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XML 101

by Jeff Jones

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Whether you're discussing e-commerce, knowledge management, or the Internet in general, you've likely seen or heard reference to eXtensible Markup Language (XML). XML is, without a doubt, one of the most heralded technologies to come across the wire in recent years (pun intended). What is XML? Why is it creating such a deluge of interest? What should you know about XML, and perhaps more importantly, why should you even care about it? In this article, I will provide a high-level description of what XML is (and what it's not), discuss the key components of an XML document, and provide a compelling argument for why it's well worth your while to learn more about XML.

To understand XML, it's helpful to compare and contrast it with another technology with which a great many of us are familiar - HyperText Markup Language (HTML).

If you've used or read about HTML, you know that it was created so that users could format and display information on the Web. HTML uses a fixed and finite set of tags, elements, and attributes that allow it to communicate to a user's browser how that browser should display the document. We see HTML everywhere, and it has for some time served as the lingua franca for displaying information on the Web. It is a proven technology that well serves its purpose in most scenarios. What if, however, the current version of HTML doesn't allow me to do something that I want to do? I have two choices: I can either write my own browser that understands my tags (bad idea) or I can put my project on hold for a year or so and hope that the next version of HTML includes the functionality that I need (even worse idea). Try selling either of these options to your boss or client and see if you still have a job by the time you end your discussion. So concludes our one-paragraph, in-depth investigation of HTML.

Now, if I may lapse into my days of standardized test taking, HTML is to displaying information as XML is to defining information. They both are text-based, and they both consist of tags, elements, and attributes. Unlike HTML, however, XML allows users to structure and define the information in their documents. While technically it is a markup language (it allows you to use tags to "mark-up" the contents of your document), it more appropriately is a meta-language. By meta-language, I mean that it allows users to create their own collection of tags, elements, and attributes as needed and in so doing to accurately describe the physical contents of a document. Unlike HTML with its finite collection of tags, XML allows users to create their own to meet their own requirements (thus the eXtensibility).

I've made several references to tags, elements, and attributes. These are the core building blocks of an XML document. Consider the following HTML fragment. It should be painfully familiar to anyone who's ever looked at an HTML document and will prove useful in understanding XML syntax.



Here is the first group of text< d>
Here is the second group of text< d>
< r>
< able>

This document contains a table element ("") with a table row element (""). The table row element, in turn contains two table cell elements (""). Each of these elements has both an opening tag ("") and a closing tag ("< able>"). While this is fairly straightforward, it also is somewhat inflexible. What if, for example, I need to create a document that describes my company's employee roster for the Annual InfoStrat softball tournament? With XML, it's as easy as replacing the element and attribute names from the previous HTML document with my own custom tags that describe my company and its employees. Here is what such a document might look like:




Hank Aaron
Babe Ruth



With this XML document, I have defined my company and two of its employees and have described the relationship between company (parent) and employees (children). I have shown that my company has two employees, but I easily could add new employee elements to reflect new hires that we bring on to ensure that we don't lose this year's tournament:

Mickey Mantle
Ty Cobb

After creating my XML document, I can display its contents in my format of choice. The same XML document could easily be displayed as HTML, a Microsoft Word document, an Adobe .pdf file, or even as text in the body of an e-mail message. As long as the XML document is well formed (meaning that it follows the appropriate XML format and syntax), you can choose your method of preference (or necessity) for displaying its content.

Let's dissect the pieces of my company roster XML document to see each piece's role and responsibility.

Header:

The header tells the document's user that this is an XML document - using version 1.0 of the XML specification in this case.




Hank Aaron
Babe Ruth



Tags (brackets, greater than, less than):

Just like in HTML, you use greater than (">") and less than ("