While immutable data structures are preferable in Scala, it's not always obvious how to utilize them inside of an Akka actor when you need the actor to be stateful. Thankfully, Akka provides some help here, by use of the become method, which is designed to update the behavior of an actor. Scala supports closures, so we can pass the new state in …

actor.scala

import akka.actor.Actor

// Following the common Akka pattern of defining the messaging 
// protocol in the companion object
object SampleActor {
  case object GetState
  case class UpdateState(key: String, value: String)
}

class SampleActor extends Actor {

  // This case class represents the actor's state
  case class State(someData: Map[String, String])

  // Initialize the actors behavior with an empty state
  def receive = updated(State(Map.empty))

  // This defines the behavior of the actor. Note that there could be 
  // multiple definitions like this, with the actor switching behaviors
  // and not just state.
  def updated(state: State): Receive = {
    case UpdateState(k, v) =>
      context.become(state.copy(state.someData ++ (k -> v)))
    case GetState => 
      sender ! state
  }
}