W3C home > Mailing lists > Public > www-rdf-interest@w3.org > December 2001

Re: Namespaces wihtout "#" Was: Few CWM Bugs

From: Tim Berners-Lee <timbl@w3.org>
Date: Sun, 9 Dec 2001 23:06:36 -0500
Message-ID: <025c01c18130$1018d240$0301a8c0@w3.org>
To: "Dan Brickley" <danbri@w3.org>
Cc: "Sean B. Palmer" <sean@mysterylights.com>, "Dan Connolly" <connolly@w3.org>, <www-rdf-interest@w3.org>

----- Original Message -----
From: "Dan Brickley" <danbri@w3.org>
To: "Tim Berners-Lee" <timbl@w3.org>
Cc: "Sean B. Palmer" <sean@mysterylights.com>; "Dan Connolly"
<connolly@w3.org>; <www-rdf-interest@w3.org>
Sent: Sunday, November 25, 2001 5:52 AM
Subject: Re: Namespaces wihtout "#" Was: Few CWM Bugs


>
> On Sat, 24 Nov 2001, Tim Berners-Lee wrote:
>
> > The second issue is more significant.   In my worldview,
> > (which I claim to be (a) consistent and (b) useful)
> > http://example.org/x is a document.  You can't reuse
> > its URI for an abstract thing without a change to HTTP.
>
> In-principle plausible, although _please_ define "document".

I uyse the term "document" because unfortunately "resource" has been
used differently in URI and RDF specs.   I mean by "document"
"resource" as in URI.   DAML uses the term "Thing" to mean what RDF
terms a resource.

A document is the abstarct thing corresponding to an HTTP URI.
It can be represented by a pair of a Content-Type and set of bits
in the entity body. A document may have >1 valid representation.
This all is defined in detail by eth HTTP spec.

When the content-type is RDF or N3, then a document can be used
to describe people and planes and ideas.  These can be identified
(in N3) by using the localname of concept within the document
as a fragment identifier.  (I think the same should be true of RDF/XML).

>.The distinction
> is only useful if it can be defined clearly enough to implement to.

Well,  you certianly can't return a person across the net, so the
distinction is
not that fine ;-)

> Is a Web
> Service a document? (CGI-based, SOAP etc)

Does it have a URI and a represnetation in binary?
I think tha web services which operate for example with POSTS should
also respond to GET, and so they should be a kind of document.
But it isn't obvious.

> An image?

Yes.

>What about a mailbox
> (eg. could http: URIs name the same things as mailto: URIs)?

> What about
> Shakespeare's "Hamlet", abstractly conceived, is that a document?

If someone coudl publish it, yes .. but i think probably no one would
maintain they were.

> Could it
> have an http: URI scheme? A "farenheit-to-centigrade conversion service"?
> What about a category of thing (eg. an RDFS class) or relationship? Are
> these sufficiently document-like? If not, whose definition do they fail?

Here is a test.  If the HTTP server responded with a 200 result and gave a
last modified date for it, would you consider the last modified date applied
to the thing in question?

Generally, you would say - no, the document retsurned is a schema, not a
Class,
even if the Class were described in the schema.

> How does an rdfs:Class like wordnet:Logo differ from the notion of 'the
> W3C logo, distinct from its concrete fileformat representations'. The
> latter has a http: URI (we can use it in web browsers).

Yes. browsers present documents. they can present the document in
a variety of ways.  It works.  It is a generic URI.  Don't confuse
genericity
(http://www.w3.org/DesignIsues/Generic) from instanciation. (sp?)


> What makes that a
> document, but not http://xmlns.com/wordnet/1.6/Logo ?

http://xmlns.com/wordnet/1.6/Logo  is an RDF document
(I know becaus emy browser displays it as XML source which is what it does
for anything in XML, and the outermost element of the XML is in the
web:RDF xmlns:web="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
namespace).  It is a very confusing document,
because it asserts that it itself is a subclass of various things,
which implies that it is a Class. So it must be a document and a class.
So I have from HTTP its last modified date, and from itself (if I were
to beleive it) a superclass heirarchy.

> Or rather, why can't
> documents have instances/members?

A document, in the semantic web, has a close relation to a forumla
in a semantic web lnguage.  (In cwm, its the log:semantics relation.)
Now, you can invent a language in which a formula can have instances,
but I think you would have to explain what it all meant, as I've never seen
it done before.
A forumla can have members in the sense that it has the RDF
statements which compose it.

> Instinctively, some notion of "bits not atoms" seems to be what we're
> after here, although http:-named services screw things up somewhat, as
> does the notion of an abstract work distinct from its concrete (paper,
> file formats etc) representation. In practice, I'm not sure what value
> their is for pursuing the distinction: it is a very high level, broad
> brush distinction. Maybe it is useful because whenever we see a resource
> whose name begins 'http:' we can conclude "that's not the sort of thing
> that can have a size-in-pounds"? (although books, an old fashioned kind of
> a document, have such a property...).
>
> There seems to be a high cost (upper ontologising is rarely fun) and
> dubious benefits (what additional inferences can we draw?) to saying that
> 'http:' URIs can only name the members of some as-yet-illdefined class.
>
> Flipping things around, here's a practical reason that I chose to use a
> '/' instead of a '#' as the final character of the URI that names my
> Wordnet-to-RDFS-class namespace:
>
> If I'd used http://xmlns.com/wordnet/1.6# instead of
> http://xmlns.com/wordnet/1.6/ then the URI reference for a class in that
> namespace would be something like: http://xmlns.com/wordnet/1.6#Image
> instead of http://xmlns.com/wordnet/1.6/Image
>
> This affects deployability in a very practical way. The Wordnet RDF Schema
> has 50,000 classes. If one attempts to dereference
> http://xmlns.com/wordnet/1.6#Image using good old fashioned HTTP/1.1, the
> actual request you do is:
>
> (remembering that #Image gets stripped off before HTTP requests are made)
>
> GET http://xmlns.com/wordnet/1.6 HTTP/1.1
>
> ...which clearly doesn't pass enough information up to my server to give
> it any useful information about wordnet. So it can't do what I currently
> do, which is return a description of the relevant portion of the class
> hierarchy. I guess it could return a dump of the _entire_ schema (huge),
> or some (as yet undefined) manifest file format, or SOAP connection
> details for talking to the server's SOAP query interface. But it can't do
> the most useful thing, which is return a description focussed on the class
> we're interested in.
>
> So, I deployed a namespace ending in '/' to see if this was a useful,
> feasible thing to do. Apart from the accusation that I've made a category
> mistake (by assuming that at least some classes and properties can be
> thought of as http:-namable 'documents') I've not run into any practical
> problems.
>
> Dan
>
>
> --
> mailto:danbri@w3.org
> http://www.w3.org/People/DanBri/
>
Received on Sunday, 9 December 2001 23:02:07 GMT

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