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Re: Uniform access to metadata: XRD use case.

From: Xiaoshu Wang <wangxiao@musc.edu>
Date: Wed, 25 Feb 2009 15:26:07 +0000
Message-ID: <49A5630F.2030606@musc.edu>
To: "Patrick.Stickler@nokia.com" <Patrick.Stickler@nokia.com>
CC: "eran@hueniverse.com" <eran@hueniverse.com>, "julian.reschke@gmx.de" <julian.reschke@gmx.de>, "jar@creativecommons.org" <jar@creativecommons.org>, "connolly@w3.org" <connolly@w3.org>, "www-tag@w3.org" <www-tag@w3.org>


Patrick.Stickler@nokia.com wrote:
>
> On 2009-02-25 11:40, "ext Xiaoshu Wang" <wangxiao@musc.edu> wrote:
>
>   
>>
>> Patrick.Stickler@nokia.com wrote:
>>     
>>> On 2009-02-25 02:00, "ext Xiaoshu Wang" <wangxiao@musc.edu> 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.  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?
>>>>
>>>>         
>>> If one wants a representation of the resource, use GET.
>>> If one wants a description of the resource, us MGET.
>>>
>>>       
>> This doesn't answer the question at all.  For me, a representation must
>> be describing something.
>>     
>
> You're definition of representation seems overly narrow.
>
> If a given URI denotes a tree, and a 200 response to an HTTP GET request for
> that URI returns an image of the tree (i.e. a representation of the tree),
> does that image "describe" the tree? One may be able to observe
> characteristics of the tree by viewing the image, but whether or not the
> image is a "description" of the tree is, I think, a matter of debate, and in
> any case, outside the scope of the protocols in question.
>   
My answer is yes.  I don't know what is your point here.
>   
>> Hence, I cannot say if something is a
>> Representation but not Description.
>>     
>
> It's the specification of the protocol that says what is returned (or should
> be).
>
> A successful response to a GET request can be presumed to be a
> representation.
>
> A successful response to an MGET request can be presumed to be a
> description.
>   
Does this help either a producer or a consumer to decide their action?
>>> There is some potential conceptual overlap between representations and
>>> descriptions for certain kinds of resources, but the distinction should be
>>> reasonably intuitive.
>>>
>>>       
>>  I don't think any protocol based on intuition is practical.
>>     
>
> Neither HTTP or URIQA are based on intuition. Some concepts are, however,
> for most folks, fairly intuitive. But the specs will say how software should
> behave and expect when using those protocols.
>   
This is really an empty answer.  You still have yet given a definition 
that helps software to behave accordingly. When should they GET and 
MGET? Please don't circular define it, such as when you needs 
Representation, GET, description, MGET.  I want to know the semantic 
difference.   
>>  The
>> concept of IR seems intuitive, but it doesn't work (at least not for me).
>>     
>>>> 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.
>>>>
>>>>         
>>> For URIQA, all that is needed is the URI. After all, you have to be able to
>>> name something to communicate effectively about it.
>>>
>>> URIQA does not presume that any representation exists. It neither posits nor
>>> requires an "Information Resource".
>>>
>>> It is perfectly complimentary to the web.
>>>
>>> GET/PUT/etc. deal with representations.
>>> MGET/MPUT/etc. deal with descriptions.
>>>
>>> If you have a URI, you can use it to either get representations or
>>> descriptions, and if you don't know anything about what resource the URI
>>> denotes, you might first want to get the description.
>>>
>>>
>>>       
>>>> 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.
>>>>
>>>>         
>>> URIQA makes none of those assumptions.
>>>
>>>       
>> Really? Try to define the distinction between your terms "description"
>> and "representation", see what you must come out.
>>     
>
> A representation is what you (should) get from a 200 response to an HTTP GET
> request. It can be expected to reflect, in some manner, the state of the
> resource denoted by the request URI. Whether the representation returned is
> useful or meaningful to the recipient (either human or machine), or whether
> it "describes" the resource in any discernable way, is outside the scope of
> the HTTP spec and lies entirely in the domain of information publication and
> consumption -- i.e the social relationship between the publisher of the
> representation and consumers of the representation.
>
> A description is what you (should) get from a 200 response to a URIQA MGET
> request. It can be expected to correspond to a graph of RDF statements,
> serialized in some manner (RDF/XML by default) where the particular
> statements of interest are those in which the request URI occurs as the
> subject (though there can be other statements in the graph in which the
> subject does not correspond to the request URI). It is intended to be
> interpreted by the recipient (usually a machine) in terms of the RDF model
> theory.
>
> Pretty distinct to me.
>   
Hence, your definition of Description is RDF.  It is fine.  Then, Conneg 
does the same thing, doesn't it?  Wouldn't that make MGET a redundant 
effort?
> HTTP GET may return a serialization of an RDF graph.
> URIQA MGET always returns a serialization of an RDF graph.
>
> Note that a description, returned by URIQA MGET, is a specific subtype of
> representation, returned by HTTP GET, and it is certainly possible for
> representations of that description to be accessible via HTTP GET. So yes, a
> representation can certainly describe a resource. But not all
> representations accessible via HTTP GET will be as explicitly descriptive as
> an RDF graph. (sorry if that is confusing, reading it several times may be
> necessary ;-)
>
>
>
>   
>>>> 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.
>>>>
>>>>         
>>> Actually, it can't. As noted on http://sw.nokia.com/uriqa/URIQA.html:
>>>
>>>       
>> The link returns a 404, so I don't know if it suppose to return
>> something meaningful or it is a metaphor.
>>     
>
> Perhaps you are including the colon at the end, which is not part of the URI
> (sorry). I.e. try
>
> http://sw.nokia.com/uriqa/URIQA.html
>
>   
>>> --
>>> Why not use a MIME type and content negotiation to request a description?
>>>
>>> Content negotiation is designed to allow agents to select from among a set
>>> of alternate encodings. The distinction between a resource description and
>>> (other kind of) resource representations is not based on any distinction in
>>> encoding.
>>>       
>> Nope.  That is perhaps the intention that conneg is designed.  But I
>> don't think that is the way it should be understood.  Content-type might
>> be signal a special encoding, but language, for instance, is also part
>> of Conneg.
>>     
>
> That is true, and the wording is perhaps imperfect, but the point made is
> valid.
>
> Content negotiation is intended to provide access to alternative
> representations where the presumption is that those representations convey,
> as much as is possible given the limitations of their form of expression,
> the same essential body of information.
>
> You may wish to use content negotiation for something else, but it's
> original intended use, and actual use, is pretty well established.
> Exploiting it to do something else, is certainly possible, but not
> necessarily optimal as a generalized solution.
>
>   
>>> In fact, a given description (which is itself a resource) may have
>>> several available encodings (RDF/XML, XTM, N3, etc.). Thus, if you use
>>> content negotiation to indicate that you want a description, you can't use
>>> it to indicate the preferred encoding of the description (if/when other
>>> encodings than RDF/XML are available).
>>> --
>>>
>>>       
>> What is the implication of your statement. That RDF (or its sort) is
>> description but others are not?
>>     
>
> No. I didn't mean that at all.
>   
Hmm., now I don't know your definition again.  See above how you defined it.
>   
>> An HTML or XML doc definitely describes
>> somethings.
>>     
>
> As noted above, representations may correspond to descriptions, but may not
> be as explicitly or formally descriptive as a serialization of an RDF graph.
>
>   
>> If you URIDL  them to an RDF, it doesn't change the nature
>> of its content.
>>     
>
> One can represent a specific RDF graph in a number of different ways, and
> content negotiation can be effectively used as intended to request
> particular variant representations of that graph.
>
> If content negotiation is (mis)used to request an explicit description of a
> resource, then it is not available to request variant representations of
> that description (at least not without potentially doubling (or more) the
> number of MIME types).
>   
Why not?
>   
>>> Content negotiation can be used as intended in conjunction with URIQA to
>>> request particular variant encodings of a description.
>>>
>>>       
>> Again, the definition of "description"?
>>     
>
> See above.
>
>   
>>>> 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.
>>>>
>>>>         
>>> I consider on of the strengths of the semantic web layer is that it is
>>> agnostic about the syntactic structure of URIs. I also think that
>>> syntactically binding the URI of a resource and the URI(s) of its
>>> representation(s) or description(s) is necessary, and would be overly
>>> cumbersome in practice.
>>>
>>>       
>> Of course.  But anyone who words with the Web should know that the Web
>> is consisted of these three kinds of things.
>>     
>
> Anyone who is familiar with the standards which serve as the foundation for
> the web, and semantic web, knows what things are defined as relevant to
> software applications and the scope of those definitions.
>
> (granted, no spec or standard is perfect, but things are defined a lot more
> clearly and precisely than the definitions you seem to be assuming for these
> particular terms)
>
>
>   
>> Hence, giving these three
>> concept some syntactic sugar doesn't violate the URI's opacity
>> principle.
>>     
>
> I'm sorry, but that statement is self-contradicting. If the URI is opaque
> for a given application, then syntax is irrelevant, hence there cannot be
> any syntactic sugar which is meaningful to that application.
>   
No.  It is not but let's don't sidetrack this to other issues.  What I 
really want is a definition of "Description".  The only one that you 
give, but later seems rescinded, can and should be CN.

Xiaoshu
> Syntax which may be relevant to the web layer is irrelevant to the semantic
> web layer.
>
> The interface between the web and semantic web layers is a shared set of
> URIs with consistent denotation, and a means for semantic web agents to
> interact with representations of descriptions accessible via those URIs
> using web protocols.
>
> The web layer is concerned with representations of resources.
> The semantic web layer is concerned with descriptions of resources.
>
> A description of a resource is a kind of representation of that resource,
> but with a formal significance to the semantic web layer, and therefore it
> is optimal if semantic web agents can easily access those particular
> representations which correspond to descriptions, or from which descriptions
> can be extracted, where such descriptions can be interpreted as RDF graphs
> according to the RDF model theory.
>
> The less bandwidth or processing needed to obtain such descriptions the
> better.
>
> URIQA is designed to provide the most optimal access to explicit
> descriptions meaningful to semantic web agents with the lowest bandwidth and
> processing overhead possible and the least amount of specialized knowledge
> (nothing more than the URI and which method to use).
>
>   
>> When I say syntactic sugar, I mean that it is not absolutely
>> necessary.  But the benefit of defining it is for convenience in practice.
>>
>>     
>
> The sheer number of software applications which would need to be modified to
> consistently support such a special URI notation is staggering. URI opacity
> is one of the most important principles of the semantic web, for the very
> reason that it allows most software and content in the web layer to remain
> unchanged and agnostic, while enabling us to make explict statements about
> any resources denoted by any form of URI.
>
> Regards,
>
> Patrick
>
>
>   
>> Xiaoshu
>>     
>>> Patrick
>>>
>>>
>>>       
>>>> 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 15:27:04 GMT

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