W3C home > Mailing lists > Public > www-tag@w3.org > July 2002

Re: Context Independent URI

From: Michael Mealling <michael@neonym.net>
Date: Mon, 22 Jul 2002 08:57:06 -0400
To: "Williams, Stuart" <skw@hplb.hpl.hp.com>
Cc: "'Miles Sabin'" <miles@milessabin.com>, www-tag@w3.org
Message-ID: <20020722085706.X207@bailey.dscga.com>

On Mon, Jul 22, 2002 at 10:01:35AM +0100, Williams, Stuart wrote:
> > -----Original Message-----
> > From: Miles Sabin [mailto:miles@milessabin.com]
> > Sent: 21 July 2002 22:50
> > To: www-tag@w3.org
> > Subject: Re: Context Independent URI
> > 
> > Williams, Stuart wrote,
> > > So... I have tried to avoid using the term absolute to avoid
> > > confusion with absolute and relative URI and tried to focus the
> > > principle on the scope of the mapping from URI to resource/concept.
> > 
> > Umm ... but that renders the "principle" pretty close to hopeless: a 
> > relative URI ISA URI, yet is quite clearly context dependent, and quite 
> > rightly so.
> 
> Yes, I agree, the resource denoted by a relative URI is also context
> dependent. What I am trying to pick up is that there are also some
> (syntactically) absolute URI (in that they start with a scheme name) that
> are also context dependent... eg. URI which use an unqualified domain name
> as the assigning authority; file: scheme URI which allow a hostname, but do
> not identify the namespace from which the hostname is taken (eg
> internet-domain name, DECNet, Novell IPX, Appletalk...).
> 
> Do each of the absolute URI file:///etc/passwd or
> file://localhost/autoexec.bat or http://cally/ identify a single resource or
> concept?

IMHO, as long as its explicit, yes. 'file:/etc/passwd' identifies
the the single concept of "the file on any machine found at /etc/passwd'.
Each individual /etc/passwd on a given machine is simply a binary 
representation of the concept. The concept is the same because the
concept includes the fact that it is relative to the resolution is
contextual...

I think its useful to note in the architecture what systems can handle
contextual resolutions and which ones can't. But in many cases you
also have the need to resolve non-contextual URIs contextually. Some
call it the Harvard problem but its where you have some local copy or
policy that dictates that you should retrieve something other than
what the 'context independent' URI would return, so you resolve
it through some special cache that uses some other metadata to determine
if a local copy is 'sufficient' for the task.

We even had a BOF on this at the IETF once. Contextual resolution is
actually pretty common place and a accomplished through some pretty
heinous hacks with DNS and not exactly compliant proxy looking abominations...

-MM

-- 
--------------------------------------------------------------------------------
Michael Mealling	|      Vote Libertarian!       | urn:pin:1
michael@neonym.net      |                              | http://www.neonym.net
Received on Monday, 22 July 2002 08:59:04 GMT

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