- From: Xiaoshu Wang <wangxiao@musc.edu>
- Date: Wed, 25 Feb 2009 15:26:07 +0000
- 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 UTC