- From: Brian McBride <bwm@hplb.hpl.hp.com>
- Date: Sat, 16 Mar 2002 13:23:38 +0000
- To: Norman Walsh <Norman.Walsh@sun.com>, www-tag@w3.org, skw@hplb.hpl.hp.com
At 12:57 15/03/2002 -0500, Norman Walsh wrote: >At a recent telcon, Stuart Williams and I agreed to publish our one >page summary of section 2 of the architecture document this week. We >are aware of a few comments that have not been addressed yet, and I >expect this publication will generate a whole lot more, so please >remember that this is a work in progress. (In fact, discussion of this >document is on the agenda for the *next* TAG meeting, so this document >cannot even purport to represent the consensus of the TAG :-). > > http://www.w3.org/2001/tag/doc/identify.html > > Be seeing you, > norm I'm really glad to see the tag taking a look at this issue. RDFCore has some dependencies on the outcome, so I hope to follow this discussion with interest. Some comments: [[ 2 What Does a URI Identify? On the web URIs identify resources."Any information that can be named can be a resource." [RFC2396]. In fact, this relationship can be taken as axiomatic: if a resource has a URI, it is identifiable on the web. If it does not, it is not. ]] I cannot find the quoted text ("Any information ...") in RFC 2396. [[ 2.2 Resources ... The set of values mapped by a resource are equivalent resource representations and/or resource identifiers (giving further indirection or redirection). Dereferencing a resource identifier yields a representation of the current value of the referenced resource. At some time, t, the set of values that a resource maps to may be empty, which allows a concept to be identified before a realisation of the concept exists (or indeed after it has been retired). ]] What notion of equivalence is meant here. How can I determine whether two values are equivalent? This para talks about "the current value". Is this the same sense of the term 'value' used in "At some time, t, the set of values that ..." or is there some notion that a resource has state, and it is the value of that state that is referred to? [[ RDF provides the ability to described resources by their relationship to one another which leads to the notion of existentally qualified resources. For example, there exists a person whose internet mailbox is identified by the URI mailto:timbl@w3.org. This identifies the person of Tim Berners-Lee by reference to the URI of his internet mailbox without it being necessary to assign a URI to identify the concept of the person Tim Berners-Lee. ]] It is not the resource that is existentially qualified. RDF has the notion of a b-node which performs a role similar to that of existentially qualified variables in first order logic. Just as in: x + 3 = 4 x is not the number 1, x is a variable, so b-nodes in RDF are not resources, they are variables. Any of the values a b-node can take can be assigned a URI. [[ 2.3 Properties of Resources ... Two different URI's may identify the same resource, but it is only the authorities that asssign those URIs that can make the commitment to them identifying the same resource. ]] Can they? Is that a proposal? The alternative notion, is that each different URI denotes a different resource, and to define a notion of equivalence between resources. Different notions of equivalence are possible; resources A and B denote the same set of values at time t, for a set of time intervals {[t1,t2]} or over all time. Consider for example, http://www.w3.org/. This web page is mirrored; I don't know what the url's of the mirrors are; lets say http://www.w3.inria.fr/ is one for the purpose of discussion. There is presumably a propagation delay between updating the master version and that change propagating, so there is a period when an HTTP GET on the two different URL's will return different values. Does this mean that these two URI's denote different resources, or is it that the implementation is an imperfect realization of the ideal. More importantly, how can we know that these two URL's will always denote the same set of values. We cannot predict the future. The French government could choose next month, to require that all web pages served from French web servers contain some metadata which depends on the origin of the page. How can we say today, that two URL's will, for all time, denote the same mapping to values. [[ We are dealing here with two time dependent mappings. Firstly a time dependent mapping between and identifier and a resource ... ]]] Oh that's horrible! Later in the document it states: [[ An absolute URI always means the same thing, regardless of the context in which it occurs. ]] and [[ The resource identified by a particular URI should always be "the same", when it is identified by that URI. ]] That seems a little contradictory. [[3.1 What about Fragment Identifiers? If a URI contains an sharp character (a " # "), the string that follows the " # " is a fragment identifier. Fragment identifiers are a mechanism for identifying part of a resource. ]] Are resources atomic, or can the parts of a resource also be resources? [[ This means that in general, it's not possible to determine what a fragment identifier means without retreiving the resource into which it points. ]] This sentence uses the term 'means' which is rather ill defined here. If this sentence is trying to say that it is not possible to determine the bytes which represent the fragment without retrieving a representation of the whole resource, then that is true given current web practise. But if that is the sense in which the word 'means' is used here, then it is also not possible to determine what http://www.w3.org/ *means* without retrieving it. [[ The fragment identifier identifies some sub-part of a resource representation. ]] I don't follow this. Consider the resource identified by http://example.org/doc/. Consider that there are two representations of this resource, one in say xhtml and the other in svg, and that each contains a fragment '#chapter1'. Can we not say that http://example.org/doc/#chapter1 names chapter one of the document? Can we not say that http://example.org/doc/#chapter1 names a resource, and that to display that resource a browser has to retrieve the resource http://example.org/doc/ and then interpret the value returned in a way that is dependent on the mimetype to compute the representation of chapter 1. The fact that computing the representation of a fragment is mimetype dependent, does not mean that a URI with a fragment identifier cannot name an abstraction which has multiple representations with different mime-types. [[ A URI that consists of only a fragment identifier (i.e, one that begins with a " # ") always points into the document that contains the URI, irrespective of the effective base URI. ]] This statement is presumably based on RFC 2396: [[ 4.2. Same-document References A URI reference that does not contain a URI is a reference to the current document. In other words, an empty URI reference within a document is interpreted as a reference to the start of that document, and a reference containing only a fragment identifier is a reference to the identified fragment of that document. ]] However, there is an escape clause. The same paragraph goes on to say: [[ 4.2. Same-document References [...] However, if the URI reference occurs in a context that is always intended to result in a new request, as in the case of HTML's FORM element, then an empty URI reference represents the base URI of the current document and should be replaced by that URI when transformed into a request. ]] Brian
Received on Saturday, 16 March 2002 08:25:41 UTC