penLaszlo, an open source framework sponsored by LaszloSystems, has attracted a significant attention after Earthlink, the second largest Internet Service Provider, announced that it will use OpenLaszlo technology for its new generation of interactive Web mail clients. OpenLaszlo is a rich, well-documented open source alternative to the popular Macromedia framework for rich Internet application delivery: Flash and Flex (it leverages the Flash runtime environment).

OpenLaszlo has very exciting presentation features for state-of-the art Web application development, a straightforward programming model, and excellent learning and development resources. And IBM Alphaworks has contributed a complete Eclipse-based open source IDE for OpenLaszlo, making this an excellent time to give it a test drive.

In this second article on rich Web clients (an earlier article looked at AJAX applications), I will look in-depth at OpenLaszlo's architecture, walking through the simple "web mail" application.

OpenLaszlo Architecture
OpenLaszlo architecture comprises three major components: the OpenLaszlo API, a server-side interpreter, and the browser runtime.

OpenLaszlo's programs are straightforward, XML-based scripts. It uses an object-oriented visual language that provides a variety of familiar structural and behavioral constructs for the composition of dynamic Web user interfaces. OpenLaszlo scripts—XML compliant files—are saved as LZX (Laszlo XML) files on the server and invoked by the clients in the runtime. The Laszlo API is well documented, with excellent, interactive examples and very usable online reference.

The server component is deployed as a J2EE Web application (a servlet) that intercepts the script requests from the client, interprets them, and serves them as Flash runtime binaries. OpenLaszlo leverages Macromedia's virtual machine for runtime execution of its scripts; applications developed in OpenLaszlo run as Flash applications, natively within the Macromedia Flash Player. This is in contrast to AJAX-based applications such as Google's Gmail Web client or Google Maps, which use JavaScript to asynchronously invoke the server in order to perform dynamic zooms and scrolls, spellchecking, and other server-side operations.

With an understanding of the basic architecture in hand, I will move on to an example application, wherein you'll get a detailed look at how OpenLaszlo UIs are developed and how all the pieces of the framework fit together.