Chapter 10 - XML Schema Basics

Abstract

The Web service interface relies on XML Schemas to define the content and context of the XML messages that are passed between service collaborators. XML Schema is defined by a set of specifications from the World Wide Web Consortium and is used as a core metadata language to describe and constrain an XML instance such as a message. XML schemas are requisite to Web services and participate in the definition of the Web service interface “contract.” The process of validation requires a validating XML parser and an XML message or similar instance with a referenced XML schema. The XML schemas language is extensive in capability and function. It includes basic data container types (elements and attributes) and allows these declarations to be further described with constraints for data types, default values, and fixed values (among several other constraints). XML schemas also allow for user-defined types declared as simpleTypes. The XML messages that move between service collaborators are not limited to simple data containers such as elements. The messages can also include complex structures with various levels of nested substructures, child elements, sibling elements, and repeating elements. These complexTypes are also defined by XML schemas. Another structural schema declaration is that of a group. While similar to a complexType, a group is specifically targeted for reuse. The elements defined by the group structure are intended to be referenced as a collection by other elements.