[SOLVED] Akka child actors return a void type and refuse to send messages

Akka child actors return a void type and refuse to send messages

I was learnning Akka Typed Actor library and tried to send a message to all child actors of a particular actor, using this.getContext().getChildren(). But it returns a Void type and not the type I initially created the child actors with -> hence I could not send any type of message to the children.

The workaround I did is to simply store the children in a hashmap at the time of creation. But this is not a scalable one.

How can I achieve this? especially under distributed scenario?

        .onMessage(GetPosition.class, message -> {

            // whenever I send a GetPosition() message to myself,
            // I get that reminder and send an AskPosition to all workers

            while (results.keySet().size() < 10) {
                // while not finished, ask for positions from child actors

                // This is not working
                this.getContext().getChildren().forEach(child -> {
                    child.tell("this is a void class, so I cannot send any message to children");
                });

                // WORKAROUND:
                // get child  actors from hashmap (hashmap lambda has 2 things in it -> bi-consumer)
                racers.forEach((id, actor) -> actor.tell(new RacerBehavior.AskPosition(this.getContext().getSelf())));
            }

            return this;
        })

Solution

Because it's not possible to statically know the types of messages accepted by the children which an arbitrary actor will spawn, getChildren() can only return ActorRef<Void>s (or ActorRef[Nothing]s in Scala, which better captures the idea that you cannot send them messages).

If all your children happen to be the same type (which would be a prerequisite for being able to iterate through them in to send messages), something like this should work:

child.unsafeUpcast<Object>.narrow<RacerBehavior.AskPosition>

If you can't be absolutely sure that all your children are the same type, tracking them explicitly with their types in the parent actor's state (e.g. having a collection per-type) is the way to go.