XML in a Nutshell, 3rd Edition

Schemas Versus DTDs

DTDs provide the capability to do basic validation of the following items in XML documents:

• Element nesting

• Element occurrence constraints

• Permitted attributes

• Attribute types and default values

However, DTDs do not provide fine control over the format and data types of element and attribute values. Other than the various special attribute types (ID, IDREF, ENTITY, NMTOKEN, and so forth), once an element or attribute has been declared to contain character data, no limits may be placed on the length, type, or format of that content. For narrative documents (such as web pages, book chapters, newsletters, etc.), this level of control is probably good enough.

But as XML makes inroads into more record-like applications, such as remote procedure calls and object serialization, more precise control over the text content of elements and attributes becomes important. The W3C XML Schema standard includes the following features:

• Simple and complex data types

• Type derivation and inheritance

• Element occurrence constraints

• Namespace-aware element and attribute declarations

The most important of these features is the addition of simple data types for parsed character data and attribute values. Schemas can enforce much more specific rules about the contents of elements and attributes than DTDs can. In addition to a wide range of built-in simple types (such as string, integer, decimal, and dateTime), the schema language provides a framework for declaring new data types, deriving new types ...