How to supervise actors in Akka and handle exceptions

Question

I am trying to improve the error handling with the actors in my system. Sometimes, when processing data, something goes wrong, and I need to stop and restart the actor, as well as log some information regarding this failure.

I have a Supervisor, which has 5 actors working for it. So I need to be able to supervise all of them. I found this link:

https://doc.akka.io/docs/akka/current/typed/fault-tolerance.html

regarding this, but I don't think it is very clear on where to implement the code:

Behaviors.supervise(behavior).onFailure[IllegalStateException](SupervisorStrategy.restart)

Where exactly is this code supposed to go?

Thanks

Answer 1

You can think of this supervisor as another behavioiur which wraps your behaviour inside of it.

Lets say you want to have following HelloWorld actor.

object HelloWorldActor {

  sealed trait Command
  case class HelloCommand(name: String) extends Command

  def apply(): Behavior[Command] =
    Behaviors.receiveMessage[Command] { 
      case msg: HelloCommand =>
        println(s"Hello ${msg.name}")
        Behaviors.same
    }

}

Now, you can "wrap" this "behaviour" with a "supervisor"

object SupervisedHelloWorldActor {

  sealed trait Command
  case class HelloCommand(name: String) extends Command

  def apply(): Behavior[Command] =
    Behaviors.supervise(
      Behaviors.receiveMessage[Command] { 
        case HelloCommand(name) =>
          println(s"Hello ${name}")
          Behaviors.same
      }
    ).onFailure(onFailure[IllegalStateException](SupervisorStrategy.restart))

}

Answer 2

Supervise normally happens in a parent-child structure, if you want to supervise n actors, create a Supervisor as a father of those n actors.

In typed, the example like this:

def parent2: Behavior[String] = {
  Behaviors.setup { ctx =>
    val child1 = ctx.spawn(Behaviors.supervise(child(1)).onFailure[IllegalStateException](SupervisorStrategy.restart), "child1")
    val child2 = ctx.spawn(Behaviors.supervise(child(2)).onFailure[IllegalStateException](SupervisorStrategy.restart), "child2")

    Behaviors.empty
  }
}