Part 2: Creating the First Actor

With an understanding of actor hierarchy and behavior, the remaining question is how to map the top-level components of our IoT system to actors. It might be tempting to make the actors that represent devices and dashboards at the top level. Instead, we recommend creating an explicit component that represents the whole application. In other words, we will have a single top-level actor in our IoT system. The components that create and manage devices and dashboards will be children of this actor. This allows us to refactor the example use case architecture diagram into a tree of actors:

actor tree diagram of the architecture

We can define the first actor, the IotSupervisor, with a few simple lines of code. To start your tutorial application:

  1. Create a new IotSupervisor source file in the com.lightbend.akka.sample package.
  2. Paste the following code into the new file to define the IotSupervisor.
Scala
package com.lightbend.akka.sample

import akka.actor.{ Actor, ActorLogging, Props }

object IotSupervisor {
  def props(): Props = Props(new IotSupervisor)
}

class IotSupervisor extends Actor with ActorLogging {
  override def preStart(): Unit = log.info("IoT Application started")
  override def postStop(): Unit = log.info("IoT Application stopped")

  // No need to handle any messages
  override def receive = Actor.emptyBehavior

}
Java
package com.lightbend.akka.sample;

import akka.actor.AbstractActor;
import akka.actor.ActorLogging;
import akka.actor.Props;
import akka.event.Logging;
import akka.event.LoggingAdapter;

public class IotSupervisor extends AbstractActor {
  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

  public static Props props() {
    return Props.create(IotSupervisor.class);
  }

  @Override
  public void preStart() {
    log.info("IoT Application started");
  }

  @Override
  public void postStop() {
    log.info("IoT Application stopped");
  }

  // No need to handle any messages
  @Override
  public Receive createReceive() {
    return receiveBuilder()
            .build();
  }

}

The code is similar to the actor examples we used in the previous experiments, but notice:

  • Instead of println() we use the ActorLogging helper trait akka.event.Logging, which directly invokes Akka’s built in logging facility.
  • We use the recommended pattern for creating actors by defining a props() method in the companion object of static method on the actor.

To provide the main entry point that creates the actor system, add the following code to the new IotApp object IotMain class.

Scala
package com.lightbend.akka.sample

import akka.actor.ActorSystem
import scala.io.StdIn

object IotApp {

  def main(args: Array[String]): Unit = {
    val system = ActorSystem("iot-system")

    try {
      // Create top level supervisor
      val supervisor = system.actorOf(IotSupervisor.props(), "iot-supervisor")
      // Exit the system after ENTER is pressed
      StdIn.readLine()
    } finally {
      system.terminate()
    }
  }

}
Java
package com.lightbend.akka.sample;

import java.io.IOException;

import akka.actor.ActorSystem;
import akka.actor.ActorRef;

public class IotMain {

  public static void main(String[] args) throws IOException {
    ActorSystem system = ActorSystem.create("iot-system");

    try {
      // Create top level supervisor
      ActorRef supervisor = system.actorOf(IotSupervisor.props(), "iot-supervisor");

      System.out.println("Press ENTER to exit the system");
      System.in.read();
    } finally {
      system.terminate();
    }
  }

}

The application does little, other than print out that it is started. But, we have the first actor in place and we are ready to add other actors.

What’s next?

In the following chapters we will grow the application gradually, by:

  1. Creating the representation for a device.
  2. Creating the device management component.
  3. Adding query capabilities to device groups.
The source code for this page can be found here.