Graceful termination - akka

Question

I am trying to implement the following use case as part of my akka learning
I would like to calculate the total streets in all cities of all states. I have a database that contain the details needed. Here is what i have so far
Configuration
akka.actor.deployment {
/CityActor{
router = random-pool
nr-of-instances = 10
}
/StateActor {
router = random-pool
nr-of-instances = 1
}}
Main
public static void main(String[] args) {
try {
Config conf = ConfigFactory
.parseReader(
new FileReader(ClassLoader.getSystemResource("config/forum.conf").getFile()))
.withFallback(ConfigFactory.load());
System.out.println(conf);
final ActorSystem system = ActorSystem.create("AkkaApp", conf);
final ActorRef masterActor = system.actorOf(Props.create(MasterActor.class), "Migrate");
masterActor.tell("", ActorRef.noSender());
} catch (Exception e) {
e.printStackTrace();
}
}
MasterActor
public class MasterActor extends UntypedActor {
private final ActorRef randomRouter = getContext().system()
.actorOf(Props.create(StateActor.class).withRouter(new akka.routing.FromConfig()), "StateActor");
#Override
public void onReceive(Object message) throws Exception {
if (message instanceof String) {
getContext().watch(randomRouter);
for (String aState : getStates()) {
randomRouter.tell(aState, getSelf());
}
randomRouter.tell(new Broadcast(PoisonPill.getInstance()), getSelf());
} else if (message instanceof Terminated) {
Terminated ater = (Terminated) message;
if (ater.getActor().equals(randomRouter)) {
getContext().system().terminate();
}
}
}
public List<String> getStates() {
return new ArrayList<String>(Arrays.asList("CA", "MA", "TA", "NJ", "NY"));
};}
StateActor
public class StateActor extends UntypedActor {
private final ActorRef randomRouter = getContext().system()
.actorOf(Props.create(CityActor.class).withRouter(new akka.routing.FromConfig()), "CityActor");
#Override
public void onReceive(Object message) throws Exception {
if (message instanceof String) {
System.out.println("Processing state " + message);
for (String aCity : getCitiesForState((String) message)) {
randomRouter.tell(aCity, getSelf());
}
Thread.sleep(1000);
}
}
public List<String> getCitiesForState(String stateName) {
return new ArrayList<String>(Arrays.asList("Springfield-" + stateName, "Salem-" + stateName,
"Franklin-" + stateName, "Clinton-" + stateName, "Georgetown-" + stateName));
};}
CityActor
public class CityActor extends UntypedActor {
#Override
public void onReceive(Object message) throws Exception {
if (message instanceof String) {
System.out.println("Processing city " + message);
Thread.sleep(1000);
}
}}
Did i implement this use case properly?
I cannot get the code to terminate properly, i get dead letters messages. I know why i am getting them, but not sure how to properly implement it.
Any help is greatly appreciated.
Thanks

Answer 1

I tested and ran your use case with Akka 2.4.17. It works and terminate properly, without any dead letters logged.
Here are some remarks/suggestions to improve your understanding of the Akka toolkit:
Do not use Thread.sleep() inside an actor. Basically, it is never a good practice since a same thread may do many tasks for many actors (this is the default behavior with a shared thread pool). Instead, you can use an Akka scheduler or assign a single thread to a specific Actor (see this post for more details). See also the Akka documentation about that topic.
Having some dead letters is not always an issue. It generally arises when the system stops an Actor that had some messages within its mailbox. In this case, the remaining unprocessed messages are sent to deadLetters of the ActorSystem. I recommend you to check the configuration you provided for the logging of dead letters. If the file forum.conf you provided is your complete configuration file for Akka, you may want to customize some additional settings. See the page Logging of Dead Letters and Stopping actors on Akka's website. For instance, you could have a section like this:
akka {
# instead of System.out.println(conf);
log-config-on-start = on
# Max number of dead letters to log
log-dead-letters = 10
log-dead-letters-during-shutdown = on
}
Instead of using System.out.println() to log/debug, it is more convenient to set up a dedicated logger for each Actor that provides you additional information such as dispatchers, Actor name, etc. If your are interested, have a look to the Logging page.
Use some custom immutable message objects instead of systematic Strings. At first, it may seem painful to have to declare new additional classes but in the end it helps to better design complex behaviors and it's more readable. For instance, an actor A can answer to a RequestMsg coming from an actor B with an AnswerMsg or a custom ErrorMsg. Then, for your actor B, you will end up with the following onReceive() method:
#Override
public void onReceive(Object message) {
if (message instanceof AnswerMsg) {
// OK
AnswerMsg answerMsg = (AnswerMsg) message;
// ...
}
if (message instanceof ErrorMsg) {
// Not OK
ErrorMsg errorMsg = (ErrorMsg) message;
// ...
}
else {
// Unexpected behaviour, log it
log.error("Error, received " + message.toString() + " object.")
}
}
I hope that these resources will be useful for you.
Have a happy Akka programming! ;)