Utypes 0.4 comments

[Norman Gray]

Mireille, hello.

Here are some comments on v0.4 of the utypes draft at <http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/Utypes>.



My only real concern is that the document could give the impression that utypes are more complicated than (I believe) they in fact are.  This is because the draft specifies more than appears to me to be necessary.

For example, you mention Object Oriented modelling at the beginning of section 2.2, and in section 3.1 say "Models are built following object oriented programming principles. They are represented in UML".  Is this necessarily a given?  Obviously, the Characterisation DM is in UML, but the SSA DM is described in text with an XSchema, STC is defined using an XSchema, and FITS-WCS (for example) is described in text (presumably we wouldn't want to exclude FITS-WCS in principle from retrospectively defining utypes).  This is not to say that object-orientation is a bad thing (of course), but that it's not clear if anything is being gained by the apparent restriction to UML.

In a similar vein, the discussion in section 4 defines a syntax for utypes, composed of models, packages, classes, references and collections.  This is starting to become intricate, and making the syntax this specific suggests that it is intended to be parsed, which goes against what I believe to be the decided unparseability of utypes.

Similarly again, Section 6, on "Generating Utypes from UML data models via their XML representation" might possibly be overspecification.  If utypes are indeed unparseable pointers to items within data models, then they could in principle be named simply 'utype1', 'utype2' and so on, without any technical ill-effects.  That would be a poor idea on usability grounds, however, because that would make any system using them hard to debug.  Thus some mechanism such as that in Section 6 would be highly recommendable.  However the text in that section seems to _require_ that utypes be generated using this specific mechanism, using a particular representation (XMI) of a particular modelling language (UML).




I hope these comments are helpful.

Best wishes,

Norman


-- 
Norman Gray  :  http://nxg.me.uk