[akka-user] Does Event Bus require Publisher and Subscriber to be started in certain order?

[kant kodali]

Hi All,

Does Event Bus require Publisher and Subscriber to be started in certain 
order? Isn't there a Listener which will push messages to subscribers 
whenever they become alive? 

I am playing with the LookUpBus Example from here 
<http://doc.akka.io/docs/akka/current/java/event-bus.html> but it doesn't 
quite work for me.  here is my sample code and I had also commented on the 
respective lines.

For now, I am just using print statements to debug. since the messages are 
getting printed out I wonder if Publisher and Subscriber to be started in 
certain order?

Thanks!


public class EventBusManager {

    private static final LookupBusImpl LOOKUP_BUS = new LookupBusImpl();

    public static LookupBusImpl getEventBus() {
        return LOOKUP_BUS;
    }
}


public class UserActor extends UntypedActor {

    private final ActorRef out;
    private final String topic;

    public UserActor(ActorRef out, String topic) {
        this.out = out;
        this.topic = topic;
    }

    public static Props props(ActorRef out, String topic) {
        return Props.create(UserActor.class, out, topic);
    }

    public void preStart() {
        EventBusManager.getEventBus().subscribe(self(), topic); // This Actor 
will become the subscriber of even bus so whenever publisher sends a message I

                                                                // Expect This 
Actor to receive the messages 

    } 

    public void postStop() {
        EventBusManager.getEventBus().unsubscribe(self());
    }

    public void onReceive(Object message) throws Exception {

        System.out.println("Received Message: " + message); // I expect the 
messages to be printed here
        if (message instanceof String) {
            out.tell(message, self());
        }
    }


}



public class LookupBusImpl extends LookupEventBus<MsgEnvelope, ActorRef, 
String> {

    // is used for extracting the classifier from the incoming events
    @Override
    public String classify(MsgEnvelope event) {
        return event.topic;
    }

    // will be invoked for each event for all subscribers which registered 
themselves
    // for the event’s classifier
    @Override
    public void publish(MsgEnvelope event, ActorRef subscriber) {
        System.out.println("LOOKUP_BUS_IMPL: " + event.payload);
        subscriber.tell(event.payload, ActorRef.noSender()); // not sure what 
exactly is going on here? But I Expect the above actor to receive the messages
    }

    // must define a full order over the subscribers, expressed as expected from
    // `java.lang.Comparable.compare`
    @Override
    public int compareSubscribers(ActorRef a, ActorRef b) {
        return a.compareTo(b);
    }

    // determines the initial size of the index data structure
    // used internally (i.e. the expected number of different classifiers)
    @Override
    public int mapSize() {
        return 128;
    }



}


public class Application extends Controller {

    public WebSocket<String> handleWebSocketConnection() {
        return WebSocket.withActor(new F.Function<ActorRef, Props>() {
            @Override
            public Props apply(ActorRef actorRef) throws Throwable {
                return UserActor.props(actorRef, "hello");
            }
        });
    }

}

public class Global extends GlobalSettings {


    @Override
    public void onStart(Application application) { 

        Logger.info("Brodcasted to event Bus started.."); // This prints out to 
console fine 

        EventBusManager.getEventBus().publish(new MsgEnvelope("hello", "Hello 
World")); // This is where I am publishing messages to event bus.
        Logger.info("Brodcasted to event Bus completed"); // This prints out to 
console fine 

    }


}

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