I already explained that I agree with Paco’s view that EPRs are not identifiers. Pankaj also provided a concrete example of why confusing references and identifiers causes problems. Paco just sent a new, better explanation of his earlier point, in the format of a formal proposal to the WS-Addressing WG. His proposal and its justification are a must read. He starts with a “what is required from an identifier” paragraph, which reads:
An identifier to be useful must allow meaningful comparison for identity or “sameness”. This requires them to overall unique and unambiguous, otherwise no meaningful comparison is possible. Moreover one could argue that to be really useful identifiers should not be reused once they’ve been made invalid.
Compare this with the specification for the ResourceId property defined in MUWS Part 1 (note: this is a link to the current working draft as a Word document, not yet a committee draft). Some highlights that match very well with Paco’s expectations for a *real* identifier:
- Globally unique: A manageability endpoint MUST create the ResourceId URI in a way that ensures that the ResourceId is unique to the resource managed through the manageability endpoint and globally unique.
- Uniqueness in time: A ResourceId MUST NOT be reused by the implementation of a manageability endpoint for another resource, even after the original resource no longer exists.
- Consistency across endpoints: An implementation of a manageability endpoint SHOULD use a ResourceId that is suggested by the characteristics of a resource.
And the spec goes on to define in more details why/how implementers should ensure that difference manageability endpoints for the same resource return the same ResourceId, persistence of the ResourceId in time and how to establish “sameness” when for some reason different manageability endpoints for the same resource are unable to return the same ResourceId (correlatable properties). Go ahead and read it or wait a couple of weeks if you want to see a committee draft as a clean PDF rather than a Word document with change tracking turned on.