Is it safe for Akka actor become method to close over immutable state?

Question

The purpose here is to implement a very simple cache for an actor that needs to call an external service (or some expensive but highly cacheable operation) without using mutable state.

class A extends Actor{
  def receive = {
    case GetCommand => 
      val response = callExternalService()
      context.become(receiveWithCache(response))
      context.system.scheduler.schedule(1 day, 1 day, self, InvalidateCache)
      sender ! response
  }
  def receiveWithCache(cachedResponse:R): PartialFunction[Any,Unit] = {
    case GetCommand => sender ! cachedResponse
    case InvalidateCache => context.unbecome
  }
}

I know there are more advanced ways to implement this, among which a fully fledged CacheSystem that can be found in the Akka patterns pages, but in some cases that really isn't required.

Plus, it's interesting to know the answer if using become like this is safe.

Answer 1

As far as I know this technique is sound and should be good for you to use. It's actually a more clever way to get around having to have a mutable var response in your code. I actully used this technique in an answer here and Viktor from the Akka team seemed to think it was a good solution. One thing though, you can change:

def receiveWithCache(cachedResponse:R): PartialFunction[Any,Unit] = {
  case GetCommand => sender ! cachedResponse
  case InvalidateCache => context.unbecome
}

to:

def receiveWithCache(cachedResponse:R): Receive = {
  case GetCommand => sender ! cachedResponse
  case InvalidateCache => context.unbecome
}

The Receive type is a shorthand alias for PartialFunction[Any,Unit].