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Re: Uniform access to metadata: XRD use case.

From: Phil Archer <phil@philarcher.org>
Date: Wed, 25 Feb 2009 10:00:01 +0000
Message-ID: <49A516A1.4050602@philarcher.org>
To: wangxiao@musc.edu
CC: Eran Hammer-Lahav <eran@hueniverse.com>, 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>


Xiaoshu Wang wrote:
> 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.

Yes you do. You know that B has something to say about A. You don't, 
however, know what format either is in or anything else. Those details 
are handled by other mechanisms, notably the content type. In this link:

Link: <foo.bar>; rel="describedby" type="application/thing";

You would probably only fetch foo.bar if you had a UA that could process 
application/thing. This is a hint - it may be superseded by the more 
authoritative headers that come back if you dereference foo.bar)

   As a URI client, how should I know when should I get A and when
> B? 

Because either:

  - you're interested in A for any of the reasons you may be interested 
in any resource (you're following a link, it's in a search result or 
whatever). Optionally, you can find out more about A by following the 
link to B.

  - you're collecting URIs of resources that have particular features. 
Therefore, you'll look for Bs and then use them to find As.

  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.

No. As an analogy: if an HTML page links to a stylesheet you can choose 
whether to fetch the stylesheet or not in order to render the page.

  Thus, I cannot
> help but wondering why they are not put together at A at the first place.

Because they are often managed by different people, subject to different 
production and editorial control etc. Take a content production 
workflow. Often there is a relatively large number of people 
(journalists, graphic artists etc.) who create the content which is then 
subject to review by an editor(ial team). There are many situations 
where the latter creates the metadata concerning resources produced by 
the former.

As a little example:

A is the homepage of a bank. It was last updated 2 hours ago.
B tells you that A is the homepage of a bank. B was last updated 2 
months ago.

Current financial crisis notwithstanding, both are accurate, both have 
been updated in a time frame that suggests they are actively managed.

> 
> 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.  

I think you're getting into a bit of a tunnel here. How do you know 
about anything on the Web? How do you discover anything? All the 
mechanisms under discussion have their As and Bs (resources and 
descriptions thereof). The current effort is all about trying to find 
some uniformity of approach.

> 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.

But even a UA doesn't live in a vacuum. It responds to input, usually 
human, sometimes automated. Either way, it is performing a task and will 
have a variety of parameters. Metadata should make its task easier.

> 
> 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.

That depends what the metadata says. If it says "this page is generated 
dynamically to suit a wide variety of devices" that says quite the 
opposite to your conjecture - namely that there are many different 
representations available at the described URI.

Others, more qualified than me, have answered your remaining issues.

Phil.

   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
>>>     
>>
>>   
> 
> 

-- 

Phil Archer
http://philarcher.org/

i-sieve technologies                |      W3C Mobile Web Initiative
Making Sense of the Buzz            |      www.w3.org/Mobile
Received on Wednesday, 25 February 2009 10:00:23 GMT

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