Actor Model: OO implementation of large scale concurrent application. It can be hierarchical.

Actor: Objects with encapsulated state and behavior, and communication happens exclusively by exchanging messages.

ActorRef: It represents a single actor during communication.


Create Actors: extend Actor and implement the receive method. The receive method is a set of case statements for all messages the actor receives and their corresponding behaviors.

Props is a configuration class to specify options for the creation of actors. An instance of it is passed to the actorOf factory method of ActorSystem and ActorContext to create actors.

Actor API:

  • self: an ActorRef of the actor
  • sender: reference sender Actor of the message
  • context
  • Overridable life-cycle hooks: preStart; postStop; preRestart; postRestart, all return Unit.

Message passing:

  • The ! operator (or tell method) to send a one-way asynchronous message (actor ! Message()) and returns Unit immediately.
  • The ? operator (or ask method) sends a message and returns a Future of possible reply. Usually the Future is pipeTo an actor to register onComplete handler when the Future returns. Note Future is a generic type.
  • Message can be forwarded and sender reference is maintained: <target> forward <message>

Messages can be defined in sender or receiver. All messages belong to an Actor share a common empty sealed trait, and the messages are declared as case classes so that the fields for message content is automatically generated:

sealed trait All<Actor_name><Description>Message
case class <description>Message(content1: type1, content2: type2, ) extends All<Actor_name><Description>Message


State * Event -> Action + State (next)

States are defined using the similar manner as messages, but use case object instead:

sealed trait State
case object <description>State extends State

Use startWith(, ) to define start state. Use initialize to start the FSM Usually there is one when() { … } declaration per state. Inside when block, each Event(, ) is matched with case statement. Inside each case statement, use goto() or stay to define next state, and use using modifier to define data used in next state:

when(<current_state>) {
 	case Event(<message_received>, <data>) => {
      	// do something here
      	stay using <new_data>
      	// or
      	// goto(<new_state>) using <new_data>

whenUnhandled { … } statement handles unhandled massages.

onTransition {
 	case <old_state> -> <new_state> => {
      	// do something here

handles operations when state changes.

Stop a FSM by the result state as stop(). It takes an optional parameter reason, which is one of Normal (default), Shutdown, or Failure. You can use onTermination handler to execute cleanup:

onTermination {
 	case StopEvent(FSM.Normal, state, data) => {  }
 	case StopEvent(FSM.Shutdown, state, data) => {  }
 	case StopEvent(FSM.Failure(cause), state, data) => {  }