Scala akka how create entry point for event source?

Question

I've been struggling to learn akka actors and I REALLY need your help guys!

So my goal is basically to write a simple auction agent. And AKKA actually has an example on how to do it https://doc.akka.io/docs/akka/current/typed/replicated-eventsourcing-auction.html! The problem is, I have no idea how to run it. Now I've spent like 3 days on trying to get it to work before asking for your help but I've completely lost it..:/

So, I started to learn about typed actors first and how they work. I've managed to actually print something on the screen by creating a simple actor system like this that I've found on the internet where I have a simple order actor (typed) and on its apply method I can print the incoming order:

//Entry point inside main(args <...>)
val orderProcessor: ActorSystem[OrderProcessor.Order] = ActorSystem(OrderProcessor(), "main")
//create a new order
orderProcessor ! Order(0, "Bananas")
//<..printing something inside the actor when receiving this message>

Now the auction example uses EventSourcedBehavior so I came to the conclusion that next step is to learn about event sourcing in Akka (hopefully this isn't confusing so far or hopefully I'm on the right path) So I went to the official documentation of Akka's event sourcing https://doc.akka.io/docs/akka/current/typed/persistence.html#module-info and I took their example:

object MyPersistentBehavior {
  sealed trait Command
  final case class Add(data: String) extends Command
  case object Clear extends Command

  sealed trait Event
  final case class Added(data: String) extends Event
  case object Cleared extends Event

  final case class State(history: List[String] = Nil)

  val eventHandler: (State, Event) => State = { (state, event) =>
    event match {
      case Added(data) => state.copy((data :: state.history).take(5))
      case Cleared     => State(Nil)
    }
  }

  val commandHandler: (State, Command) => Effect[Event, State] = { (state, command) =>
    command match {
      case Add(data) => Effect.persist(Added(data))
      case Clear     => Effect.persist(Cleared)
    }
  }

  def apply(id: String): Behavior[Command] =
    EventSourcedBehavior[Command, Event, State](
      persistenceId = PersistenceId.ofUniqueId(id),
      emptyState = State(Nil),
      commandHandler = commandHandler,
      eventHandler = eventHandler)
}

Now that is great! Very simple and concise.

The problem is, I don't understand where the entry point is? Like for orderProcessor (first example that I've shown) it is obviously just to create a new ActorSystem and thats it, but I can't find any information on this example. I've tried soooo many different projects from github and none of them very simple enough for me to understand. To be fair most of them had tests, but tests didn't really help me much.

Please, any help, any tips would be SOOO much appreciated, I'm really struggling guys!

Love Yall !<3

Answer 1

You have to obtain ActorRef[MessangeType] to be able to send MessageType to it.

Existing actor has context (passed as an argument with Behavior definition) where you can create a child like:

val actorRef = context.spawn(behavior)

but you can also create it top-level from ActorSystem (then the system is the parent directly)

val actorRef = system.systemActorOf(behavior)

So in your case it could be something like:

val actorRef = system.systemActorOf(MyPersistentBehavior("test"))
actorRef ! MyPersistentBehavior.Add("test")

alternatively you can context.spawn it inside Behavior[OrderProcessor.Order] defined for your ActorSystem, then you'll talk to actorRef through system which would send commands to persistent actor.

Answer 2

I understand your paint, unfortunately Akka Documentation give only Snippets as samples in documentation and they are most of the time not complete and hard to understand.

You can find in their Github complete examples I guess you can understands things better this way.

If you want to see a full fledged Proof of Concept application with Akka, I have a blog about it which you can find here.