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Re: MGET and machine processing

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Wed, 26 Nov 2003 11:31:28 +0200
Cc: www-rdf-interest@w3.org
To: "ext Mark Baker" <distobj@acm.org>
Message-Id: <501EF6AC-1FF3-11D8-9FAA-000A95EAFCEA@nokia.com>


On Wednesday, Nov 26, 2003, at 04:02 Europe/Helsinki, ext Mark Baker 
wrote:

> On Tue, Nov 25, 2003 at 07:21:49PM +0200, Patrick Stickler wrote:
>> On Monday, Nov 24, 2003, at 16:55 Europe/Helsinki, ext Mark Baker 
>> wrote:
>>
>>> On Mon, Nov 24, 2003 at 02:41:34PM +0200, Patrick Stickler wrote:
>>>> Well, while I consider it acceptable to treat a description as
>>>> a representation, it is nonetheless necessary to be clear about
>>>> the distinction when interacting with the server.
>>>
>>> Right.  Using a different URI would be another way to do that! 8-)
>>
>> Both representations *and* descriptions are distinct resources
>> from the resource denoted by a request URL (except for the special
>> case where the resource is a digital entity and the representation
>> is a bit-equal copy of it).
>
> Absolutely.
>
>> So whether you are asking the server for a representation or
>> a description, you should (usually) expect to get back *something
>> else* and have the URI denoting that something else specified
>> in the response header.
>>
>> But one cannot know, from some URI, what some *other* URI denoting
>> a description of the thing denoted by the first URI might be, no
>> more so than one can know what URI might denote a representation
>> returned for some request having a *different* request URI.
>
> Yup.  You use a new method on a URI to return a description, which
> is given a URI upon which subsequent GETs can return it.  I use a
> GET or HEAD on the original URI to return a new header which
> communicates the URI of the description resource, upon which I can
> invoke GET.
>
> The trade off is an extra round trip and a new header, for a new 
> method.
> IMO, that's akin to trading a pawn and a bishop (respectively) for a
> queen.
>

Right.

It is this double request that I consider unnacceptable.

Imagine if, e.g. HTTP 2.0 came out, and for whatever reason, every
request by every browser would first have to submit a HEAD request
to get some little bit of information before it could submit a GET
request. What kind of reaction do you think the web community would
have? I expect that someone would get lynched.

I don't know how many others share my view on this, but I refuse to
accept any standardized solution for SW agent interaction which
requires such overhead -- no matter how committed I am to having
maximal synergy with the existing web architecture (and I'm *very*
committed to that, I asure you).

My #1 requirement is that, for any arbitrary URI which is meaningful
to the HTTP protocol, and contains a web authority component, a SW agent
should be able to send a request to that web authority to obtain a
concise bounded description of the resource denoted by that URI, with
a single request, and without any further information than the URI 
itself
(and the generic protocol by which the request is made) and recieve in
response either a description, or an error indication of some fashion.

While there are other requirements that I consider also to be essential,
the above is, for me, the very heart and core of how (atomic level
of) the SW should work, regardless of all other more capable
protocols, apis, services, etc.

>> No. Content negotiation does *not* do the job. And we will want
>> to use content negotation *as* content negotiation, for requesting
>> different possible encodings of descriptions.
>>
>> I've pointed this out before.
>>
>> In short, content negoation does not work, nor should it be overloaded
>> in this fashion, as the semantic distinction between description and
>> (other form of) representation has nothing whatsoever to do with
>> encoding (even if RDF/XML is a default encoding for descriptions).
>
> What you've pointed out before - and presumably what you're referring 
> to
> there - was that it is incorrect to use content negotiation to 
> negotiate
> for descriptions from the URI which is being described, and I agree
> completely.  But I can safely negotiate for RDF/XML *representations*
> using the URI.
>

Absolutely. And if you have a URI denoting a concise bounded description
of some other resource, you could use content negotiation to request
any number of supported encodings/serializations of that description.

Content negotiation and the distinction between description and 
representation
are orthogonal, and should remain so.

It appears that we agree on that point. Yes?

Cheers,

Patrick


> Mark.
> -- 
> Mark Baker.   Ottawa, Ontario, CANADA.        http://www.markbaker.ca
Received on Wednesday, 26 November 2003 04:57:26 GMT

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