Chapter 1
Introduction to XML

Example

• Example 1-1

What XML Is

XML is a metamarkup language. That is, it's a markup language where you make up all the tags yourself. XML is the set of rules that this made-up markup language must follow. This ability to create your own tags is called "extensibility." XML stands for "Extensible Markup Language."

XML is also open, which means that no one owns it. Anyone can use XML without having to ask anyone else for permission, and anyone can create any XML tags they want.

What XML is Good For

XML was created to allow applications to send complex, richly structured data (such as arrays, structures, recordsets and the like) over the Internet in simple text form. The expectation in creating XML was that certain open standards would emerge, and developers would adhere to the ones they liked the best. This has happened in the case of SOAP, a flavor of XML that's being rapidly implemented to first-generation web services.

Brief History of XML

XML began life as SGML (Standard Generalized Markup Language), a powerful and complicated markup language that's been in use for years. XML is a simplification of SGML—the most useful parts of SGML are part of XML, and the more obscure features were dropped.

HTML is a super-simple version of SGML. While HTML is simple to learn, it's only good for displaying information, not describing it. In other words, HTML isn't information, it's a picture of information. XML strives to actually describe information.

The creators of XML had some distinct goals in mind:

1. XML shall be straightforwardly usable over the Internet.
2. XML shall support a wide variety of applications.
3. XML shall be compatible with SGML.
4. It shall be easy to write programs that process XML documents.
5. The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
6. XML documents should be human-legible and reasonably clear.
7. The XML design should be prepared quickly.
8. The design of XML shall be formal and concise.
9. XML documents shall be easy to create.
10. Terseness in XML markup is of minimal importance.