Appropriate Future Handling in Akka Actors Typed

Question

What is the proper way to handle Futures from inside an Akka (typed) Actor?

For example, assume there is an Actor OrderActor that receives Commands to place orders... which it does by making an http call to an external service. Since these are http calls to an external service, Futures are involved. So, what is the right way to handle that Future from within the Actor.

I read something about the pipeTo pattern. Is that what needs to happen here or something else?

class OrderActor(context: ActorContext[OrderCommand], orderFacade: OrderFacade)
    extends AbstractBehavior[OrderCommand](context) {

  context.log.info("Order Actor started")

  override def onMessage(msg: OrderCommand): Behavior[OrderCommand] = {

    msg match {

      case PlaceOrder(
            referenceId: OrderReferenceId,
            ticker: Ticker,
            quantity: Int,
            replyTo: ActorRef[OrderResult]
          ) =>
        orderFacade
          .placeOrder(ticker, quantity) //this returns a Future
          .map(res => {

            //transform result
            //book keeping / notification (affects state)
            replyTo ! transformed

            //Can/Should we map like  this? I tried adding a log statement in here, but I never see it... and the replyTo doesnt seem to get the message. 

          })

        this

user_5641244 · Answer

It's generally best to avoid doing Future transformations (map, flatMap, foreach, etc.) inside an actor. There's a distinct risk that some mutable state within the actor isn't what you expect it to be when the transformation runs. In Akka Classic, perhaps the most pernicious form of this would result in sending a reply to the wrong actor.

Akka Typed (especially in the functional API) reduces a lot of the mutable state which could cause trouble, but it's still generally a good idea to pipe the Future as a message to the actor.

So if orderFacade.placeOrder results in a Future[OrderResponse], you might add subclasses of OrderCommand like this

// also include fields from the PlaceOrder which will be useful
case class OrderResponseIs(resp: OrderResponse, replyTo: ActorRef[OrderResult]) extends OrderCommand
// TODO include fields
case class OrderFailed() extends OrderCommand

And then pipe the Future to yourself with:

import scala.util.{ Failure, Success }

context.pipeToSelf(orderFacade.placeOrder) {
  case Success(resp) => OrderResponseIs(resp, replyTo)
  case Failure(_) => OrderFailed()
}

You then have to handle those messages:

case OrderResponseIs(resp, replyTo) =>
  // transform resp
  val transformed = ???

  replyTo ! transformed

  this

case OrderFailed() =>
  context.log.warning("Stuff is broken")

  this

There's not actually much overhead in doing this versus map and friends (both will typically involve scheduling a task for asynchronous execution on the dispatcher).

Appropriate Future Handling in Akka Actors Typed

1 Answers