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   Re: [xml-dev] Re: Flexible Schemas (was RE: [xml-dev] The task to be sol

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----- Original Message -----
From: "Elliotte Rusty Harold" <elharo@metalab.unc.edu>
To: <xml-dev@lists.xml.org>
Sent: Saturday, January 19, 2002 11:35 PM
Subject: Re: [xml-dev] Re: Flexible Schemas (was RE: [xml-dev] The task to
be solved by RDDL)

> At 4:57 PM -0800 1/19/02, Dare Obasanjo wrote:
> >I believe the thrust of Nicolas' question is where to find the RDDL
> >for an XML file that utilizes names from multiple namespaces. No one has
> >proposed a *concrete* answer to that question yet although the designers
> >RDDL may consider this to be beyond the design goals of RDDL which is
> >reasonable.
> >
> There is no such document. One more time: A RDDL document describes a
> namespace name, not a document.

Well, I said I wouldn't reply to each message, but... One more time : WE
KNOW that a RDDL document only describes a namespace. What we are trying to
explain from the beginning is that this is not sufficient, and that in its
examples RDDL try to make us think that RDDL could be used to automatically
fetch schemas for unknown XML document types. Hence the 'silver-bullet'
effect, and the enthusiasm of people about RDDL. If people weren't made to
think that describing a namespace would help them to describe document
types, I'm sure they would not be so enthusiastic about RDDL.

Not that I'm jealous or anything ;). It is just that while the hype grows on
RDDL, we are losing precious time that could be dedicated to the problem of
describing document type. I think that describing document types should be
addressed before describing namespaces, not the contrary.


> >Such as obtaining the W3C XML schema for validating this document or
> >all the normative references that apply to the document.
> It may not be possible to do this. RDDL makes no guarantees about
> which resources exist for any given namespace. It simply provides a
> convenient way to list those resources that do exist. However, you
> could try this, which would work provided all the namespace
> maintainers have put RDDL documents locating schemas for their
> namespace at the namespace URLs:
> 1. Start at the root element of the document.
> 2. Load the RDDL document for its namespace. Search that document for
> the rddl:resource element whose xlink:role and xlink:arcrole
> attributes indicate a schema of the type you want.
> 3. Use the xlink:href attribute of that rddl:resource element to
> locate the URL for the schema that applies to the root element.
> 4. Apply that schema to the root element.

This won't work for http://www.rddl.org/. You'll try to validate the
document against the XHTML schemas (since the root element is from the XHTML
namespace), and you'll get a validation error as soon as the validator will
encounter RDDL elements. Please forget about trying to find an algorithm.
There is none. You summed it up perfectly : RDDL is for namespaces only. It
won't work for document types. So let's try to solve the problem of document
types together.

> At this point with some schema languages, e.g. DTDs, we're
> effectively done, because validity is equivalent to validity of the
> root element. However, for those schema languages where this is not
> the case, recursively apply the algorithm to all the child elements
> of the root element (and their children, and their children's
> children, and so forth)

If I only load the RDDL document matching the root element's namespace, I'll
validate the document against the XHTML DTD. Missed. If I load all the RDDL
documents of all namespaces that I can find, I'll have two DTDs with the
same root element : the XHTML DTD, and the RDDL DTD. Which one do I use ?
Once again, namespaces are not a substitute to document types, neither is
the QName of the root element of a document.

> This all assumes that the necessary RDDL documents are in the right
> place and have the right rddl:resource elements that point to schemas
> written in the desired schema language; that these schema documents
> are syntactically correct, that the network hasn't gone down, and a
> dozen other constraints, none of which RDDL addresses, but that's OK.
> RDDL, like XML, is an enabling technology. It allows namespace
> purveyors to implement this scheme if they choose. It does not
> mandate that they do so.
> Liek XML itself, we can't force people to use RDDL. Also like XML, if
> a lot of people are doing similar tasks, then they'd probably rather
> use RDDL and standard tools than reinvent their own solutions.

I hope you have understood that I'm OK with RDDL for namespace description
(apart a few critics). But it is only provided that RDDL is not used for
what is not meant, i.e. a substitute for a resource directory for document

Best regards,

> +-----------------------+------------------------+-------------------+
> | Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
> +-----------------------+------------------------+-------------------+
> |          The XML Bible, 2nd Edition (Hungry Minds, 2001)           |
> |              http://www.ibiblio.org/xml/books/bible2/              |
> |   http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/   |
> +----------------------------------+---------------------------------+
> |  Read Cafe au Lait for Java News:  http://www.cafeaulait.org/      |
> |  Read Cafe con Leche for XML News: http://www.ibiblio.org/xml/     |
> +----------------------------------+---------------------------------+
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