PubSub DeadLetters issues

Lagom
1

Lagom Version: 1.3

Attempting to do: Notify all actors in the cluster that an event has been received from a message queue, this prompts them all to update in memory caches.

Issue encountered: When publishing my akka message, my code on the subscription never seems to fire. though i receive the following:

Message [java.lang.String] from Actor[akka://my-impl-application/deadLetters] to Actor[akka://my-impl-application/system/distributedPubSubMediator#855479769] was not delivered. [2] dead letters encountered. This logging can be turned off or adjusted with configuration settings ‘akka.log-dead-letters’ and ‘akka.log-dead-letters-during-shutdown’.

My Investigation: I have tried to find examples and found I was getting similar issues even using example code such as https://github.com/knoldus/lagom-pub-sub.g8

I’ve noted almost all examples seems to suggest subscription be done async to publish logic (in the example the subscription is only done when a stream is opened), which doesn’t suit my needs. An example of the overall logic of my code is below (this isn’t the code, it’s a nice simplification):

public class MyService {
    @Inject
    //KafkaService is just an instance of com.lightbend.lagom.javadsl.api.Service with a single Topic
    public MyService(KafkaService service, PubSubRegistry pubSubRegistry) {
        PubSubRef<String> pubSubRef = pubSubRegistry.refFor(TopicId.of(String.class, "my_topic"));
        service.myTopic().subscribe()
            .atLeastOnce(Flow.<String>create().fromFunction(msg -> {
                pubSubRef.publish(msg);
                return Done.getInstance();
            });
        pubSubRef.subscriber().subscribe().map(msg -> {
            System.out.println(msg);
            return msg;
        });
    }
}

application.conf

play.akka.actor-system = "my-impl-application"
akka {
  actor {
    provider = "cluster"
  }
  remote {
    log-remote-lifecycle-events = off
    netty.tcp {
      hostname = "127.0.0.1"
      port = 2552
    }
  }
}
akka.cluster.seed-nodes = [
  "akka.tcp://my-impl-application@127.0.0.1:2552"]

Expected: When an actor in my cluster receives a kafka event, an akka msg is published which ALL other actors consume and then do something.

Current Workaround: Currently as per https://doc.akka.io/docs/akka/2.4/java/distributed-pub-sub.html I’m just building my own akka pubsub structure, though I would much prefer to use the built in Lagom feature

2

Where are you running your subscription graph ? Seems like you just build it.

3

How do you mean? As in it looks like in my code I’m just building the service but never user it.
I should probably have clarified I am injecting the service into a controller for the purposes of being created.

4

@Maraket
When you do

pubSubRegistry.refFor(TopicId.of(String.class, “my_topic”));

this ill return a PubSubRef[T] rigth ? After that you subscribe to the topic

pubSubRef.subscriber().subscribe()

this ill return a Source[T,Mat] rigth ? Then you need to run the source graph. If you can ignore you sink do that:

.runWith(Sink.ignore)(Keep.left)

This ill execute your graph until that you ill not be abble to read any messages because you are just building the graph

5

How would that be in Java for lagom 1.3? As far as I can tell Akka 2.4 requires runWith with 2 parameters not 1.

6

@Maraket
With akka 2.4.20 you can use that =)

Screenshot from 2020-02-19 12-46-16
Screenshot from 2020-02-19 12-46-161817×934 133 KB

The second parameter is an Generic type for the materializer, you can use the runWIth function as i show before

7

That was it, thank you VictorBLS, it works thanks for all the help

1 Like