X-Schema concerns

Erik Freed ejfreed at infocanvas.com
Wed Jun 16 18:18:30 BST 1999

The latest arguments regarding the usefulness of XML schema languages versus
DTDs are the most interesting. I have long used what are sometimes called
'reflective' object models, where the 'meta-data' can be thought of as a
specialization of 'data'. This unified approach in my experience is more
than a conceptual nicety, it provides long term implementation advantages. A
single model for creation, storage, queries, fetches, modification,
browsing, editing, translating, etc means that your large scale mature
system is simpler to understand and has much less implementation code in it.
For instance, the reflective system I have most recently constructed, has a
browsing tool, where with only one tree view model, can browse through
meta-data and data 'boundaries'. This unified approach, I find, pays off
immediately in very practical terms.

So while programming languages have long had a 'programming' language to
handle meta-data constructs and a 'run-time' environment to handle data
constructs, this separation is in fact an implementation choice, to support
higher performance static executables. XML is not currently, and may never
be, a high performance static approach, so why not enjoy the benefits?

I heartily vote for moving quickly on getting meta-data into normal XML
format and moving away from DTD's.

On the other hand, I have major concerns regarding the current the
XML-SCHEMA efforts. It reminds me a little of SQL3, where the spec has the
'kitchen sink' feel to it. This is for understandable and hard to resist
reasons, but none the less a little scary.

XML is or soon will be, the synthesis of the document and data modeling
worlds. However the current efforts seem to me at least to be highly skewed
towards document expertise, and the data modeling expertise is not as
evident. I believe that the data modeling world, having to live in the rough
and tumble of high performance large scale reliable delivered systems
(object and relational environments/servers), has schema that are much less
ambitious than the document world. I personally would start with a much
simpler XML schema, implement carefully, live with the constraints, and
slowly learn what is *required*, as opposed to what is conceptually
interesting or complete. I have never seen a system that is that complex
start serving a useful purpose quickly.



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