W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: resources and URIs

From: pat hayes <phayes@ihmc.us>
Date: Fri, 18 Jul 2003 13:13:18 -0500
Message-Id: <p0600122dbb3dd010219f@[]>
To: Dan Connolly <connolly@w3.org>
Cc: www-tag@w3.org

>On Tue, 2003-07-15 at 18:20, pat hayes wrote:
>>  Gentlemen, I would like to ask you to please clarify the meaning of
>>  the terms 'resource' and 'representation' in
>>  http://www.w3.org/TR/2003/WD-webarch-20030627/.
>It seems to me that your request is pretty much a request
>to resolve the httpRange-14 issue.
>   http://www.w3.org/2001/tag/ilist#httpRange-14

Well, it is relevant to that issue, but it was more of an editorial 
request for clarification of meanings of the text.

>We would sure like to. We've been working on it, slowly,
>since July 2002 (and who knows how long related
>discussions that predate the TAG issue have been going on).
>We tried ignoring it and hoping
>it would go away for a while (Sep 2002 to Feb 2003)
>but that didn't work, so we put it back on the table.
>But we haven't made much progress on it since.
>That said, a few comments from my perspective on the issue...
>>  Allow me to elaborate.  Your introductory example asserts the
>>  following:
>>  "Objects in the networked information system called resources are
>>  identified by Uniform Resource Identifiers ( URIs ). "
>>  and later the document says:
>>  "URIs identify resources. When a representation of one resource refers
>>  to another resource with a URI, a link is formed between the two
>>  resources. The networked information system is built of linked
>>  resources, and the large-scale effect is a shared information space.
>>  The value of the Web grows exponentially as a function of the number
>>  of linked resources (the "network effect").  "
>>  These, and other pieces of text concerning 'resources' published by
>>  other W3C authorities,  seem to clearly indicate that the word
>>  "resource" is intended to refer to the entities *in* the networked
>>  information system: they are the kind of thing we use words like
>>  'website', 'client' and 'server' to describe; they are things with a
>>  computational state, things with which one can communicate, things
>>  which send and receive information which can be transmitted along
>>  optical fibers and twisted pairs, things than can be linked to one
>>  another.
>By 'resource' I mean not only those, but also lots of other

I know that you do, Dan: I got my own understanding of this sense of 
'resource' from you.  But I don't see how to reconcile that 'broad' 
sense of "resource" with what is said in the document (sparse though 
it is) about "resources". Hence my request for clarification, and my 
attempt to pick out what seem to be the inconsistencies in the 
current text.

>  > So far this is clear; and the account of 'representation' given in the
>>  document is also then reasonably clear:
>>  "Agents (such as servers, browsers and multimedia players) communicate
>>  resource state through a non-exclusive set of data formats, used
>>  separately or in combination (e.g., XHTML, CSS, PNG, XLink, RDF/XML,
>>  SVG, SMIL animation). In the travel scenario, Dan's user agent uses
>>  the URI to request a representation of the identified resource. In
>>  this scenario, the representation consists of XHTML with embedded
>>  weather maps in SVG. "
>>  On this picture, the information (which Dan, in your introductory
>>  example, reads on his screen, and which is in some sense all about the
>>  weather in Oaxaca) is a representation of the (current state of) some
>>  entity *in the WWW itself*: a resource in the global information
>>  network: the state of some computer system, or maybe some abstraction
>>  of a computer system.
>>  However, it is also clear that neither the weather in Oaxala, nor
>>  Oaxala itself, are entities of this kind:
>it is?

Yes, it is. That is, the category of things that can be actually "on" 
an information network in the sense of being physically connected to 
it does not encompass things like regions of Mexico, or weather.

>  >   weather and cities in Mexico are not the kind of entities which can
>>  be thought of as 'objects on the networked information system'.
>Yes, they are; I think of them that way.

OK, then you are apparently able to think in a way that is not 
accessible to me. Could you elucidate this way of thinking, at all? 
For example, is there *anything* that, in your way of thinking, could 
not be part of an information network? Is there any distinction, in 
your way of thinking, between being in some sense physically attached 
to an information system, being an entity transferred over an 
information network, and being referred to by a symbolic expression 
stored in an information system?

>  >  Other examples abound,
>>  eghttp://chandra.harvard.edu/photo/2003/ngc1068/index.html  is in
>>  clearly about a galaxy containing a supermassive black hole, which is
>>  also not something one would expect to find as part of an networked
>>  information system, given the likely physical constraints on network
>>  architecture.
>I think that particular identifier refers to a document about
>a galaxy, not the galaxy itself; if you want to refer to
>the galaxy itself, you should use a URI with a # in it.

I know this general rule: it makes some sense of the usage which has 
emerged for the SW languages. But you are talking about *referring 
to*; whereas my point was about being *part of* a network. I agree 
that a URI reference might well refer to a galaxy: but - and this was 
my point in sending the message - that doesn't seem to be anything 
like the same claim that a galaxy is *part of* an information network 
or (to refer for a moment to the language used in RFC 2396) that by 
having a URI reference which refers to the galaxy one is thereby 
enabled to *perform operations on* the galaxy.

>[folks with other opinions on httpRange-14 disagree,
>I believe.]
>>  It seems that there is a systematic ambiguity between two senses of
>>  'resource' (or maybe two senses of 'representation') here. In your
>>  first example, I doubt very much that Dan, when looking at his screen
>>  after telling his browser to retrieve
>>  http://weather.example.com/oaxaca, thinks of what he is reading as in
>>  any sense about the state of something on the WW information network.
>If 'Dan' refers to me, I can say with authority that I do.

I was using the Dan mentioned in the example, but that may be you for 
all I know :-).

Again, if you really do think this way then I am unable to comprehend 
how you think.  The weather is part of the Web?? What can that 
possibly mean? How would weather fit into the REST model?  What Web 
protocols can I use to retrieve pieces of weather?

>In particular, I think http://weather.example.com/oaxaca refers
>to a document about the weather, not the weather itself.
>But http://weather.example.com/oaxaca#weather might refer
>to the weather itself.
>>   Certainly if I were in his shoes, I would be reading it as being
>>  about Oaxala and weather: that is why he is reading it, presumably: to
>>  find out something about the weather in Oaxala.  So what this
>>  representation is *about* is not, apparently a resource:
>In this particular case, the representation is a representation
>of a document about the weather.

Ah, OK. But that isn't what you said above. If its a representation 
of a document then the DOCUMENT is the resource (I presume?), and its 
the document that is 'part of' the information network. I have no 
problem with that; but that is not the same as saying that the 
weather described by the document in on the Web.

>  >  so it is not a representation of a resource, in the usual sense of
>>  'representation' and what is apparently your sense of 'resource'.
>>  Similarly,http://chandra.harvard.edu/photo/2003/ngc1068/index.html
>>  sure reads to me like it is about NGC 1068. But this means that either
>>  it is a 'representation' which is not about what it is 'of', or else
>>  that NGC 1086 is an 'object in the networked information system';
>>  neither of which seem to me to be remotely plausible as factual claims
>>  using the ordinary senses of the words, and kind of brain-damaged as
>>  attempts at a formal definition of some kind of architectural/semantic
>>  theory.
>No, it's pretty plain:
>The URI http://chandra.harvard.edu/photo/2003/ngc1068/index.html
>refers to (i.e. denotes in many/most interpretations)
>a document about a galaxy.

Again, I have no problem with that. If that is what the TAG 
architecture document said then I would not be asking for 

>  > Now, this could be just a matter of philosophical opinion, were it not
>>  for the fact that semantic web languages like RDF and OWL have been
>>  given *formal* semantic theories which have direct architectural
>>  consequences for Web agents, and which depend crucially on notions
>>  like the term 'about' I have used rather loosely above.  RDF uses URI
>>  references as *names* to *refer* to entities. So if a web page such
>>  ashttp://chandra.harvard.edu/photo/2003/ngc1068/index.html were to
>>  include RDF markup, one might expect to find things like this in it:
>>  <rdf:Description
>>  rdf:about="http://chandra.harvard.edu/NGC/ngc1068"
>>  rdf:type="http://chandra.harvard.edu/AOtype/Activegalaxy7"
>>  </rdf:Description>
>That would be... wierd.

But would it violate the spec? Surely that is the point.

><http://chandra.harvard.edu/NGC/ngc1068> foaf:topic
>   [ rdf:type [
>     is foaf:topic of
>       <http://chandra.harvard.edu/AOtype/Activegalaxy7> ]].
>using foaf:topic (cf http://xmlns.com/foaf/0.1/)
>which is kinda like containsInformationAbout-Focally
>in the cyc vocabulary.

Well, THIS seems weird to me. If a galaxy can be a resource and a URI 
can (must?) indicate a resource, then why can't I use a URI to 
indicate a galaxy? Why do I need to get all tangled up in subjects 
and topics and all this stuff? I'm an astronomer, not a linguist, in 
this example.

But more to the point, suppose I just DO use URIs in this way, for 
reasons I'm not obliged to tell you about.  Even if it does violate 
some stylistic rules and cause y'all to stretch your eyes, will my 
website still actually *work* ? And if it fails to work, why? What 
W3C specifications have I violated or misunderstood?

>In full RDF/XML glory, that's...
><rdf:RDF xmlns:foaf="http://xmlns.com/foaf/0.1/"
>     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
>     <rdf:Description 
>         <foaf:topic rdf:nodeID="_g1"/>
>     </rdf:Description>
>     <rdf:Description rdf:nodeID="_g0">
>         <rdf:type rdf:nodeID="_g1"/>
>     </rdf:Description>
>     <rdf:Description rdf:about="http://chandra.harvard.edu/NGC/ngc1068">
>         <foaf:topic rdf:nodeID="_g0"/>
>     </rdf:Description>

Whats with all the blank nodes?? Just to say that a galaxy is in a class??

>  > where the URIs refer respectively to a galaxy and an RDFS class of
>>  galaxy types. This is completely incompatible with what your document
>>  says about resources and representations.
>No, it's just a wierd way to model it... you're saying that
>a galaxy is something you can get at with the HTTP

Ah!!  Good, at last. So (contrary to what you say above and the TAG 
architecture description together imply) I am NOT allowed to use URIs 
to denote or refer to anything at all: I have to use them to *refer 
to* the things that can be *retrieved* by a suitable protocol, 
specified by the URI prefix.  Things that really are *on* the 
information network, in fact, in the sense that they can be emitted 
by something which is communicated with by a transfer protocol. I'm 
quite happy with that, but I havn't seen that stated ANYWHERE until 

>You don't have to do that.
>>    Using the URI in this way does not create any kind of link between
>>  anything on this planet and NGC 1086 (which is, fortunately, about 50
>>  million light-years away).  But RDF/RDFS/DAML/OWL/OIL and all the
>>  other emerging Semantic Web formalisms *require* that URIs be used in
>>  this way,
>nope. See counterexample above.

I said URIs, not URIrefs.  Your workaround is obviously unworkable as 
a general technique.

>>   as *referring expressions*, not as informational links in a global
>  > architecture.
>>  The RDF/RDFS/OWL semantics assumes that URI references refer to
>>  "resources" , but it explicitly denies that this word "resource" is
>>  limited to the kinds of resource that you seem to be talking about. On
>>  the SW view, *anything* is a resource: galaxies, regions of France,
>  > kinds of wine, sodium atoms, classes, mathematical abstractions, even
>>  fictional entities: anything that can be referred to by a name
>Yup. And I'm pretty sure nothing in the webarch doc says otherwise.

Apprently not, with your remarkable powers of interpretation. They do 
say otherwise in my reading, however. Hence, please can we have some 

>>  . None of these can possibly be "objects in a networked information
>>  system".


>>    So whatever you are talking about, and whatever they are talking
>>  about, y'all cannot possibly be using the words "resource" and
>>  "representation" in the same sense.
>>  As a result, several of the assertions you make in this document are
>>  not correct. For example
>>  2.8.2
>>  "merging Semantic Web technologies, including "DAML+OIL" [ DAMLOIL ]
>>  and "Web Ontology Language (OWL)" [OWL10 ], define RDF properties such
>>  asequivalentToandFunctionalProperty to state -- or at least claim --
>>  formally that two URIs identify the same resource. "
>>  is incorrect.
>I don't think so.
>>   These assertions claim that two URI references *denote* the same
>>  entity in all interpretations.
>>   That is not the same notion as 'identify'.
>Well, it's pretty close... try taking 'identify' to mean
>"denote in many/most useful interpretations".

It can't possibly mean that and still have the TAG document (or RFC 
2396) make sense.

>The a claim that they denote the same entity in all
>interpretations is also a claim that they denote
>the same thing in many/most useful interpretations
>(though it doesn't work the other way around).
>>  In fact, there is no such notion as 'identify' in RDF/RDFS/OWL
>>  semantics; and the first principle in section 2 ("All important
>>  resources SHOULD be identified by a URI ") is meaningless when taken
>>  literally in the context of semantic web languages, as URIs there
>>  typically cannot be said to identify anything: they act as names whose
>>  possible referents are constrained by the assertions made using them,
>>  but they are not 'linked' to anything, not 'bound' to anything, and
>>  are not obliged to 'identify' anything; and the universes of discourse
>>  may contain entities which cannot possibly be all identified or even
>>  referred to by URIs, since there are too many of them, or it is
>>  physically impossible to identify them with enough precision, or
>>  simply because it is impractical to do so.
>>  ------
>>  Sorry this comes across so negatively, but there seems to be a central
>>  misunderstanding right at the center of several architectural accounts
>>  of the Web, and I think it is important to get it sorted out.
>[... I don't see any discussion of the text in the architecture
>document below here...]
>I agree that talking about formal systems and distributed
>systems in the same document is tricky... my intuition
>is that formally, the same URI can identify different
>things in different interpretations. But there's
>a sort of economic/architectural benefit to having
>lots of interpretations agree.

Sure, I agree. BUt the issue Im naggling at isn't really to do  with 
this issue, its more to do with the distinction between referring 
(denoting, naming) on the one hand and being part of an information 
network, providing access to, specifying a route to a source of 
information about, etc., on the other hand. Those are completely 
different notions, but the architecture documents try to conflate 
them, and only manage to get confused, because in one place they say 
things that make perfect sense for one notion but are crazy when read 
relative to the other.

>Or something... I'm
>still trying to figure out how to say it.

OK, maybe I'll wait until there is a text to naggle at.


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Received on Friday, 18 July 2003 14:13:22 UTC

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