Writing a publisher/subscriber (PubSub) is quite simple with Atmosphere using the atmosphere-jersey module.

The main idea here is to use Comet for suspending the response when a client subscribe to a topic, and use REST for publishing messages to the those suspended responses. First, let’s bind our application to the root uri using the @path annotation

package org.atmosphere.samples.pubsub;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import org.atmosphere.annotation.Broadcast;
import org.atmosphere.annotation.Schedule;
import org.atmosphere.annotation.Suspend;
import org.atmosphere.cpr.Broadcaster;
import org.atmosphere.jersey.Broadcastable;

@Path("/") public class PubSub {

Next, let’s implement the subscribe operation by using the @Suspend and inject our Atmosphere’s Broadcaster using the @PathParam annotation:

    @Suspend
    @GET
    @Path("/{topic}")
    @Produces("text/plain;charset=ISO-8859-1")
    public Broadcastable subscribe(@PathParam("topic") Broadcaster topic) {
        return new Broadcastable("",topic);
    }

The code above will be invoked when we want to create a new topic (@Path("/{topic}"), we will return a Broadcastable instance which will tell Atmosphere to use the passed Broadcaster, who got injected using the @PathParam, when broadcasting {topic}. Finally, the underlying response will be suspended forever via @Suspend annotation. Next, let’s implement the publish operation:

    @GET
    @Path("/{topic}/{message}")
    @Produces("text/plain;charset=ISO-8859-1")
    @Broadcast
    public Broadcastable publish(@PathParam("topic") Broadcaster topic,
                                 @PathParam("message") String message){

        return new Broadcastable(message,topic);
    }

To publish, we just need to send a uri that takes the form of "/{topic}/{message}" as defined by @Path("/{topic}/{message}") and again, the Broadcaster we want to use to broadcast messagse will be injected based on the {topic} value. Finally, we just return a Broadcastable object and let Atmosphere broadcast the value to all suspended connections. That’s it! Now we can see it in action by doing:

Create a topic
curl http://localhost:8080/atmosphere-pubsub/myAtmosphereTopic
Publish to that topic
curl http://localhost:8080/atmosphere-pubsub/myAtmosphereTopic/Atmosphere_is_cool

The source for the entire sample can be viewed here. Really simple, is it?

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technorati: atmosphere framework rest comet jersey ajax push