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Re: Uniform access to metadata: XRD use case.

From: Xiaoshu Wang <wangxiao@musc.edu>
Date: Wed, 25 Feb 2009 00:00:21 +0000
Message-ID: <49A48A15.6040303@musc.edu>
To: Eran Hammer-Lahav <eran@hueniverse.com>
CC: Julian Reschke <julian.reschke@gmx.de>, "Patrick.Stickler@nokia.com" <Patrick.Stickler@nokia.com>, "jar@creativecommons.org" <jar@creativecommons.org>, "connolly@w3.org" <connolly@w3.org>, "www-tag@w3.org" <www-tag@w3.org>
The critical flaw of all the proposed approach is that the definition of 
"metadata/descriptor" is ambiguous and hence useless in practice.  Take 
the "describedBy" relations for example.  Here I quote from Eran's link.

      The relationship A "describedby" B asserts that resource B
      provides a description of resource A. There are no constraints on
      the format or representation of either A or B, neither are there
      any further constraints on either resource.

As a URI owner, I don't know what kind of stuff that I should put in A 
or B.  As a URI client, how should I know when should I get A and when 
B?  Since I don't know what I might be missing from either A or B, it 
seems to suggest that I must always get both A and B. Thus, I cannot 
help but wondering why they are not put together at A at the first place.

The same goes for MGET, how a user knows when to GET and when to MGET? 
PROFOUND is different because when people use it, they have already 
known that the resources is defined by WebDAV.   Hence, these kind of 
ideas only works when the client already have some knowledge about A.  
But, to propose it as a general framework for the Web, it won't work.  
At the most fundamental level, we only know three things about the Web 
-- URI, Representation, Resource.  The concept of metadata is 
ill-conceived at this level because as data about data, to say metadata 
implies that we already know something about the resource we tries to 
access, a piece of knowledge that we don't have.

There are a lot of implicit assumptions under the so-called "uniform 
access to metadata/descriptor" approach.  It either requires the 
definition of IR or a one-on-one relationship between Resource and 
Representation.  As the former implies that non-IR cannot have a 
representation, it makes the "descriptor/metadata" necessary.  The knock 
on this assumption is that the definition of IR is impossible to work with.

The 1-on-1 relationship gives rise to the so-called "legacy resource".  
But the word "legacy resource" is wrongly named too.  In the Web, there 
might be something as "legacy representation" but there should NOT be 
such thing as "legacy resource" because the latter implies that the 
Resource is closed and no more semantics will be added. 

But the so-called "metadata/descriptor" problems can be solved by using 
HTTP Content Negotiation, making any other proposal a redundant one. The 
actual issue, as I have discussed in [1], is about the incomplete syntax 
of the URI specs, which  currently does not have a syntactic notation 
the other two foundation objects in the Web, i.e., URI and 
Representation.  Once we supplement URI spec with those syntactic sugar, 
such as the one I proposed in [2], then, we can have a uniform approach 
to (1) describe URI along with standard resources and (2) to 
systematically discover the possible representation types, i.e., 
Content-Type/MIME types, associated with a Resource (either URI or 
standard Resource). As a particular content-type is equivalent of a 
particular *service*, hence, the approach in effect establishes a 
uniformed approach to service discovery. 

What is required is to define Content-Type in URI.  Once we have these, 
not only Data/Resource are linked but DataType/Service.  The best of 
all, it works within the conceptualizations defined in AWWW, and does 
not require any other ambiguous conceptualization, such as, IR, 
metadata, and description, etc.

1. http://dfdf.inesc-id.pt/misc/man/http.html
2. http://dfdf.inesc-id.pt/tr/uri-issues

Xiaoshu

Eran Hammer-Lahav wrote:
> Both of which are included in my analysis [1] for the discovery proposal.
>
> EHL
>
> [1] http://tools.ietf.org/html/draft-hammer-discovery-02#appendix-B.2
>
>   
>> -----Original Message-----
>> From: Julian Reschke [mailto:julian.reschke@gmx.de]
>> Sent: Tuesday, February 24, 2009 1:45 AM
>> To: Patrick.Stickler@nokia.com
>> Cc: Eran Hammer-Lahav; jar@creativecommons.org; connolly@w3.org; www-
>> tag@w3.org
>> Subject: Re: Uniform access to metadata: XRD use case.
>>
>> Patrick.Stickler@nokia.com wrote:
>>     
>>> ...
>>> Agents which want to deal with authoritative metadata use
>>>       
>> MGET/MPUT/etc.
>>     
>>> ...
>>>       
>> Same with PROPFIND and PROPPATCH, btw.
>>
>> BR, Julian
>>     
>
>   
Received on Wednesday, 25 February 2009 00:01:15 GMT

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