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Re: Uniform access to descriptions

From: Tim Berners-Lee <timbl@w3.org>
Date: Sat, 12 Apr 2008 22:20:41 -0400
Cc: Michaeljohn Clement <mj@mjclement.com>, "www-tag@w3.org WG" <www-tag@w3.org>
Message-Id: <FA92C202-6A83-4B13-BDCE-86E7490ED2C2@w3.org>
To: wangxiao@musc.edu


On 2008-04 -11, at 19:54, Xiaoshu Wang wrote:

>
>
>
> Michaeljohn Clement wrote:
>> Hi Xiaoshu,
>>
>>
>>> We agree that there are legacy data, yes?  Let's make its URI x,  
>>> whose
>>> owner is Joe.
>>> Case 1. Joe is lazy.
>>> Then, no LINK, no Conneg. Is this fair?
>>> Case 2: Joe is not lazy.
>>> (a) Joe makes LINK(x)=metadata.
>>> (b) Joes make Conneg(x)=metadata (can easily GET x Accept
>>> application/rdf+xml).
>>>
>>
>> (b) would be wrong, because the metadata is not an alternative  
>> variant of the resource identified by x.
>>
> Why wrong? First define metadata?  Say _:x _:b _:y.  Is this  
> assertion metadata of _:x or _:b or _:y?  You assume it is wrong  
> because of an arbitrary definition of metadata. In your proposal,  
> any RDF transformation is the metadata of an HTML, they should be  
> put in LINK too.

The point is that when conneg is used to return two different  
representations of a document, with different content types, the use  
is ONLY to allow negotiation of different formats for the SAME  
information.

(Sometimes different formats cannot express all the content of the  
original, so sometimes there is quality degradation. But this should  
be avoided where possible).


>> Surely there may be more than one application/rdf+xml resource that  
>> might be associated in some way with the resource identified by x,  
>> right?  It's impossible to distinguish between these by using conneg.
>>
> This is the reason.  Perhaps it is not I who have failed to  
> understand the <LINK> problem, it is you who have failed to  
> understand Conneg.
>> If the only purpose of the Link: header would be the same as the  
>> purpose of the HTML <link rel=alternate>, then surely (b) would be  
>> more relevant, but the Link: header can express other relations as  
>> is being discussed here, viz "describedBy".
>>
>> The only way (b) can be correct here is if the result of a GET with  
>> Accept: application/rdf+xml is actually just a variant  
>> representation of the same resource.
>>
> What do you mean *just* a variant?  All representations bound to the  
> resource 'x' identifies x.  It may be of different format, different  
> language, but they are all talking about the same resource - that is  
> what matters.

No, what matters is that they same the same thing.  I'm sorry this  
wasn't clear to you.

>
> Of course, this goes back to my request for TAG to delineates the  
> meaning of /representation/ vs. /resource/.

The TAG attempetd to so it. The

> Without it, any discussion on <LINK> is a waste of time.
>
> Xiaoshu
Received on Sunday, 13 April 2008 02:21:15 GMT

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