This slide shows you how to use Akka cluster in Java. Source Code: https://github.com/jiayun/akka_samples If you want to use the links in slide, please download the pdf file.

1. #JCConf
Akka Cluster in Java
Jiayun Zhou
jiayun@gmail.com

2. Introduction

3. Akka

4. Concurrent & Distributed

5. Actor Model

6. http://blog.shiftehfar.org/?p=431

7. Scala

8. Java?

9. Scala is better

10. Java

12. Maven

13. Basic

14. pom.xml
<dependencies>
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-actor_2.11</artifactId>
<version>2.4.1</version>
</dependency>
</dependencies>
<repositories>
<repository>
<id>typesafe</id>
<name>Typesafe Repository</name>
<url>http://repo.typesafe.com/typesafe/releases/</url>
</repository>
</repositories>

15. Main.java
public static void main(String[] args) {
// akka.Main.main(new
String[]{HelloWorld.class.getName()});
ActorSystem system = ActorSystem.create("Hello");
ActorRef a =
system.actorOf(Props.create(HelloWorld.class), "helloWorld");
}

16. HelloWorld Actor
public class HelloWorld extends UntypedActor {
@Override
public void preStart() {
final ActorRef greeter =
getContext().actorOf(Props.create(Greeter.class), "greeter");
greeter.tell(Greeter.Msg.GREET, getSelf());
}
@Override
public void onReceive(Object msg) {
if (msg == Greeter.Msg.DONE) {
getContext().stop(getSelf());
} else
unhandled(msg);
}
}

17. Greeter Actor
public class Greeter extends UntypedActor {
public static enum Msg {
GREET, DONE;
}
@Override
public void onReceive(Object msg) {
if (msg == Msg.GREET) {
System.out.println("Hello World!");
getSender().tell(Msg.DONE, getSelf());
} else
unhandled(msg);
}
}

18. Sending message
• tell() – Fire and forget
• ask() – Send and receive

19. Avoid Ask
• https://www.safaribooksonline.com/library/view/effective-
akka/9781449360061/ch02.html#_avoiding_ask
• https://www.safaribooksonline.com/library/view/effective-
akka/9781449360061/ch03.html#_tell_don_8217_t_ask

20. Remoting

21. pom.xml
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-remote_2.11</artifactId>
<version>2.4.1</version>
</dependency>

22. Local application.conf
LocalSys {
akka {
actor {
provider = "akka.remote.RemoteActorRefProvider"
}
remote {
enabled-transports = ["akka.remote.netty.tcp"]
netty.tcp {
hostname = "127.0.0.1"
port = 2551
}
}
}
}

23. Remote application.conf
RemoteSys {
akka {
actor {
provider = "akka.remote.RemoteActorRefProvider"
}
remote {
enabled-transports = ["akka.remote.netty.tcp"]
netty.tcp {
hostname = "127.0.0.1"
port = 2552
}
}
}
}

24. Local Main
public static void main(String[] args) throws
Exception {
ActorSystem _system =
ActorSystem.create("LocalNodeApp",ConfigFactory
.load().getConfig("LocalSys"));
ActorRef localActor =
_system.actorOf(Props.create(LocalActor.class));
localActor.tell("Hello", null);
Thread.sleep(5000);
_system.shutdown();
}

25. Local Actor
ActorRef remoteActor;
@Override
public void preStart() {
//Get a reference to the remote actor
remoteActor = getContext().actorFor(
"akka.tcp://RemoteNodeApp@127.0.0.1:2552/user/remoteActor"
);
}
@Override
public void onReceive(Object message) throws Exception {
Future<Object> future = Patterns.ask(remoteActor,
message.toString(),
timeout);
String result = (String) Await.result(future,
timeout.duration());
log.info("Message received from Server -> {}", result);
}

26. Actor Path
akka.<protocol>://<actorsystemname>@<hos
tname>:<port>/<actor path>
• http://doc.akka.io/docs/akka/2.4.1/general/addressing.html

27. Remote Main
public static void main(String[] args) {
final ActorSystem system =
ActorSystem.create("RemoteNodeApp", ConfigFactory
.load().getConfig("RemoteSys"));
system.actorOf(Props.create(RemoteActor.class),
"remoteActor");
Runtime.getRuntime().addShutdownHook(new Thread()
{
@Override
public void run() {
system.shutdown();
}
});
}

28. Remote Actor
@Override
public void onReceive(Object message) throws
Exception {
if (message instanceof String) {
// Get reference to the message sender and
reply back
log.info("Message received -> {}", message);
getSender().tell(message + " got something",
null);
}
}

29. Router

30. Router Main
public static void main(String[] args) throws
InterruptedException {
ActorSystem _system =
ActorSystem.create("RemoteRouteeRouterExample",
ConfigFactory.load().getConfig("MyRouterExample"));
Address[] addresses = new Address[]{
new Address("akka.tcp", "RemoteNodeApp",
"10.211.55.6", 2552),
new Address("akka.tcp", "RemoteNodeApp",
"10.211.55.5", 2552)
};
ActorRef router = _system.actorOf(new RemoteRouterConfig(new
RoundRobinPool(5), addresses).props(
Props.create(RemoteActor.class)));
for (int i = 1; i <= 10; i++) {
// sends randomly to actors
router.tell("Hello " + Integer.toString(i), null);
}
_system.shutdown();
}

31. Cluster

32. pom.xml
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-cluster_2.11</artifactId>
<version>2.4.1</version>
</dependency>

33. application.conf
akka {
actor {
provider = "akka.cluster.ClusterActorRefProvider"
}
remote {
log-remote-lifecycle-events = off
netty.tcp {
hostname = "127.0.0.1"
port = 0
}
}
cluster {
seed-nodes = [
"akka.tcp://ClusterSystem@10.211.55.5:2552",
"akka.tcp://ClusterSystem@10.211.55.6:2552"]
auto-down-unreachable-after = 10s
}
}

34. Seed Nodes

35. Member States

36. Backend Main
public static void main(String[] args) {
// Override the configuration of the port when specified as
program argument
final String hostname = args.length > 0 ? args[0] : "127.0.0.1";
final String port = args.length > 1 ? args[1] : "0";
final Config config =
ConfigFactory.parseString("akka.remote.netty.tcp.hostname=" +
hostname).
withFallback(ConfigFactory.parseString("akka.remote.netty.tcp.port="
+ port)).
withFallback(ConfigFactory.parseString("akka.cluster.roles =
[backend]")).
withFallback(ConfigFactory.load());
ActorSystem system = ActorSystem.create("ClusterSystem", config);
system.actorOf(Props.create(TransformationBackend.class),
"backend");
}

37. Backend Actor - 1
Cluster cluster = Cluster.get(getContext().system());
//subscribe to cluster changes, MemberUp
@Override
public void preStart() {
cluster.subscribe(getSelf(), MemberUp.class);
}
//re-subscribe when restart
@Override
public void postStop() {
cluster.unsubscribe(getSelf());
}
void register(Member member) {
if (member.hasRole("frontend"))
getContext().actorSelection(member.address() +
"/user/frontend").tell(
BACKEND_REGISTRATION, getSelf());
}

38. Backend Actor - 2
@Override
public void onReceive(Object message) {
if (message instanceof TransformationJob) {
TransformationJob job = (TransformationJob) message;
getSender().tell(new
TransformationResult(job.getText().toUpperCase()),
getSelf());
} else if (message instanceof CurrentClusterState) {
CurrentClusterState state = (CurrentClusterState) message;
for (Member member : state.getMembers()) {
if (member.status().equals(MemberStatus.up())) {
register(member);
}
}
} else if (message instanceof MemberUp) {
MemberUp mUp = (MemberUp) message;
register(mUp.member());
} else {
unhandled(message);
}
}

39. Frontend Main - 1
final String hostname = args.length > 0 ? args[0] :
"127.0.0.1";
final String port = args.length > 1 ? args[1] : "0";
final Config config =
ConfigFactory.parseString("akka.remote.netty.tcp.host
name=" + hostname).
withFallback(ConfigFactory.parseString("akka.remote.n
etty.tcp.port=" + port)).
withFallback(ConfigFactory.parseString("akka.cluster.
roles = [frontend]")).
withFallback(ConfigFactory.load());
ActorSystem system =
ActorSystem.create("ClusterSystem", config);

40. Frontend Main - 2
final ActorRef frontend = system.actorOf(
Props.create(TransformationFrontend.class),
"frontend");
…
system.scheduler().schedule(interval, interval, new
Runnable() {
public void run() {
Patterns.ask(frontend,
new TransformationJob("hello-" +
counter.incrementAndGet()),
timeout).onSuccess(new OnSuccess<Object>()
{
public void onSuccess(Object result) {
System.out.println(result);
}
}, ec);
}
}, ec);

41. Frontend Actor - 1
List<ActorRef> backends = new ArrayList<ActorRef>();
int jobCounter = 0;
@Override
public void onReceive(Object message) {
if ((message instanceof TransformationJob) &&
backends.isEmpty()) {
TransformationJob job = (TransformationJob)
message;
getSender().tell(
new JobFailed("Service unavailable, try again
later", job),
getSender());

42. Frontend Actor - 2
} else if (message instanceof TransformationJob) {
TransformationJob job = (TransformationJob) message;
jobCounter++;
backends.get(jobCounter % backends.size())
.forward(job, getContext());
} else if (message.equals(BACKEND_REGISTRATION)) {
getContext().watch(getSender());
backends.add(getSender());
} else if (message instanceof Terminated) {
Terminated terminated = (Terminated) message;
backends.remove(terminated.getActor());
} else {
unhandled(message);
}
}

43. Run
java -cp .:./*
sample.cluster.transformation.TransformationBackendMa
in 10.211.55.5 2552
java -cp .:./*
sample.cluster.transformation.TransformationBackendMa
in 10.211.55.6 2552
java -cp .:./*
sample.cluster.transformation.TransformationFrontendM
ain 10.211.55.2 2552

44. Best Practices
• At least two seed nodes
• Use fixed port if possible

45. Something not mentioned
• Supervision
• Persistence
• …

46. ZooKeeper

47. 3 ZooKeeper Servers

48. pom.xml
<dependency>
<groupId>com.sclasen</groupId>
<artifactId>akka-zk-cluster-seed_2.11</artifactId>
<version>0.1.2</version>
</dependency>

49. application.conf
akka {
loglevel = "DEBUG"
stdout-loglevel = "DEBUG"
actor {
provider = "akka.cluster.ClusterActorRefProvider"
}
remote {
log-remote-lifecycle-events = off
netty.tcp {
hostname = "127.0.0.1"
port = 0
}
}
cluster {
// seed-nodes = [
// "akka.tcp://ClusterSystem@10.211.55.5:2552",
// "akka.tcp://ClusterSystem@10.211.55.6:2552"]
auto-down-unreachable-after = 10s
}
}

50. reference.conf
akka.cluster.seed.zookeeper {
url = "10.211.55.2:2181,10.211.55.5:2181,10.211.55.6:2181"
path = "/akka/cluster/seed"
}

51. Join Cluster
ActorSystem system = ActorSystem.create("ClusterSystem", config);
new ZookeeperClusterSeed((ExtendedActorSystem)system).join();

52. Cluster & Router

53. factorial.conf - 1
include "application"
# //#min-nr-of-members
akka.cluster.min-nr-of-members = 3
# //#min-nr-of-members
# //#role-min-nr-of-members
akka.cluster.role {
frontend.min-nr-of-members = 1
backend.min-nr-of-members = 2
}
# //#role-min-nr-of-members

54. factorial.conf - 2
# //#adaptive-router
akka.actor.deployment {
/factorialFrontend/factorialBackendRouter = {
router = adaptive-group
# metrics-selector = heap
# metrics-selector = load
# metrics-selector = cpu
metrics-selector = mix
nr-of-instances = 100
routees.paths = ["/user/factorialBackend"]
cluster {
enabled = on
use-role = backend
allow-local-routees = off
}
}
}
# //#adaptive-router

55. Backend Main
final String port = args.length > 0 ? args[0] : "0";
final Config config =
ConfigFactory.parseString("akka.remote.netty.tcp.port=" +
port).
withFallback(ConfigFactory.parseString("akka.cluster.roles
= [backend]")).
withFallback(ConfigFactory.load("factorial"));
ActorSystem system = ActorSystem.create("ClusterSystem",
config);
new
ZookeeperClusterSeed((ExtendedActorSystem)system).join();
system.actorOf(Props.create(FactorialBackend.class),
"factorialBackend");
system.actorOf(Props.create(MetricsListener.class),
"metricsListener");

56. Backend Actor

57. Frontend Main
final int upToN = 200;
final Config config = ConfigFactory.parseString(
"akka.cluster.roles = [frontend]").withFallback(
ConfigFactory.load("factorial"));
final ActorSystem system =
ActorSystem.create("ClusterSystem", config);
new
ZookeeperClusterSeed((ExtendedActorSystem)system).join();
Cluster.get(system).registerOnMemberUp(new Runnable() {
@Override
public void run() {
system.actorOf(Props.create(FactorialFrontend.class, upToN,
true),
"factorialFrontend");
}
});

58. Frontend Actor
ActorRef backend =
getContext().actorOf(FromConfig.getInstance().props(),
"factorialBackendRouter");
…
void sendJobs() {
log.info("Starting batch of factorials up to
[{}]", upToN);
for (int n = 1; n <= upToN; n++) {
backend.tell(n, getSelf());
}
}

59. Resources

60. Java Sample Code

61. https://github.com/jiayun/
akka_samples

62. https://www.safaribooksonlin
e.com/search/?query=Akka&
highlight=true

63. Thanks