UType proposals

[Norman Gray]

Doug, hello.

[I realise that this discussion has moved over to the semantics list,  
so I'll copy this there as well]

On 2009 Jun 28, at 02:55, Douglas Tody wrote:

> On Fri, 26 Jun 2009, Norman Gray wrote:
>
>> I think this is a key point.  The corollary of this is that SSA  
>> UTYPEs will _lose_ their meaning once they're separated from their  
>> context, and thus if you want to store the UTYPEs outside of the  
>> context of a SSA transaction (such as in a database, or a FITS  
>> file, or some other format yet to be invented), then you are  
>> absolutely required to retain the link to (the version of) the SSA  
>> transaction which initially retrieved the object.  In other words,  
>> it means that the SSA _data model_ is closely tied to the SSA  
>> _protocol_, which will make it at least inconvenient to reuse it in  
>> some different application.
>
> Again, data model attributes are not generally used separately without
> any knowledge of the overall data model of which they are a part.
> If we could usefully reduce a complex data model to a single value
> we could merely define a new UCD and we would not need UTYPEs.

Indeed.  I entirely agree.  I have at no point suggested otherwise,  
and goal 1 of the utype-proposals document that started this thread  
restated your formulation of the problem -- that UTypes are to be used  
as a group, in a set of key-value pairs where the keys are UTypes and  
the values are numbers or strings as appropriate.

> This discussion still misses the point that it is more important to
> specify the version of the entire data model than that of a single
> attribute, since we are dealing here with data models, not single
> quantities.  Whatever solution we adopt should take this as the first
> priority.

Quite apart from anything else, including the datamodel version in the  
UType string means it cannot get lost.  If you have a UType string and  
a version somewhere else in the 'context', then the two things _will_  
get separated somehow.

Best wishes,

Norman


-- 
Norman Gray  :  http://nxg.me.uk
Dept Physics and Astronomy, University of Leicester, UK