Chapter 7. XML Data Transformation

XML was first introduced as a metalanguage for data description. Why is it a metalanguage and not just a language? In general, the prefix meta indicates an evolutionary transformation process. A metalanguage represents a well-defined interface that evolves and is transformed into derived languages. XML is simply the foundation interface for a number of specific markup languages, each of which is based on its own vocabulary and schema.

The schema syntactically differentiates XML languages from each other. XML is key for data exchange and interoperability, and the schema is essential for providing XML documents with a typed and well-defined structure. Unfortunately, in the imperfect world in which we live, schemas often express the same semantics through different syntaxes.

An XML transformation is simply the XML workaround for this relatively common situation. An XML transformation is a user-defined algorithm that attempts to express the semantics of a given document using another equivalent syntax. A transformation is much like a type cast in programming. You can always try to coerce the type, but in doing so you could face and accept compromises like syntax adaptations and, sometimes, loss of data and logic.

In XML, the transformation process is seen as the application of a style sheet to the source document. The style sheet is a declarative and user-defined document that is referred to as extensible. The term Extensible Stylesheet Language (XSL) indicates a metalanguage designed for expressing style sheets for XML documents. An XSL file contains the set of rules that will be used to transform a document into another, possibly equivalent, document.

XSL files were originally conceived as the XML counterpart of HTML’s cascading style sheets (CSS). In this context, XSL files were simply extensible and user-definable tools to render an XML markup in HTML for display purposes. The growing complexity of style sheets, as well as the advent of XML schemas, changed the perspective of XSL and led to XSL Transformations (XSLT).