Giving your clients a kick

Aren't distributed events a fun problem to solve? It was nearly ten years ago that I read and was profoundly inspired by a HotWired (remember them?!) article, Writing Chat Systems with Java 1.1's RMI, which showed how to build a chat system that used RMI to asynchronously send new messages to all the participants (or let two participants exchange messages directly, without touching the server). In fact, a year or two later, I designed a client-server system that put a nice, clean event listener metaphor around RMI communication, so the Swing clients would get asynchronous updates from the server when new media arrived that they'd be interested in. It worked great, so long as you didn't get hosed by RMI's inexplicable and evil reverse-DNS lookup, which is a rant for another time. If I had it to do over, I'd probably use Jini, since it would be more robust and tolerant of failure (and I was already pretty pro-active about dead clients). But suffice to say that asynchronous updating of clients is one of those interesting problems you see over and over again.

To wit, what if instead of a nice thick Swing client on the LAN, your client is a web page somewhere halfway across the internet? And what if the nature of your application still calls for the server to send asynchronous notifications to the client? This is handily addressed by the Direct Web Remoting Ajax framework, and its new "Reverse Ajax" feature. Katherine Martin has details in our Feature Article, Developing Applications Using Reverse Ajax.

DWR, hosted on java.net, is a Java open source library that helps developers to write websites that include Ajax technology. When DWR broke onto the Ajax scene with release 1.0, its announced mantra was "Easy Ajax for Java." It aids the developer in many ways, not least by allowing code in the web browser to use Java functions running on the web server as if they were in the browser.

With release 2.0, DWR continues on the same theme, removing from the developers shoulders the pesky problem of "pushing" information. It introduces the term reverse Ajax to describe this asynchronous transfer of messages from server to browser or browsers. While, as mentioned above, techniques for doing this already exist, the beauty of "reverse Ajax" is that it wraps them all up neatly and then automatically selects the best method to use transparently to the user.

In a nice bit of irony, the sample code for Katherine's code, included as part of the DWR 2.0 application, is a chat application... just like the HotWired article from 10 years ago. The more things change, the more they stay the same.