Finite State Machine in Akka can be modeled as: 1) A state machine consists of states, events, and actions where an event triggers a state transition and actions. 2) In Akka, a FSM actor encapsulates the state in its behavior so that getting the state is not an action. 3) States are represented as sealed traits or objects, events are regular messages, and state transitions are triggered through goto or stay functions along with optional state data.

1. Finite State Machine in Akka
Dmytro Bischak 2017

2. Mathematical Model of Computation
State(S) x Event(E) -> Actions (A), State(S')

3. Blackbox
Event #1
Event #2
Event #3
State A
Behaviour A
State B
Behaviour B
State C
Behaviour C

4. States, Events, Data
Turnstile

5. How It Works

6. State
Only one State at time
State is Behaviour
Getting state is not an Action

7. State Example
sealed trait State
case object Locked extends State
case object UnLocked extends State
when(Locked) {
case Event(Push, _) =>
println("<Locked!>")
stay()
}

8. Event
val turnstile = system.actorOf(Props[Turnstile])
turnstile ! Coin
Just a regular message to FSM Actor

9. Data
FSM doesn’t have data fields
State encapsulation
when(States.UnLocked) {
case Event(Coin(money), StateData(total)) =>
println(s"<Even more Coins, money: $money, old total: $total>")
stay() using StateData(money + total)
}

10. Timeout
StateTimeout Event
Only when no other Events occur!
when(Locked) {
case Event(Coin, _) =>
goto(UnLocked) forMax TIMEOUT
}
when(States.UnLocked) {
case Event(StateTimeout, _) =>
goto(Locked)
}

11. State Transition
onTransition callback on state changes
Occur only with goto(StateA), not stay()
onTransition {
case UnLocked -> Locked =>
println("<Transition UnLocked -> Locked>")
}

12. Examples
https://github.com/bischak/akka-fsm

13. Conclusions
High level DSL
Easy to describe discrete models
Omitting shared state issues

14. Thank You
Questions?