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Re: RDDL and XML Schema instances are not valid representations of namespaces

From: Sandro Hawke <sandro@w3.org>
Date: Thu, 30 Jan 2003 14:46:19 -0500
Message-Id: <200301301946.h0UJkJc28531@wadimousa.hawke.org>
To: Patrick.Stickler@nokia.com
cc: www-tag@w3.org

> If an HTTP GET returns a representation of a resource, and RDDL or
> XML Schema instances are considered valid representations of an
> XML Namespace, then I see no useful value to the concept of 
> representation, since there apparently are no bounds as to what it
> might be, and very well might be random.


Each URI string can be used to point to several different things.  In
thinking about what a URI string points to, while working with RDF or
namespaces, I find it useful to ask:

   1.  What knowledge base might it be pointing to (if any)?
       For every successful GET, over time, will I get a
       serialization of the information in that knowledge base at that
       time?  If GET doesn't get me anything, or what it gets can't be
       thoughts of as the contents of a knowledge base, then the URI
       is not identifying a knowledge base.  

   2.  What subject (as in topic maps) might the URI be pointing to
       (if any)? Does it seem like the text or pictures returned via
       GET are conveying information about some one thing, every time,
       all the time?  Alternatively, has the URI's owning authority
       made it clear in some other way what the URI identifies?

This isn't too different from how I think about URIs in general.  In
writing HTML or talking to people, I mostly use URIs to point to
reliable, authoritative sources of information.  Often that
information is about some particular subject (like a book, e-mail
message, or world event), but I still have to pick a good URI for that
subject.  I do so based on the qualities of the information source.
But humans jump quickly to the subject, so when I say "look at
http://yyy" where that URI points them at a news story about a virus,
we'll often talk directly about the virus (with no need to focus on
the news story itself).  

Still, if I say "check out http://yyy", and it's a web page about a
book, you might wonder if it's the web page that's interesting, the
book that's interesting, or even the subject of the book that's
interesting.  I try to straighten this out in RDF by making the
URI-to-whatever mapping explicit and very well documented.  

The problem in RDF is when people use URIs directly as node labels [as
almost every does] because then it can be very hard to tell which
mapping (which kind of pointing) they had in mind.  TimBL is the main
force here arguing for what mapping everyone should have in mind, but
with the WGs sitting out on this issue, consensus seems unlikely, and
URIs in RDF will continue to be only marginally better signifiers than
English words.

Back to XML: XML Namespace Names are URI strings for which sense #2
always holds; to me they always identify an XML Namespace[1].  They
may also work in sense #1, where the identified knowledge base is the
collection of information, which you talked about, about schemas,
tools, etc for working with XML documents using the namespace.

    -- sandro

[1] But what is an XML Namespace?  It's often described as a
    collection of strings, but I find that insufficient.   Two
    namespaces which conceptually have exactly the same strings in
    them still may have different semantics and so are different
    namespaces.  I think of it more like a mapping to absolute XML
    lexemes/tokens, which have their own semantics in the overall
    mixed-namespace XML language.  But that's another issue.
Received on Thursday, 30 January 2003 14:46:41 UTC

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