[wg11] Part 28 teleconferences

[Ed Barkmeyer]

David Price wrote:

>>1)  a simple non-validating schema with a simple data mapping,
> 
> An XML schema driven from EXPRESSS designed for a non-STEP- and
> non-EXPRESS-literate community of users who do not have access to any
> EXPRESS or Part 28 tool.

I'm not sure that the literacy of the modeling literacy of the community 
is what is at issue here.

I would have said: An XML schema derived from the EXPRESS model by 
fairly simple rules with no options, where necessary eliminating 
restrictions and features that make the XML schema difficult for people 
to read and tools to use.

>>2)  a schema that provides type checking, and
> 
> An XML schema designed for XML use by the STEP community in a more-or-less
> traditional STEP implementation environment with access to EXPRESS/Part 28
> tools.

This is exactly the opposite of what is wanted, as Tom indicated.  The 
idea here is to produce a schema that allows XML-only tools to validate 
the data, by incorporating in the XML schema as many of the constraints 
and implications of the EXPRESS schema as possible.

If the XML is processed in an EXPRESS-aware environment, then the 
EXPRESS schema is the basis for validation, and the XML schema can be 
much simpler, as in case (1).

>>3)  a configurable schema.
> 
> A pre-existing XML schema is defined somewhere and Part 28 is used to
> specify the relationship between it and a STEP EXPRESS schema covering the
> same scope. 

Yes.  This is what I understood, and I think this matches Günter's (2b).

> The pre-existing schema creator did not take the STEP EXPRESS
> schema structures into account when creating the XML Schema.

This part is either totally wrong or totally irrelevant.  What Martin 
and Günter both have in mind is that the AP developers (or some 
consortium of AP users) will specify a normative XML schema that covers 
exactly the same underlying data model as does the normative EXPRESS 
schema.  So they are fully aware of the normative EXPRESS schema.

The purpose of the configuration language is to define the mapping 
between the data elements modeled by the normative EXPRESS schema and 
the data elements specified by the XML schema.  The expectation should 
be that there is a one-to-one onto mapping for all the conceptual data 
elements in the EXPRESS ARM, but they may be somewhat reorganized in the 
XML.  In effect, the configuration language is the language of a 
"mapping table" between the EXPRESS schema and the XML schema.

This mapping can be phrased "forward" -- from the EXPRESS schema 
elements to the XML schema elements -- or "backward", from the XML 
schema elements to the EXPRESS schema elements.  If we state the mapping 
forward, it allows the XML schema to be generated from the EXPRESS 
schema, and tells a processor how to reorganize the EXPRESS-modelled 
data into the XML.  If we state the mapping "backward", it tells the 
processor how to generate the Part 21 file from the XML document.

The current configuration language approach is "forward" and "by 
exception".  That is, it assumes the default mapping (one of (1) or (2) 
above) and specifies only the mapping for elements that do not match the 
default mapping.  I would suggest that we continue with this approach to 
(3) in order to save time.

I agree with Günter that, for this purpose, the current configuration 
language is "too much and not enough".  It isn't quite powerful enough 
to cover all the organization and naming transforms that are wanted, and 
it includes directives that have nothing to do with the target XML schema.

I think Günter is also right that (1) and (2) are orthogonal to (3). 
The amount of EXPRESS constraint that is captured in the AP-specific XML 
schema will be what it is, presumably somewhere between (1) and (2), and 
the AP developers/users only need to say in the configuration language 
which kinds of EXPRESS constraints the XML schema should capture.

-Ed

-- 
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."