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How a web application works 221

In the case of web applications, certain lines of code are undetermined

when the visitor requests the page. These lines must be determined by

some mechanism before the page can be sent to the browser. The

mechanism is discussed in the following section.

Processing dynamic pages

When a web server receives a request for a static web page, the server sends

the page directly to the requesting browser. When the web server receives a

request for a dynamic page, however, it reacts differently: It passes the page

to a special piece of software responsible for finishing the page. This special

software is called an application server.

The application server reads the code on the page, finishes the page

according to the instructions in the code, and then removes the code from

the page. The result is a static page that the application server passes back

to the web server, which then sends the page to the requesting browser. All

the browser gets when the page arrives is pure HTML. Here’s a view of the

process:

Application server

Request

Web browser

<HTML>

<code>

</HTML>

Response

<HTML>

<p>Hi

</HTML>

WEB SERVER

Step 1 - Web

browser requests

dynamic page.

Step 2 -

Web server

finds page

and passes it

to application

server

Step 3-

Application server

scans page for

instructions and

finishes page.

Step 4-

Application server

passes finished

page back to web

server

Step 5 - Web

server sends

finished page to

requesting browser