[wg11] Re: [step-os] Using STEPMod XML Representation of EXPRESS

[Ed Barkmeyer]

David Price wrote:

> [snip]
> So, my question is … Are people generally comfortable with using the 
> STEPMod XML representation of EXPRESS as the de facto standard for 
> open-source STEP work? 

Couldn't care less, subject of a previous email.

> I’m asking because I’ve been thinking that we could steal some of the 
> ideas from UML/XMI and apply them to EXPRESS/STEP by extending the 
> STEPMod DTD for EXPRESS. A simple example is that we could implement the 
> UML tagged value or stereotype concepts as a way of marking up EXPRESS 
> in a way that’s similar to what the OMG Model Driven Architecture people 
> are trying to do as part of their model-to-model mappings (e.g. to 
> control the mapping from EXPRESS into SQL DDL). 

Or to XML schema, or to Java, to choose examples for which we already 
have SC4 standards.  Or to WSDL, or (to please JTC1/SC34) to RELAX NG, 
or to Schematron or to CAMS or to the XML modeling technology of the week.

> Perhaps we could even 
> agree between ourselves on some of the specific stereotypes/tagged 
> values for interoperability between the various open-source efforts.

Well...

First, stereotypes and tagged values are a feature of the UML language 
that is described in the language standard.  There are no corresponding 
features in EXPRESS.  I would think the logical choice would be to add 
those features as markups to the EXPRESS or EXPRESS-G languaages.

So what is David proposing here?  From an EXPRESS schema, you convert to 
a UML v2 meta-model representation using Part 25.  And then what?  Since 
you have a meta-model representation of the EXPRESS model, the OMG MDA 
idea is that you make a formal mapping to a representation using some 
other meta-model, presumably the meta-model for SQL or XML schema or 
Java or whatever.  That is pretty much what the 20-series standards do. 
  Of course, they don't do it using MOF meta-model formalisms, but that 
doesn't seem to be what David is proposing, either.

Of course, you can take the UMLv2 metamodel representation of a given 
EXPRESS schema, convert it to a UML static structure diagram, and mark 
up that diagram with the "refinement" stereotypes for the implementation 
language, thereby specifying the mapping using current UML technology.
Is that what David intends?  Then, when we talk about agreement on the 
"implementation" stereotypes, we are in fact talking about agreement on 
a weak metamodel for (the interesting elements of) the implementation 
language.  (The meta-model implied by the implementation stereotypes is 
"weak", because it contains the terms from the implementation language, 
but not their relationships or constraints.)

But I don't understand what the XML version of the EXPRESS schema would 
have to do with that.  Are we talking about devising a kind of 
stereotype feature for EXPRESS that tools will implement by modifying 
the XML representation of the EXPRESS schema?  Why not markup the 
EXPRESS language text, or the EXPRESS-G diagram?  In those cases, at 
least, the stereotyping will be easier to read and comprehend.  Or do we 
envisage a browser app that presents the EXPRESS model in yet another 
form (not EXPRESS text, not EXPRESS-G, not UML static structure), 
accepts the markups and creates the marked up XML?

I don't think the "implementation markup" idea is a bad one; I just 
don't understand it in this context.

-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."