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Re: URI ref comparison - clarification requested

From: Graham Klyne <GK@ninebynine.org>
Date: Mon, 30 Aug 2004 17:41:36 +0100
Message-Id: <>
To: "Roy T. Fielding" <fielding@gbiv.com>, Danny Ayers <danny.ayers@gmail.com>
Cc: uri@w3.org

This is really off-topic, and not relevant to RFC2396bis, but to try and 
clarify something...

As it happens, RDF abstract syntax will treat (say) http://example.org/ and 
http://example.org:80/ as different URIs which might be used semantically 
to denote different things.  This is not recommended practice, just a 
consequence of the way the abstract syntax+semantics are defined.

The fact that when used to perform retrieval using HTTP they always return 
a representation of the same resource is a reason why such practice would 
not be recommended with RDF, but doesn't of itself invalidate the RDF or 
URI specifications.


At 15:30 27/08/04 -0700, Roy T. Fielding wrote:

>>There is still an aspect of this that makes me a little uneasy, though
>>I doubt that it's significant even if my reasoning makes sense. If a
>>spec like RDF says it's using URIs but provides its own comparison
>>mechanism (such as the first approximation of string equiv), then
>>applications built to that spec may systematically, as a group, behave
>>differently than apps built directly to the URI spec (possibly
>>including support for better approximations). That systematic aspect
>>seems a step beyond different apps implementing different variations
>>of the original options.
>>Where the primary practical use of the URI is in the process of
>>obtaining a representation of the resource identified,  the
>>comparisons only (potentially) producing false negatives seem to
>>preclude problems. I suspect it might not be such a failsafe in the
>>general case when is used in constructing logical statements (though I
>>might well be mistaken, IANAL).
>I am kind of curious how a system constructing logical statements
>could somehow fail in a non-safe way just because two equivalent URI
>are considered different.  I think, at most, it just adds one to the
>number of aliases, and thus the admonishment against creating
>arbitrary aliases for a resource still applies.  If the RDF graph
>contains conflicting assertions for two equivalent URIs, then those
>assertions are broken regardless of the comparison algorithm; that
>brokenness is simply made harder to discover due to the lax method
>of comparing URIs -- merely declaring the URIs to be different
>does not cause the assertions to be true.
>However, it is important to note that the reason RDF specifies
>it that way is because the probability of encountering two
>equivalent but not string-equal URIs in the same RDF graph is
>quite small, and easily avoided by use of canonical URI forms.

Graham Klyne
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Received on Wednesday, 1 September 2004 12:17:24 UTC

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