Creating the Actors
So far we have looked at the definitions of Actors and their messages. Now let’s dive a bit deeper into the power of location transparency and see how to create Actor instances.
The power of location transparency
In Akka you can’t create an instance of an Actor using the new
keyword. Instead, you create Actor instances using a factory spawn
method. Spawn does not return an actor instance, but a reference, akka.actor.typed.ActorRef
, that points to the actor instance. This level of indirection adds a lot of power and flexibility in a distributed system.
In Akka location doesn’t matter. Location transparency means that the ActorRef
can, while retaining the same semantics, represent an instance of the running actor in-process or on a remote machine. If needed, the runtime can optimize the system by changing an Actor’s location or the entire application topology while it is running. This enables the “let it crash” model of failure management in which the system can heal itself by crashing faulty Actors and restarting healthy ones.
The Akka ActorSystem
An ActorSystem
is the intial entry point into Akka. Usually only one ActorSystem
is created per application. An ActorSystem
has a name and a guardian actor. The bootstrap of your application is typically done within the guardian actor.
The guardian actor of this ActorSystem
is GreeterMain
.
val greeterMain: ActorSystem[GreeterMain.SayHello] = ActorSystem(GreeterMain(), "AkkaQuickStart")
It uses Behaviors.setup
to bootstrap the application.
object GreeterMain {
final case class SayHello(name: String)
def apply(): Behavior[SayHello] =
Behaviors.setup { context =>
val greeter = context.spawn(Greeter(), "greeter")
Behaviors.receiveMessage { message =>
val replyTo = context.spawn(GreeterBot(max = 3), message.name)
greeter ! Greeter.Greet(message.name, replyTo)
Behaviors.same
}
}
}
Spawning child actors
Other actors are created using spawn
methods on ActorContext
. The GreeterMain
creates a Greeter
actor this way on startup as well as a new GreeterBot
each time it receives a SayHello
message.
val greeter = context.spawn(Greeter(), "greeter")
val replyTo = context.spawn(GreeterBot(max = 3), message.name)