.model._

import scala.collection.mutable.{HashMap, HashSet}
import scala.actors.Actor
import scala.actors.Actor._

import net.liftweb.util.Helpers._

//Message types
case class AddListener(listener: Actor)
case class RemoveListener(listener: Actor)
case class SendFriends
//Data Types
case class Authority(usr: Actor, role: String)
case class Channel(channelName: String, password: String, creator: String, motd:   String, users: HashSet[Authority])

object ChatController extends Actor {
    // The Channel List  - Shows what actors are in each Chan
    val chanList = new HashMap[String, Channel]
    // The Actor List - Shows what channels its in
    val actorList = new HashMap[Actor, HashSet[String]]

    def notifyListeners = {

    }

    def act = {
        loop {
            react {
                case AddListener(listener: Actor)=>
                    actorList += listener -> new HashSet[String]
                    reply(SendFriends)

            }
        }
    }
    start //Dont forget to start
}

Though its not complete it does return the Sendfriends case class as expected.