A new WS-* specification from BEA Systems, IBM, Microsoft, and SAP defines messages to retrieve specific types of metadata associated with an endpoint: Web Services Metadata Exchange (WS-MetadataExchange).
The purpose of the specification is to "define a bootstrap mechanism for metadata-driven message exchange," as required in the Web Services Policy Framework (WS-Policy), in Web Services Description Language (WSDL) 1.1, and in W3C XML Schema. Supporting SOAP 1.1, SOAP 1.2 Envelopes and WSDL 1.1 description, the WS-MetadataExchange specification "leverages other Web service specifications for secure, reliable, transacted message delivery." The associated namespace URI is http://schemas.xmlsoap.org/ws/2004/02/mex.
"Specifically, WS-Policy describes the capabilities, requirements, and general characteristics of Web services; WSDL 1.1 describes abstract message operations, concrete network protocols, and endpoint addresses used by Web services; W3C XML Schema describes the structure and contents of XML-based messages received and sent by Web services. To bootstrap communication with a Web service, this WS-MetadataExchange specification defines three request-response message pairs to retrieve these three types of metadata. One retrieves the WS-Policy associated with the receiving endpoint or with a given target namespace, another retrieves either the WSDL associated with the receiving endpoint or with a given target namespace, and a third retrieves the XML Schema with a given target namespace. Together these messages allow efficient, incremental retrieval of a Web service's metadata."