part4_messaging.md

Messaging & Events

In this part we learn how to use Ignite messaging & event system.

Messaging

A node can subscribe to messages published on a topic from local or remote nodes.

Subscribing and publishing messages to a topic is as simple as:

import org.apache.ignite.*;

public class App {

    public static void main(String[] args) {

        Ignite ignite = Ignition.start();

        ignite.message().remoteListen("my-topic", (nodeId, message) -> {
            System.out.println("Received message: " + message);
            return true; // keep on listening to new messages
        });

        ignite.message().send("my-topic", "Hello");
    }
}

Complete TODOs in Step1_Messaging to fix all tests in Step1_MessagingTest.

Events

A node can also subscribe to events triggered on local or remote nodes.

Events can be things such as: value put in cache, job execution end or failure, node leaving or joining cluster...

Subscribing to events is pretty easy:

import org.apache.ignite.*;

public class App {
    public static void main(String[] args) {

        Ignite ignite = Ignition.start();

        ignite.events().remoteListen(
            (nodeId, event) -> {
                System.out.println("Received event: " + event);
                return true; // keep on listening to new events
            },
            event -> event.type() == EventType.EVT_CACHE_OBJECT_PUT
        );
    }
}

Here put cache events are filtered on remote nodes before being sent to listener.

/!\ A lot of events can be generated by application or internally by Ignite Kernel. A special care has to be taken when listening to events, and that's why they are disabled by default. For this hands on, only 2 events were enabled in Config class to illustrate event system.

Complete TODOs in Step2_Event to fix all tests in Step2_EventTest.

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