Validating XML with the Document Type Definition (DTD)
XML is a meta-markup language that is fully extensible. As long as it is well formed, XML authors can create any XML structure they desire in order to describe their data. However, an XML author cannot be sure that the structure he poured so much time and effort into creating won’t be changed by another XML author or for that matter an application. There needs to be a way to ensure that the XML structure cannot be changed at random. This type of assurance for XML document structure is vital for e-commerce applications and business-to-business processing, among other things. This is where the Document Type Definition (DTD) steps in. A DTD provides a roadmap for describing and documenting the structure that makes up an XML document. A DTD can be used to determine the validity of an XML document.
In this chapter we will start with several examples and a brief overview of the DTD and what it does. Then we will break down the different items that make up the structure of the DTD. The coverage of the DTD structure will begin with a discussion of the Document Type Declaration. Then we will move on to the functional items that make up the DTD. The DTD includes element definitions, entity definitions, and parameters. Finally, before closing the chapter, we will explore some of the drawbacks of DTDS and emerging alternatives for validation. Now, let’s start by defining the Document Type Definition.