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Thursday, June 2, 2011

Pusher - websockets made simple and easy

While visiting the Falsy Values Conference in Warsaw I stumpled upon Pusher offers an interesting websocket infrastructure which makes jumping into websocket application development easy.

Currently websocket technology on the server side is not as widely available and common as traditional HTTP server technology. In the Java ecosystem there are pleinty of  well known http server implementations available which use non-blocking I/O. However, non-blocking HTTP is rather new in the Java world and has just been added to the most recent JEE6 specification.  A particular websocket implementation is not part of the current Java spec.

Grizzly (Glassfish) offers a Websockets implementation, Jetty does as well and Atmosphere even aims for a portable solution. Tomcat, the most popular Servlet container does not.

Traditional webservers bind each connection to a thread. This limits scalability for websockets and look-a-likes tremendously. NIO implementations like node.js or Java Servlets 3 are event driven. One (node.js) or more worker threads are dispatching events. That way many connections are handled by a single thread which allows for better scalability especially for connection held open for a long time.

It seems that he protocol and NIO technology required for doing websockets add complexity to web applications which is currently not common for the community and hence not that easy to achieve. However, applications based on websockets mostly need nothing more than some kind of publish-subscribe mechanism. Publish-subscribe is easy to use, but very powerful and flexible.

Thats exactly what Pusher offers. A Websocket (or Flash-socket) based service to publish and subscribe JSON messages. On the browser side a JavaScript provided by Pusher is included to do the the websocket work. Browsers not offering Websocket support are supported by a Flash fallback. This is of great value and not a snap to achieve by a homebrew solution. Today it's essential to have a good and proven third-party library to achieve solid websocket applications targeted to a main-stream web audience.

An application subscribes for a given event and gets its callback called when a given event occurs.

var pusher = new Pusher('KEY');
  function(data) {
    document.getElementById("news").innerHTML = "<p>" + data.msg + "</p>";

Trigger events is not only possible from within the browser. Pusher offers a REST interface to trigger events from any language supporting HTTP. It's not a suprise to find implementations in many languages listed on the Pusher website. There is a Java library for Google App Engine for Pusher available at github. As I'm not using Goolge App Engine and required a adapted solution so I forked the project and provided a implementation based on Jakartas HttpClient. The refactored project added some more OO style eg. to have it easily available in a Spring environment or other IOC containers. This has been mostly achieved by removing the rather static nature of the GAE solution. Have a look at my fork at github.  The GAE implementation of Stephan Scheuermann was of great value for me. The code was well-structured and his implementation of creating appropriate hashed signatures worked for me like plain vanilla and helped me having a nice solutions quickly. Thx for that nice work!

Sending an event to a Pusher application from within Java is easy that way:

Pusher Transport httpClientTransport = new HttpClientPusherTransport();
PusherChannel channel = new PusherChannel("iam", APPLICATION_ID,
          APPLICATION_KEY, APPLICATION_SECRET,  httpClientTransport);

PusherResponse response = channel.pushEvent("news", JSON_STRING);
PusherResponse response = channel.pushEvent("news", ANOTHER_JSON);

I like that Pusher stuff. I can easily create a Websocket implementation without going into other solutions as node.js, kaazing or Glassfish offer and for which it's is hard to find hosting providers. I can stick to the server environment which I and my server administrator is used to and opt-in websockets with Pusher for those applications relying on websockets (which are not that many by now). Having a solid client-side solution with support for many browsers is another big plus.

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