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Re: resource equivalence was: Re: fragment identifiers

From: Michael Mealling <michael@neonym.net>
Date: Wed, 24 Jul 2002 11:01:38 -0400
To: Jonathan Borden <jonathan@openhealth.org>
Cc: "Roy T. Fielding" <fielding@apache.org>, www-tag@w3.org
Message-ID: <20020724110138.A1825@bailey.dscga.com>

On Wed, Jul 24, 2002 at 09:40:23AM -0400, Jonathan Borden wrote:
> 
> Roy T. Fielding wrote:
> >
> > A URI is an identifier.  The semantics of the resource it identifies
> > are defined by the sameness of representations of that resource over
> > time, not by any property of the identification system used to create
> > that identifier. In short, a URI reference has the semantics that
> > other people assign to it when they persist in trying to use it to
> > refer to something useful.
> 
> Terrific. This para should (IMHO) be included in the "arch doc".
> 
> BTW: I've suggested equating resources by the equality of their
> representation sets as axioms [10,11] in
> http://www.openhealth.org/RDDL/SchemaAlgebra
> 
> [[
> [10] equivalent(URIa,URIb) := Entities(URIa) = Entities(URIb) and
> cardinality(Entities(URIa)) > 0
> 
> Two URIs are equivalent when they map to the same set of entities.
>
> [11] equivalent(A,B) <=> exists URIa such that A = resource(URIa) and exists
> URIb such that B = resource(URIb) and equivalent(URIa,URIb)
> 
> Two resources a and b are equivalent if the set of entities given the URIa
> and URIb are equal where URIa identifies a and URIb identifies b.
> ]]
> 
> where Entities(URI) is the set of entities which the URI maps to across
> media-types, time, and other e.g. HTTP request parameters.

Wrong. Two URIs are equivalent when they match each other according to 
the canonicalization rules in RFC 2396. Certain _systems_ may, in 
a system specific way, provide other methods for determining some
aspect of equality between resources but that sense of equality is
only valid within that system.

You may make the statements above but they are only valid for your 
system in which you have control over the semantics/policies/laws/etc
that determine the _entire_ set of semantics (client and server side).
But you can't make that statement for all URIs.

The only uniform statement you can make about the equality of two or more
Resources is whether or not the URIs in question are syntactically
equal according to RFC 2396's normalization rules. Period.

-MM

-- 
--------------------------------------------------------------------------------
Michael Mealling	|      Vote Libertarian!       | urn:pin:1
michael@neonym.net      |                              | http://www.neonym.net
Received on Wednesday, 24 July 2002 11:03:38 GMT

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