Just as other service metadata languages (say XML Schema and WSDL), Web Services Policy does not mandate any specific policy retrieval mechanism or specify a mechanism to indicate a (or a sequence of) retrieval mechanism (s).
Any combination of any retrieval mechanisms in any order may be used for referencing policy expressions. Choice of one or more retrieval mechanisms (or a sequence of retrieval mechanisms) is a prerogative of the application that references a policy expression. Example retrieval mechanisms are:
- Do nothing. A policy expression with the referenced IRI is already known to be available in a local cache or chip (embedded systems).
- Use the referenced IRI and retrieve an existing policy expression from the containing XML document: a policy element with an XML ID.
- Use the referenced IRI and retrieve a policy expression from some policy repository/registry (local or remote) or catalog. Policy tools may use any protocols (say Web Services Metadata Exchange) for such metadata retrieval. These protocols may require additional out of band information.
- Attempt to resolve the referenced IRI on the Web. This may resolve to a policy element or a resource that contains a policy element.
These illustrative examples are briefly described in the Web Services Policy Primer.
By not mandating any specific policy retrieval mechanism or specifying a mechanism to indicate a (or a sequence of) retrieval mechanism (s), the Web Services Policy Framework offers a level of abstraction from any underlying concrete application context. This way, a policy expression is usable in different environments.
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