Overview

Although exhibiting many advantages and capabilities, XML may not always be the best metadata technology for a project. Determining whether XML should be applied requires careful evaluation of the project's data and functional requirements and the characteristics of the data that are to be described. When it has been determined that XML is advantageous, one of the initial architectural tasks is to ascertain whether a constraining schema should be applied (i.e., as the set of definitions, rules, and constraints applied during parsing and validation). Several types of XML schemas exist, resulting in one or more possible choices. However, each type of schema has distinct advantages and disadvantages. The three most common schema types include the following:

• Document type definitions (DTD or dtd)

• XML data reduced (XDR or xdr)

• W3C XML Schemas (XSD or xsd)

It is important to note that for some applications, an XML document (i.e., an XML file, document, transaction, or message) can be processed without any type of schema. When the XML document is clearly described, is represented as a simple structure, does not require the application of business rules and constraints, and is of limited size, parser validation to a schema may not be required (Fig. 2.1) However, XML documents without a referenced schema cannot be validated to ensure that contained data are of a particular data type or that the organization of the data elements in the document fits an expected or prescribed structure.

 12345 Sally M. Johnson 2002-05-27