Structured Document Types
In order to span heterogeneous environments, many distributed systems have looked to means of expressing data in a way that can be interpreted unambiguously and without loss of information by remote nodes irrespective of their operating system or hardware. The Standard Generalized Markup Language (SGML) forms the basis of the most popular of such mechanisms. A markup language is a vocabulary that combines content with notation, or markup. The markup provides a context for the content. SGML has two important properties that make it suitable for use in distributed environments. First it is based on descriptive markup; the markup is designed to annotate the data in terms of its structure, not in terms of what one should do with the content. This means no particular processing, and therefore underlying software or hardware, is presumed. Such markup separates data from structure and, by implication, presentation as well because the structural markup can be processed for presentation independently of the data. Second SGML provides a means of associating a marked-up document with a document type. Document types are defined in a Document Type Definition (DTD). The DTD is a template against which the marked-up content can be understood, interpreted and validated using an SGML parser.
SGML has spawned several languages that are widely used, in particular Hypertext Markup Language (HTML) and Extensible Markup Language (XML). These are referenced extensively in this book because they are so commonplace. The following sections give an overview of these technologies. Readers may skip these sections now if they are familiar with these languages, or return to them later for clarification.
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