XML 101

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).

To summarize, XML is a text-based meta-language that uses tags, elements, and attributes to add structure and definition to documents. It is similar to HTML in syntax and implementation, but different with regard to functionality. Where HTML allows users to control how documents are displayed, XML allows them to describe the actual contents of the documents. It is a markup language because it uses tags to mark-up documents and it is a meta-language because it uses these tags to give structure to documents that it in turn uses as a means of communication. XML is extensible because it enables users to create their own collection of tags (unlike HTML).

Now, why should you care about XML? If for now other reason, consider that the World Wide Web Consortium (W3C), the Internet's governing body, is considering a proposal to rewrite the HTML 4 language in XML 1.0. As of the time this article was written, XHTML had received endorsement by the director of the W3C as a recommendation. This proposal, known as XHTML will require well formedness in all HTML documents. The W3C is a neutral standards body responsible for defining the future of the Internet. They do not support every new idea that comes along, and we should view their full support of XML (or any technology), as a harbinger of where tomorrow's Internet will take us. Ignore XML if you will, but know that it is most definitely a legitimate technology that will revolutionize the way that we program applications for the Web

About the Author

Jeff Jones
For more information on XHTML, XML, and the W3C, check out the W3C website at http://www.w3c.org.