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Re: ISSUE-57: The use of HTTP Redirection

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 29 Aug 2007 20:06:37 +0200
Message-Id: <AEFAC229-24CF-440A-8A85-DF47B603792F@cyganiak.de>
Cc: "'Booth, David \(HP Software - Boston\)'" <dbooth@hp.com>, "'Tim Berners-Lee'" <timbl@w3.org>, "'Ed Davies'" <edavies@nildram.co.uk>, "'Technical Architecture Group WG'" <www-tag@w3.org>
To: "Rhys Lewis" <rhys@volantis.com>

Rhys and others,

I'll try to articulate my questions more clearly.

I assume that X is an information resource, such as “Tim's homepage”,  
and that different people can allocate different URIs <x1>, <x2>, ...  
to identify that resource (e.g. to make RDF statements about it),  
without necessarily knowing about each other.

1. *MUST* the owner of each <xi> configure it to answer requests with  
a 200 and a representation?

2. Or *MAY* they answer requests to <xi> also with 404 or 303?

3. Assuming there is reason to believe that other people have minted  
<x1> and <x2> already, and serve representations there: Does WebArch  
in any way constrain what the owner of <x3> can serve as a  
representation of X at <x3>?

4. What if the owner of <x3> is legally prohibited from serving  
representations of X, e.g. by copyright law? Can they still mint  
their own URI for X? How would they configure it?

5. Assuming X is “Tim's homepage”, is there a process (technical or  
social) that allows me to determine if <x1> does identify X, assuming  
that <x1> 200-responds with a certain representation "abc"?

6. Assuming X is “Tim's homepage”, is there a process (technical or  
social) that allows me to determine if <x2> does identify X, assuming  
that <x2> 404-responds?

7. Assuming X is “Tim's homepage”, is there a process (technical or  
social) that allows me to determine if <x3> does identify X, assuming  
that <x3> 303-redirects to <y3>?

What do you think?


On 29 Aug 2007, at 15:03, Rhys Lewis wrote:

> Hello Richard,
>> -----Original Message-----
>> From: www-tag-request@w3.org [mailto:www-tag-request@w3.org]
>> On Behalf Of Richard Cyganiak
>> Sent: 29 August 2007 13:15
>> To: Booth, David (HP Software - Boston)
>> Cc: Tim Berners-Lee; Ed Davies; Technical Architecture Group WG
>> Subject: Re: ISSUE-57: The use of HTTP Redirection
>> On 28 Aug 2007, at 22:19, Booth, David (HP Software - Boston) wrote:
>>>> From:  Tim Berners-Lee
>>>> [ . . . ]
>>>> Often, people use the term "non-information resource" to
>> mean "some
>>>> resource, not necessarily an information resource".
>>> Good Lord, I hope not!  Please chastise anyone who is doing so!  We
>>> have enough chaos without changing the English meaning of "non-".
>> An example perpetrated by, uh, me:
>> [looking at an HTTP response] "This is a 303 redirect, which
>> tells the client that the requested resource is a
>> non-information resource, and its associated description can
>> be found at the URI given in the
>> Location: response header." [1]
>> This is incorrect, because 303 doesn't really tell us
>> *anything* about the nature of the resource, except that
>> associated information might be found somewhere else.
>> My incorrect statement was motivated by the question why hash URIs or
>> 303 URIs or 200 URIs should be used in a particular case.
>> Usually, the answer goes like this: If you have an IR, you
>> can use 200. If you have a non-IR, you must use 303 or a hash
>> URI. Unfortunately, that's not the whole story. There is this
>> grey area where you *may* or even
>> *must* identify normal IRs using hash URIs or 303 URIs. That
>> grey area is a mess.
>> There's a lot of discussion currently about the distinction
>> between IRs and non-IRs. Is it about "essential
>> characteristics conveyable in a message", about "can we
>> attach an HTTP endpoint to it", about "document-ness"?
>> To me, this all misses the point. Even if we can nail down
>> objective criteria to distinguish these buggers, this will
>> *still* not tell us if we have to serve them up using 303/hash or  
>> 200.
>> So, can anyone shed light on this? If I mint a URI, what's
>> the criterion for setting up 200 vs. 303/hash? Keep in mind Tim's
>> example: CERN could set up a 303 redirect from some early-day
>> WWW URIs of historic interest, to a "museum page" hosted at
>> today's W3C site.
> Let me try and tease this apart to see if I understand why you  
> think this
> is a problem.
> In Tim's example, the CERN 303 redirect simply says there is no
> representation for the early day WWW URI. That URI identifies a
> non-information resource. There is no representation available. The  
> nature
> of the URI that CERN gives back in the 303 is completely  
> indeterminate. It
> could be an information resource, or a non-information resource, or  
> could
> lead to another redirect, for example. (And of course it could lead  
> to a
> plethora of other response codes indicating various form of error that
> I'll ignore here).
> Assuming the URIs are set up correctly, and the URI provided by  
> CERN in
> the 303 does indeed identify an information resource, a  
> representation can
> be retrieved and everything has worked out as intended.
> Surely the criterion for minting URIs is straightforward. If the  
> URI is
> for an information resource (provides representations) you return a
> suitable representation, if you have one, and a 200 response coed  
> (let's
> ignore content negotiation for the purposes of this discussion). If,
> however, the URI is for a non-information resource, you have two  
> options.
> You can return a 303 and a helpful URI. You're not allowed to return a
> representation according to HTTP. If you prefer, you return a
> representation of the racine, which is not a representation of the  
> hash
> URI.
> You cover all this in your paper. The only statement that appears  
> to go
> beyond what has been generally accepted to date is to say that the  
> URI in
> the 303 necessarily provides some kind of description of the
> non-information resource. At the moment, the httpRange-14 draft  
> finding
> [2] says that authors SHOULD do this when using the 303 mechanism with
> non-information resources. Some people would like this statement to be
> stronger. We think that we can encourage people to use 303 this way  
> for
> the particular purpose of associating additional information with
> non-information resources. We have to be careful about revising the
> meaning of 303 to mean only this, however. It's been in the wild for a
> long time.
> Apologies if I'm missing something deeper.
> By the way, the range14 draft has taken a bit of a public beating  
> over the
> last week or so. There will be some changes. I sense that there is  
> quite a
> bit of interest in stronger statements about certain redirections,
> including 303, though.
> Best wishes
> Rhys
> [2]
> http://www.w3.org/2001/tag/doc/httpRange-14/2007-08-31/ 
> HttpRange-14.html
>> Richard
>> [1] http://sites.wiwiss.fu-berlin.de/suhl/bizer/pub/
>> LinkedDataTutorial/#ExampleHTTP
>>> David Booth, Ph.D.
>>> HP Software
>>> +1 617 629 8881 office  |  dbooth@hp.com
>>> http://www.hp.com/go/software
>>> Opinions expressed herein are those of the author and do
>> not represent
>>> the official views of HP unless explicitly stated otherwise.
Received on Wednesday, 29 August 2007 18:07:40 UTC

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