W3C home > Mailing lists > Public > uri@w3.org > February 1997

Re: Resources and Identifiers

From: Michael Mealling <michaelm@rwhois.net>
Date: Fri, 21 Feb 1997 11:09:59 -0500
Message-Id: <330DC8D7.307C@rwhois.net>
To: Daniel LaLiberte <liberte@ncsa.uiuc.edu>
Cc: Dan Connolly <connolly@w3.org>, uri@bunyip.com
Daniel LaLiberte wrote:
>  > > ... A single object could have multiple URLs, so
>  > > different URLs do not necessarily mean different objects.
>  >
>  > That's an interesting assertion. Is it the conclusion of some
>  > argument, or just an assertion?
> It is the conclusion of a fairly short argument.  Consider that a
> document may have an http URL and an ftp URL for the very same
> document.  The question is how do you, the client, *know* that it is
> the same document?  You could do an md5 hash of what you get back if
> it is static.  Some explicit metadata could specify that they are the
> same.  Otherwise, you don't know.  When is an object really a replica
> of an object?  In fact, each time you get bits in the result of a
> request, you are getting a replica of some object (in my world view).
> I suppose this problem with replicas is the reason you like to have
> a reference to the object instead of the object itself.

Right. Which is the entire reason everyone wants a URN. So they
can tell whether or not two replicas are the 'same'.

>  > But I don't think there's any universal concept of 'same
>  > objects' other than 'objects referred to in exactly the
>  > same way'.
> Same bits means same object,

Not necessarily. There is other metadata about an object (resource)
that can determine sameness. Take Ron Daniels employer, Los Alamos
National Labs, their security is very tight. (I'm guessing here
but I would bet money I'm right). I'm sure their security
people would consider two objects different even though their
bits were the same depending on where you got the object. Their
reasons are their reasons. When your talking nuclear bombs they
get to have all of the arbitrary reasons they want. 

The only entity that gets to really define sameness is the entity
that owns the resource(s).


Michael Mealling	| 505 Huntmar Park Drive       | Phone:  (703)742-0400
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Received on Friday, 21 February 1997 11:10:41 UTC

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