Programming C#, Third Edition

SOAP, WSDL, and Discovery

What is needed to make web services possible is a simple, universally accepted protocol for exposing, finding, and invoking web service functions. In 1999, Simple Object Access Protocol (SOAP) was proposed to the World Wide Web Consortium. SOAP has the advantages of being based on XML and of using standard Internet communications protocols.

SOAP is a lightweight, message-based protocol built on XML, HTTP, and SMTP. Two other protocols are desirable, but not required, for a client to use a SOAP-enabled web service: a description of the methods provided by a particular service that can be understood and acted upon by clients, and a description of all such services available at a particular site or URL. The first of these is provided in .NET by the Web Service Description Language (WSDL) protocol, jointly developed by Microsoft, IBM, and others. Two other protocols have been proposed for discovery: UDDI, a joint effort by a number of companies including IBM and Microsoft, and Discovery, a proprietary offering from Microsoft.

WSDL is an XML schema used to describe the available methods—the interface—of a web service. Discovery enables applications to locate and interrogate web service descriptions, a preliminary step for accessing a web service. It is through the discovery process that web service clients learn that a service exists, what its capabilities are, and how to properly interact with it. A Discovery (.disco) file provides information to help browsers ...