You know that, as an XML document author, you can create the XML document in whatever structure you decide on. You are able to decide on your own element names, you can determine how the data within these elements is represented, and you can even dictate the complete hierarchy of the data represented in the document. The structure you decide on is referred to as a vocabulary. This open set of rules may seem like anarchy, but this is what gives XML its power. It is a creative environment that allows you to build a true representation of your data.
This openness of XML vocabulary does, however, require a set of rules defined on the structure of XML documents. This set of rules, once in place, can then be used to validate XML documents that are created or being read. If you want to consume an XML document, you must have a means to run the document through a validation process to make sure it abides by the established rules to ensure easy processing. Otherwise, you must ensure this by laboriously parsing the XML document line by line.
The XML validation process is an important one. This book covers the three main ways to validate an XML document. Document Type Definitions, also known as DTDs, are ways you can apply this validation process. Other means include XML Schemas and RELAX NG. This chapter takes a look at DTDs and how you can create and work them.
Validation is important. If you plan to share information ...