Web Services Essentials

XML Schema Data Typing

In order for a SOAP client to communicate effectively with a SOAP server, the client and server must agree on a data type system. By default, XML 1.0 does not provide a data type system. In contrast, every programming language provides some basic facility for declaring data types, such as integers, floats, doubles, and strings. One of the greatest challenges in building web services is therefore creating a common data type system that can be used by a diverse set of programming languages running on a diverse set of operating systems.

WSDL does not aim to create a standard for XML data typing. In fact, WSDL is specifically designed for maximum flexibility and is therefore not tied exclusively to any one data type system. Nonetheless, WSDL does default to the W3C XML Schema specification. The XML Schema specification is also currently the most widely used specification for data typing.

The more you know about XML Schemas, the better you can understand complex WSDL files. A full discussion of XML Schemas is beyond the scope of this chapter. However, two facts are crucially important.

First, the XML Schema specification includes a basic type system for encoding most data types. This type system includes a long list of built-in simple types, including strings, floats, doubles, integers, time, and date. This list, shown in Table 6-1, is excerpted from the XML Schema Part 0: Primer (http://www.w3.org/TR/2000/WD=xmlschema=0=20000407/). If your application sticks ...