..OO.. Wheeling thru - by Asir

How to think about Web Service Policy Assertions?

A Policy Assertion identifies a visible domain specific behavior that is a capability or requirement. Typically, the provider of a Web service exposes a Policy Assertion to convey a condition under which the provider offers a Web service. A Policy Assertion is supported by a requestor (or requestors) if and only if the requestor (or requestors) satisfies the corresponding requirement. In most cases: there is more than one participant in the capability or requirement expressed by a Policy Assertion; and the semantics of a Policy Assertion are reflected in messages. For example, say to indicate that messages are protected using mechanisms provided by the underlying transport (say HTTPS), a transport security policy assertion can be used. This assertion is supported by a requestor if and only if the requestor uses transport level security, and the semantics of this assertion are reflected in messages because they are encrypted over the wire.

Service Metadata and WS-Policy

The Service Metadata is an expression of visible aspects of Web service behavior. Service Metadata is represented by a mixture of machine- and human-readable languages. Machine readable languages enable tooling. Service Metadata is a subset of the spectrum of general metadata. There are many popular points in this Service Metadata subset.

First popular point in the Service Metadata is the message interchange format within the SOAP Message Construct. That is, representing SOAP Body Children and SOAP Header Blocks. Industry uses XML Schema language for representing message interchange format.

Second popular point in the Service Metadata is ‘how’ and ‘where’ messages are exchanged. That is, representing the concrete message format, what headers are used, transmission protocol, message exchange pattern, and endpoint. Industry uses WSDL for representing ‘how’ and ‘where’ messages are exchanged. WSDL has extensible points that can be used to expand on ‘how’ and ‘where’ representations.

Third popular point in the Service Metadata is the representation of capabilities and requirements for interacting with Web service endpoints. That is, representing whether and how a message must be secured, whether and how a message must be delivered reliably, etc.

WS-Policy is a new machine-readable language for representing Web service capabilities and requirements as Policies.

Web Services Policy?

Web services are being successfully used for interoperable solutions across the industry. One of the key reasons for interest and investment in Web services is that they are well suited to enable service-oriented systems. XML-based technologies such as XML Schema, WSDL and SOAP provide a broadly-adopted foundation on which to build interoperable Web services. The WS-Policy and WS-PolicyAttachment specifications extend this foundation and offer mechanisms to represent capabilities and requirements of Web services as Policies.