How to represent that an Akka actor is in a long running operation using FSM-s

Question

My actor that is described with an FSM is waiting for a trigger (in Idle state). When it gets it it starts processing some data (and goes to Running state) and when it is done it goes back to Idle state.

If I understand the FSM model correctly, from this point of view there were two events: processing started (Idle->Running) and processing completed (Running->Idle).

But from the actor point of view there was only one message.

One possibility is to delegate the processing itself to another actor. So I can forward the triggering event and go to Running state, then when got the result I go to Idle. It has the advantage that the FSM itself can quickly react to requests (e.g. asking what's the current state), but it makes the design more complicated.

Another one is to send a completed message to self when the actor has completed processing, which will trigger the Running -> Idle transition, but it looks a bit strange for me.

What other options do I have?

Note: there are several other states with various transitions so I'd like to stick to the FSM model.

Susan Potter Susan Potter · Accepted Answer · 2013-09-30T12:47:34

Since it seems like you have an actor that needs to do processing and make transitions to an FSM, I suggest you follow these guidelines (some basic code outline follows this list):

Actor A receives the message in question that will trigger some processing
Actor A transitions a separate FSM actor, say F, (akka.actor.FSM) to the appropriate state. Actor A on startup would spawn the specific FSM to track the state for the corresponding context (e.g. for all transactions or state per transaction or some other context). The code outline below uses all processing or completed transactions as the context for the example but may need to be changed.
Actor A then triggers whatever processing should be triggered for the message. Remember actors shouldn't block generally, but here is an answer that provides more guidelines on when an Akka actor may block.
Alternative: If you can trigger long running processing without blocking and ensure you receive the necessary events after processing stages by the other party, then you could eliminate the front Actor A and just have the FSM actor F. You should look at onTransition in this case.

So my code outline suggestion is based on what I understood from the question is:

/* Events */
sealed trait MyEvents
case class ProcessingStarted(txnId: Long) extends MyEvents
case class ProcessingFinished(txnId: Long, result: Result) extends MyEvents

/* Valid states for your FSM */
sealed trait MyStates 
case object Idle extends MyStates
/* Constructor arguments could be anything, I randomly chose a Long for a transaction ID which may be specific to a job */
case object Processing extends MyStates
/* define more case classes or objects depending on the rest of the states */

/* Valid internal state data types for FSM */
sealed trait MyDataTypes
case object Uninitialized extends MyDataTypes
case class StateData(processingIds: Seq[Long], resultMap: Map[Long, Result]) extends MyDataTypes

import akka.actor.{ Actor, ActorRef, FSM }
import scala.concurrent.duration._

 class ActorF extends FSM[MyStates, MyDataTypes] {
   startWith(Idle, Uninitialized)

   when(Idle) {
     case Event(ProcessingStarted(txnId), Uninitialized) =>
       goto(Processing) using StateData(Seq(txnId), Map.empty[Long, Result])
     case Event(ProcessingStarted(txnId), StateData(emptyIds, resultMap)) =>
       goto(Processing) using StateData(Seq(txnId), resultMap)
   }

   when(Processing) {
     case Event(ProcessingFinished(txnId, result), StateData(processingIds, resultMap)) => {
       val remainingIds = processingIds diff Seq(txnId)
       val newResultMap = resultMap + (txnId -> result)
       if (remainingIds.isEmpty) {
         goto(Idle) using StateData(remainingIds, newResultMap)
       } else {
         stay using StateData(remainingIds, newResultMap)
       }
     }
   }

   initialize()
 }

 // inside Actor A do something like this for creating the FSM actor (just like other actors)
 val f = system.actorOf(Props(classOf[ActorF], this))

 // send an event message to it just like other types of actors
 f ! ProcessingStarted(txnId)

If you choose to trigger off non-blocking processing requests to other parts of your code you can use onTransition to add the trigger code in as necessary. You may also want to update the MyEvents case classes to a different tense. The event naming was used above was to show that something else was responsible for triggering that off (e.g. Actor A the received the initial message to do some things).

Also take note of Supervision capabilities of Akka which could be used here to supervise the actors in question.

For more details read the following which might help further with building your FSM, testing it, using non-blocking methods to trigger long-running processing externally. All of which might be useful for your needs:

How to represent that an Akka actor is in a long running operation using FSM-s

1 Answers