Translation between DTDs, schemas, UML, and the like

[Paul Prescod]

"A. G. McDowell" wrote:
> 
> There seem to be a large number of languages devoted to listing (for
> instance) the fields that make up a customer order and describing their
> data types. To comprehend a (hypothetical) ecommerce system I might have
> to follow a relational schema for the underlying database, a UML model
> of the application classes and logic, and a schema or DTD for the XML
> used to exchange data with its customers.
> 
> Is there any hope of a product that could be used to automatically
> generate some part of this? My chosen format would be UML, but I'm open
> to reasons why not. 

My primary argument would be that AFAIK UML is not a language in the
computer-science/chomsky sense! It is a graphical notation. You can't
email me the UML code for "a class" (though you could mail me the mutually
incompatible output of various UML modeling programs). As I understand
it, there are initiatives afoot to make standard serializations for UML
diagrams, but those are not UML itself. If my understanding is out of
date, perhaps someone could fill me in.

Now even if we presume the existence of a standard UML serialization, I am
not confident that our goals in DTD creation and object modeling are the
same. UML describes how a system functions. Database schemas describe how
its data should be persisted. DTDs describe how the data should be
interchanged.

System function modeling is focussed on increasing reliability, usability
and reusability. Persistence modeling is focussed on reducing duplication
and providing efficient navigation and querying. Interchange is focused on
expressing very careful, very explicit constraints on what is and is not
allowed, and in what order.

I fear that if you tried to build a single schema to accomplish all of
these things, it would have to be very complex and sophisticated. It would
need to incorporate features of IDL, SQL, STEP and DTDs. Perhaps it's
possible, but I don't think that UML is anywhere close. I fear greatly
that this overcomplicated monster is what many participants expect as the
result of the W3C XML schema language effort.

Also note that XML still has a major role to play in the document world.
This implies many usability concerns that would need to be recognized by
this UBER-Schema.

 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself
 http://itrc.uwaterloo.ca/~papresco

"You have the wrong number."
"Eh? Isn't that the Odeon?"
"No, this is the Great Theater of Life. Admission is free, but the 
taxation is mortal. You come when you can, and leave when you must. The 
show is continuous. Good-night." -- Robertson Davies, "The Cunning Man"

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