Application Servers

As explained earlier, Web servers don't do that muchthey serve content that is read from disk. Application servers are software applications that extend Web servers, allowing them to do things they couldn't do on their own. These include:

• Accessing databases

• Sending and receiving email

• Manipulating XML data

• Personalizing content

• Building ecommerce applications

• Powering rich Internet applications

• Much more

Application servers usually rely on the underlying Web servers for all host-to-host communication (sending data to and from the browser, for example). When a request is to be processed by an application server, the Web server receives the request from the client, hands it to the application server for processing, and then returns the output from that processing to the client.

Web servers are primarily used to serve static content. Static content is usually stored in HTML files directly (and supporting image files perhaps). These files are created in editors, and they are rendered and displayed just as they were created.

Application servers are used to provide dynamic content. Dynamic content is built at run time based on external criteria. Information could be retrieved from databases, user input could be solicited, and more. All this information is then used to build an application that generates output that could be different each time it is requested.

Static content is essentially the electronic form of print-based publishing. The true power of the Web is realized in dynamic content.