How to remove a topic when it has no subscribers?

Question

I am building a Diffusion solution that create a tree of topics.

I am creating topics on demand to reflect values received from a RabbitMQ feed. Each topic has a memory cost, so I am looking to remove the topic once it has had no subscribers for some time.

How can this do done with unified Java API?

Answer 1

The TopicEventListener (part of the TopicControl feature) provides this functionality. Providing individual callbacks for when a topic has 0 subscribers, as well as when a topic has at least 1 subscriber.

Example:

    public TopicEventListenerClient() {
        session = Diffusion.sessions().principal("admin").password("password").open("ws://localhost:8080");
        topicControl = session.feature(TopicControl.class);
        topicControl.addTopicEventListener("rabbitMQ/foo", new TopicEventListener() {

            @Override
            public void onClose(String arg0) {
                LOG.info("Listener closed");
            }

            @Override
            public void onError(String arg0, ErrorReason arg1) {
                LOG.info("Error on listener: " + arg1);
            }

            @Override
            public void onRegistered(String arg0, Registration arg1) {
                LOG.info("Listener registered");
            }

            @Override
            public void onHasSubscribers(String arg0) {
                LOG.info("Topic: " + arg0  + " has at least 1 subscriber");
            }

            @Override
            public void onNoSubscribers(String arg0) {
                LOG.info("Topic: " + arg0  + " has no subscribers");
            }
        });
    }

Hope this helps!

EDIT: 04/10/2019

TopicEventListener is deprecated (since Diffusion version 6.1). The preferred method of removing a topic with no subscribers is Automatic Topic Removal. This can be achieved by specifying a Removal Policy as part of the topic's specification.

For example:

final Session session = Diffusion.sessions().principal("admin").password("password").open("ws://localhost:8080");

final TopicControl topicControl = session.feature(TopicControl.class);

final TopicSpecification specification =
            topicControl.newSpecification(TopicType.JSON)
                .withProperty(TopicSpecification.REMOVAL, "when subscriptions < 1 for 10s");

The above code creates a topic specification for a JSON topic that will be automatically removed if it has no subscribers for 10 seconds.