Introduction to Active Server Pages, Continued

Normally, HTML files are "interpreted" on the client side (in a user's web browser). With Microsoft's Active Platform, the server can also interpret files. Interpreting to a server means that before displaying a file in a user's browser, the server will complete some initial steps:

• It will look at the file's extension. If the file has a standard extension, such as .html or .htm, the server will pass the file to the browser. If the file has an extension of .asp (or .asa), the server will open the file and look for tags that mark ASP code. ASP code is contained inside special tags: <% before the code begins and %> after it ends.

• The server evaluates the code contained inside these tags and replaces it with HTML code. This HTML code is generated based on information about the user's setup or other conditions on the client side.

• The resulting page, which contains some of the file's original markup and some markup that has been generated with ASP, is sent to the browser that requested it.

Let's take the example of a shopping cart. Imagine a customer has selected five items from the store's catalog. These selections are listed together on one "shopping cart" page, which contains a table with rows and columns. At the top of each column is a heading, such as quantity, description, stock number, or price. How will you construct this page, considering that parts of it are dependent on what the customer has chosen?

The column headings can be included in the original HTML layout, since they won't change. The table's position probably won't change either, so it can also be incorporated into the original HTML file. These elements of the page will remain the same, regardless of how many times you come back and view the page.

The information inside the table (the list of what has been selected for purchase) will change, however. Each person viewing the site may select a different set of items. To display this information, ASP pulls the item information from a database and adds that item information to the HTML layout before it is sent back to the user's browser. Programmers use the term "on the fly" for this type of page, because it doesn't exist on its own. Instead, it is created on demand ("on the fly") when needed. The ASP script creates the entire page, by combining the static page elements with selected information from the database.

You can view a diagram showing how this works.

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