sender inside a future

I have an actor which on receiving a message, searches the filesystem for a file and returns the full path of the File.

To keep it asynchronous, I have done:

def receive = {
  case s:String => {

    val f = future{
      val ans = search(s)
      println("Input Request: "+s+" output:"+ans+" "+sender.path)
    }
    f.onComplete{
      case Success(x) => sender ! x
      case Failure(y) => println("Could not complete it")
    }
  } 
}

But I have observed that it returns the message to akka://FileSystem/deadLetters and not the sender. Documentation says that:

Only valid within the Actor itself, so do not close over it and * publish it to other threads!

So does it mean, I will have to necessarily keep it synchronous? Is there any other way?

Solution 1:

You are making a very common mistake of "closing over mutable state". The closure you pass to onComplete does not make a copy of this.sender, so when your onComplete gets called, you are sending the message to whatever this.sender happens to point to at that time, not what it pointed to when you created the closure.

You can avoid this problem by creating your own local, immutable copy of the current contents of this.sender, and reference that value in the closure:

val origSender = sender
f.onComplete {
    case Successs(x) => origSender ! x
    ...
}

Solution 2:

import akka.pattern.pipe

Does the trick. Doing:

val reply = sender
future {
  val ans = searchAndCache(s)
  println("Input Request: "+s+" output:"+ans+" "+reply.path)
  ans
} pipeTo reply

replies back to the sender

Solution 3:

I know this is old but I've got to add this,
the pipeTo is the right way but you don't need to copy your sender,
you're already in the same context.
actually the pipeTo does this for you.
it will take the current sender ref (by passing to it's argument)
and use it to resolve the future for you (look at it's implementation)
just do:

future {
  val ans = searchAndCache(s)
  println("Input Request: "+s+" output:"+ans+" "+reply.path)
  ans
} pipeTo reply