Secure XML: The New Syntax for Signatures and Encryption

5.1. Overview

A schema is a model for describing the structure of information. An XML schema is an XML language that defines a class of XML documents and describes the possible arrangement of tags and text in a valid document. Schemas can be the basis for machine validation of document structure. An “instance” of a schema is an XML document that meets the constraints of that schema. As an XML schema may describe another XML schema, it can be, somewhat confusingly, referred to as an instance of a schema.

The W3C Schema Recommendation language uses namespaces and reconstructs and refines the capabilities found in XML 1.0 DTDs (see Chapter 4). Schemas, unfortunately, do not fully replace DTDs. DTDs are a lower-level mechanism. According to the fundamental ...