Web Services Description Language (WSDL)

Web Services Description Language (WSDL) is in many ways what makes Web services interesting. Although SOAP is a message format that everyone understands in the Web services world, WSDL is what everyone uses to tell others what they can do.

WSDL is an XML vocabulary to describe Web services. It allows service authors to provide crucial information about the service so that others can use it. WSDL is designed to be highly extensible and adaptable to enable the description of services using different type systems (such as XML Schema, RelaxNG, or even Java) and services that communicate over SOAP and various other protocols (such as RMI/IIOP or in-memory calls).

A WSDL document consists of two parts: a reusable abstract part and a concrete part. The abstract part of WSDL describes the operational behavior of Web services by recounting the messages that go in and out from services. The concrete part of WSDL allows you to describe how and where to access a service implementation.

WSDL carefully does not cross into describing semantics of Web services. A WSDL document tells, in syntactic or structural terms, what messages go in and come out from a service. It does not provide information on what the semantics of that exchange are. That limitation was intentional in WSDL; semantic descriptions must be described in natural language or other evolving semantic description languages, such as DAML+OIL and OWL-S.

The World Wide Web Consortium (W3C) is standardizing the widely used version of WSDL, which is version 1.1. Although the standard version, 2.0, will be ready shortly, it probably won't become widely deployed for several years.

This chapter discusses both versions of WSDL, considering the differences between them and how to use WSDL in a future-proof manner.