[wg11] Part 28 Issue : Do we need all this configurability?

[Ed Barkmeyer]

David Price wrote:

> Is there support for removing or vastly reducing and simplifying the ISO 
> standard EXPRESS to XML Schema configuration capability in Part 28? For 
> example, could we agree on an EXPRESS to XML schema mapping for each 
> usage scenario (e.g. data exchange vs. SOAP) and standardize those instead?

For what it's worth, David, it is the NIST position that most of the 
elaborate configuration language is unnecessary and hinders 
interoperability.  Whether the U.S. ballot comments will reflect such a 
position is "quite another thing entirely", in that two other U.S. 
organizations have crafted most of the options that NIST considers least 
useful.

What we are facing, however, is three problems:
- The default rendering of an EXPRESS AIM schema into an XML schema will 
not produce an XML schema intended for use by human engineers who are 
not STEP experts.  It will not result in increased use of STEP models in 
ebXML and other business exchanges, because it will be ugly, 
counterintuitive, and difficult for non-STEP-expert analysts and 
programmers to work with.  And if it can only be used by STEP experts, 
why should we not just continue using Part 21 for all STEP exchanges?
- When an XML schema is developed from STEP models with the express 
intent of reaching this business data exchange community, "it would be 
nice if" there were a standard mapping language in which the 
relationship between the XML schema and the normative EXPRESS schema 
could be formally described.  Several participants see this as the 
primary purpose of the configuration language.
- Several XML schemas based on STEP models have been constructed and are 
in pilot or production use in certain communities.  The toolsmiths and 
user communities who have invested in this effort would like the 
standard to support 90+% of what they have in "product". 
Unsurprisingly, there is nearly no consistency in their various 
approaches to mapping EXPRESS reference models into useful XML schemas. 
  Which of them do we choose as the only conforming one?

In my personal view, these three problems demand three different solutions:
- Use more intuitive EXPRESS ARM schemas for XML exchanges.  The mapping 
tables will tell you how to construct the AIM equivalent, for those of 
you who still believe that AIMs are good for something.  The problem is 
that only a few APs have useful, normative and complete ARM schemas.
- Include a formal EXPRESS-to-XML-schema mapping language in Part 28 (or 
some spinoff) that allows complex transformations.  Require its use as a 
documentary element of any hand-crafted XML schema that becomes part of 
an SC4 standard.
- In Part 28, pick one general intuitive approach to mapping EXPRESS 
schemas to XML schemas, even if it is somewhat complicated (both EXPRESS 
and XML schema are).  This will probably render 10-20% of every "in 
product" implementation invalid and send people who made certain bizarre 
choices back to the drawing board.  Standards not drawn from active 
practice have a way of doing that.

The problem with my approach, of course, is that Part 28 itself can only 
really address the last of these (and perhaps half of the second).  But 
without the other two, it will be the solution to no real problem, other 
than allowing us to say STEP and XML in the same sentence.

-Ed

P.S. Sometimes doing the wrong thing well actually begets a useful 
solution to the problem, albeit indirectly.  Thus far in Part 28, it has 
not proved to be the case, but now we come to the first ballot.

-- 
Edward J. Barkmeyer                        Email: edbark at nist.gov
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8264                Tel: +1 301-975-3528
Gaithersburg, MD 20899-8264                FAX: +1 301-975-4694

"The opinions expressed above do not reflect consensus of NIST,
  and have not been reviewed by any Government authority."