Actors.kt

interface Package
object Echo : Package
object Broadcast : Package
data class Children(val response: Receptionist.Listing) : Package

val childrenServiceKey: ServiceKey<Echo> = ServiceKey.create(Echo::class.java, "children")

val father: Behavior<Package> = Behaviors.setup<Package> { ctx ->
    val receptionistAdapter = ctx.messageAdapter(Receptionist.Listing::class.java) { Children(it) }

    repeat(2) {
        val child = ctx.spawnAnonymous(child)
        ctx.system.receptionist().tell(Receptionist.register(childrenServiceKey, child))
    }
    Behaviors.receive<Package> { context, msg ->
        when (msg) {
            is Echo -> println("Echo from ${ctx.self.path()}")

            is Broadcast -> {
                println("Echo from ${ctx.self.path()}")
                context.system.receptionist().tell(Receptionist.find(childrenServiceKey, receptionistAdapter))
            }
            is Children -> msg.response.serviceInstances(childrenServiceKey).foreach { it.tell(Echo) }
        }
        Behavior.same()
    }
}

val child: Behavior<Echo> = Behaviors.receive<Echo> { ctx, _ ->
    println("Echo from ${ctx.self.path()}")
    Behavior.stopped()
}

fun main() {
    val system = ActorSystem.create(father, "test")
    system.tell(Echo)
    system.tell(Broadcast)
}

output

Echo from akka://test/user
Echo from akka://test/user
Echo from akka://test/user/$b
Echo from akka://test/user/$c