W3C home > Mailing lists > Public > www-tag@w3.org > September 2006

RE: fragment identifiers and media types (was RE: XPointer considered incomprehensible)

From: Dan Connolly <connolly@w3.org>
Date: Tue, 05 Sep 2006 15:54:19 -0500
To: "Booth, David (HP Software - Boston)" <dbooth@hp.com>
Cc: simonstl@simonstl.com, noah_mendelsohn@us.ibm.com, Bjoern Hoehrmann <derhoermi@gmx.net>, www-tag@w3.org, www-xml-linking-comments@w3.org, Jonathan Marsh <jmarsh@microsoft.com>
Message-Id: <1157489659.9288.1036.camel@dirk>

On Tue, 2006-09-05 at 12:37 -0400, Booth, David (HP Software - Boston)
wrote:
> > From: Simon St.Laurent
> > 
> > . . .  For a URI reference like:
> > 
> > http://simonstl.com/#news
> > 
> > The interpretation of the fragment identifier depends on the 
> > media type returned.  URI philosophers will likely wave their 
> > hands and say this isn't a problem. 
> 
> As a side comment, this is precisely why, in my opinion, hash URIs are
> unsuitable as *general purpose* identifiers: the meaning of the URI is
> tied to the media type that is returned when the racine is dereferenced.
> (The racine is the part before the hash.)  I.e., the meaning of this
> URI:
> 	http://simonstl.com/#news
> depends on the media type that is returned when this other, related URI:
> 	http://simonstl.com/
> is dereferenced.  This may be fine if one is willing to limit one's self
> to certain media types, but it is not very general purpose.

Why not?

The meaning of *every* URI in the web is, practically, connected to
protocol messages involving that URI, and pretty much all the
Web protocols use MIME types somehow.

The meaning of URIs that include #'s is no more or less dependent
on media types than other URIs. For example, I can mint
a URI right now... http://dm93.org/2006/09/not-very-helpful#sixtythree
and tell you that it refers to the integer 63.

I did that without using any representations of
http://dm93.org/2006/09/not-very-helpful , and hence without
any dependence on media types.

Now this URI is not a very good one, because when you look it up, you'll
get a 404; so you don't get any useful information when
you use standardized protocols to look it up. But as the
owner of dm93.org, I have the right to allocate that URI
and say what it means, whether I use standard protocols or not.

(You could argue that I'm conflicting myself when I say that
I have allocated that URI and yet give out a 404 error.
OK, so change it to "403 unauthorized"; in other words:
I know what /2006/09/not-very-helpful means, but
but I'm not telling; not via HTTP, anyway. Please grant this line of
argument even though I have not, actually, configured the
dm93.org web server to act this way.)



> Slash URIs using 303-redirects do not have this limitation.

Sure they do. The meaning you get back after you follow your
nose thru the redirection certainly depends on the media type
of what you get back.

>   For
> example, if
> 	http://simonstl.com/news
> does a 303 "See Other" redirect to
> 	http://simonstl.com/
> then
> 	http://simonstl.com/news
> could identify any resource, independent of the media type returned when
> 	http://simonstl.com/
> is dereferenced.

No, it's not independent; the whole transaction tells you about
the meaning of http://simonstl.com/news , right? And that meaning
is determined by looking at the media type of what you
get back from http://simonstl.com/ .


-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
Received on Tuesday, 5 September 2006 20:54:34 GMT

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