WS-Addressing Metadata

EPRs and Metadata Any Web services endpoint, or more generally, virtually any distributed systems endpoint, incorporates various forms of metadata. For example, an endpoint typically supports some kind of interface, accepts certain forms of data as inputs, and produces the same or different forms of data as outputs. Endpoints might have requirements related to security or transactions, or they might support particular quality-of-service (QoS) guarantees. Although such metadata has traditionally been hard-coded directly into applications, leading to tightly coupled systems, numerous research efforts are currently exploring how applications might dynamically discover and use metadata in practical ways that let them be more loosely coupled and flexible. The original WS-Addressing member submission to the W3C specified several EPR metadata properties.1 The selected-port-type and service-port properties, for example, were intended to convey information about the Web Services Description Language (WSDL) definition associated with a given endpoint. Meanwhile, the policies property was intended to be a general container for Web Services Policy Framework (WSPolicy)2 information associated with an endpoint. Unfortunately, these metadata properties have some problems. First, WS-Policy is currently only a specification, not an actual standard. When the draft WSAddressing standard becomes final (currently scheduled to occur sometime in late 2005) it will no longer be able to normatively reference WSPolicy. The WS-Addressing working group realized this shortly after its inception in October 2004 and quickly eliminated the policies property from the working draft. Second, in WSDL, a port type specifies a collection of input and output messages — essentially, an interface — that a Web service supports, and a WSDL binding associates a port type with a particular protocol and message format (SOAP bindings are commonly used, for example). A WSDL service combines a binding (and thus a port type) with an actual service address or location. In the original WS-Addressing specification, the selected-port-type property let an EPR creator specify an endpoint’s WSDL port type or interface, and the service-port property let the creator specify the endpoint’s WSDL service. Unfortunately, applications can’t use these properties unless they’re already aware of the endpoint’s WSDL definition. What’s more, the properties don’t let EPRs include information about multiple WSDL bindings that a Web service might support. Although these properties enhance certain types of flexibility, they limit others. The fact that the selected-port-type and service-port properties assume that applications already know the Web service’s complete WSDL definition, or