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Re: XML Schema working with DTDs?

From: Dan Connolly <connolly@w3.org>
Date: Fri, 28 Apr 2000 20:44:40 -0500
Message-ID: <390A3E88.ADAA34D9@w3.org>
To: Noah_Mendelsohn@lotus.com
CC: cmsmcq@acm.org, ht@cogsci.ed.ac.uk, reagle@w3.org, www-xml-schema-comments@w3.org
Noah_Mendelsohn@lotus.com wrote:
> Dan, I think you're presuming answers to a versioning architecture for XML
> and namespaces.

I'm observing an answer. Not the only answer, but one that is
known to work (i.e. to avoid the problem Joseph ran into).

>  I believe that to be a known hard problem which, in spite
> of my suggestions to the contrary [1], the XML activity has so far
> declined to formally consider (I believe it was discussed informally at a
> CG meeting, perhaps in Montreal last year.)

Declined to consider versioning? Hardly!
Evolution of specs is one of W3C's core values.
cf "Web Architecture: Extensible Languages"

I guess it could depend on your definition of 'formal'. But...
I look hard at forward/backward compatibility
of all the specs, and I'm not the only one. We insisted on some
changes to XSLT (a way to make xslt:message act as a
halt-and-catch-fire instruction) for exactly this reason.

> Everything you propose, i.e. immutable namespaces, makes sense in
> isolation.  The problem I see is that none of the other necessary XML
> machinery has been developed.  Let's assume that some particular
> vocabulary undergoes within a year 20 minor modifications, mostly bug
> fixes,introducing little incompatibilities that are not of concern to the
> vast majority of users.  So, over the course of the year, 100,000
> documents are written to this vocabulary, 5,000 in each of the 20
> namespaces.  Question: how do I build and maintain 30 XSL stylesheets that
> do the right thing with these documents?

I think you've answered your own question. You just do. It's
hard and awkward, but clearly it's possible.

The answer I'm talking about meets some requirements (namely,
that you can write a document and be assured that it will
be interpreted consistently henceforth) but doesn't meet
others, i.e. easy maintenance of stylesheets.

>  For the sake of discussion, none
> of the stylesheets happen to make use of any of the features that were
> affected by the 20 bug fixes.  Were it not for the decision to make
> namespaces immutable, a single set of 30 stylesheets would suffice, and
> none of the 30 would have required change through the year.  Presuming
> immutable namespaces, which do indeed have many desirable architectural
> properties, I either need 600 stylesheets (30 useful sheets x 20
> namespaces used in the instances), or some rather messy disjunctions in
> each of my XPaths.


> I do not propose that we go into an extensive discussion of versioning
> here.  I merely wish to agree with Henry that the answers are far from
> clear, and in that sense we are feeling our way.

I agree that the whole general problem of language evolution is
messy. But I maintain that there's one mechanism, immutable
resources, that's known to avoid the problem Joseph ran into.

>  I think we are far from
> having worked out the practical ramifications of any particular fixed
> design for versioning, including any that might be based on immutable
> namespaces.  It is my opinion that almost anything practical we do for
> robust versioning of XML vocabularies will require some serious
> engineering in one or another of our existing XML specifications (e.g.
> XPath, if you believe the analysis above).  Pending such developments, I
> think we in the schemas group will have to make decisions that are
> somewhat ad hoc at times, perhaps republishing minor fixes as changes to
> the same namespace, with some means of deploying new ones for major
> changes.

Sure... I just disagree that henry's approach of "we'll put the
old one someplace that you can find it" is very useful.

>  In short, I think we are about to get bitten by an overall lack
> of investment in figuring out how to do namespace and vocabulary
> versioning in a robust manner.  Maybe I am just being too pessimistic.

If we had to design all parts of the Web before we deployed anything,
where would we be? I guess I have a little more faith that economical
solutions will present themselves in a timely fashion ;-)


Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Friday, 28 April 2000 21:45:05 UTC

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