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Doing More With XML Schemas (part 1)
Learn to apply extensibility and inheritance to your schemas

| Revving Up |

Back in the old days, when XML was still a dark and nebulous cloud on the horizon, the only way to verify the integrity of XML-encoded data was with a Document Type Definition (DTD). DTDs were incomprehensible beasts consisting of strange symbols and tangled acronyms, and it took a tremendous amount of patience (not to mention a fair amount of alcohol) to successfully write one that worked as it was supposed to.

Realizing that the arcane syntax used to construct DTDs was hindering rather than helping its efforts to make XML the de facto standard for data markup on the Web, the W3C came up with a kinder, gentler way of validating XML data. It was called XML Schema, and it offered developers all the capabilities of current DTDs while simultaneously adding a number of new capabilities designed to improve maintainability and extensibility.

As the name suggests, a "schema" is a blueprint for a specific class of XML document. It lays down rules for the types of elements and attributes allowed within an XML document, the types of values that accompany such elements, and the order and occurrence of these elements. It also addresses a number of issues which cannot be handled by DTDs: datatyping (including the ability to derive new datatypes from existing ones), inheritance, grouping, and database linkage.

Specific XML documents (referred to by the Working Group as "document instances") can be linked to a schema and validated against the rules contained within it. The XML Schema specification specifies the process by which document instances and schemas are linked together, and a number of tools are now available to perform this validation.

Now, if you've been paying attention to previous columns, you probably already know the basics of how schemas work (in case you don't, drop by http://www.melonfire.com/community/columns/trog/archives.php?category=XML and get yourself up to speed). In this series of articles, I'll be building on that basic knowledge to demonstrate some of the more advanced capabilities available to you via XML schemas, in the hope that it will assist you in fully exploiting the powers of this new tool. Keep reading!

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