This back-and-forth process creates a bottleneck effect on the server and almost always creates delays in application response. To work around this architecture flaw, companies have invested heavily in server clusters and performed load-testing on their applications to predict throughput and response time. While this solution works well for some corporate applications, it does not make sense to build that infrastructure for every application that requires heavy user interaction.

To make Flash more structured and easier for corporate developers to master, Macromedia introduced the Flex application architecture, a server-side runtime environment specifically designed to create sophisticated client-side Web applications. In addition to using proprietary ActionScript, the Flex architecture uses a Markup XML (MXML) language, which is a declarative programming model designed to create user interfaces. Essentially, MXML abstracts ActionScript code so users do not work directly with the ActionScript language to develop interfaces. The integrated Flex presentation server converts MXML into ActionScript code without user interaction. Since MXML is easy to learn and conforms to traditional server-side development standards, Flash programming now is easier to learn-and thus more attractive to corporate developers.

Developers are still able to write ActionScript class files and invoke them inside MXML. While MXML code is a good presentation wrapper for most applications, ActionScript is sometimes a better option when logic is required to generate client-side code.

When developers create complex online interfaces for e-commerce or business-intelligence applications, they must write and execute extensive configuration code on the server side to interact with HTML on the client side. Every time a user makes a choice in an application, such as selecting the color of a car on an automaker's Web site or viewing an inventory item on a clothing e-tailer, a server-side request must be made to process that selection and a server-side response must be sent back to the client. Completing this process is time-consuming and can be a drain on many e-commerce applications. For instance, searches can be slow and tedious, since every item description or indication of customer preference requires that a server-side request be made and returned.

To combat the bottleneck effect, some savvy developers have used Macromedia's Flash to develop more effective client-side code to increase application response speed. However, using Flash requires developers to invest considerable time learning the software's authoring environment and programming framework.

To extend its functionality, Flex arrives with a set of prebuilt components that generate sophisticated interfaces with little MXML coding. These components allow developers to build graphics-intensive interfaces without learning MXML code or Flash programming techniques.

Because this technology is new, application providers and independent consultancies have lots of opportunities to create components and custom applications for their customers. Software developers can also sell applications through Macromedia's Exchange sites, though there is not a site dedicated to Flex code.

Flex is used in combination with Flash to generate Web application user interfaces. Once downloaded, Flash applications provide connectivity to the server through XML, SOAP Web service or Macromedia's proprietary Active Message Format (AMF). Flex resides on the server and becomes a proxy for enterprise applications to interact with clients through XML messages or SOAP methods. Any server-side language that generates XML or SOAP Web services can work in conjunction with Flex.

The Flex server can also execute Java classes through an internal broker that uses remote object invocation to marshal data to clients. Flex translates Java interfaces into SOAP Web services or AMF calls so that Flash applications can invoke Java code directly. Flex relies on middle-tier code to connect to databases, legacy systems and packaged applications that rely on .Net-managed code.