utype questions

[Matthew Graham]

Hi,

Just to pitch in here with my understanding of utypes. We're using  
them in the Simple Event Access Protocol to identify our keywords for  
parameter queries, e.g.

http://nvo.caltech.edu/seap?seap:pos.ra=120&seap:pos.dec=14

I guess that this is representation of our data model in a way.

	Cheers,

	Matthew

On May 12, 2009, at 10:57 AM, Norman Gray wrote:

>
> Doug, hello.
>
> On 2009 May 12, at 16:54, Doug Tody wrote:
>
>> The purpose of utypes is to "parameterize a data model", that is,
>> assign unique tags to each field of a data model.  The data model
>> in question may have some hierarchical structure, and in the process
>> of parameterizing the data model we "flatten" it, reducing it to
>> a set of name-value pairs.
>>
>> The reason we do this is to separate the semantics of the data model
>> from the representation, to allow the same semantic content to be
>> reliably represented in many different ways, both externally and in
>> program structures and containers.  Hence, we can take a data model
>> instance, parameterize it via utypes, and store the resultant data
>> in the fields of a table, in a parameter set, in a hashmap in Java,
>> or even in a FITS header
>
> Then that turns into a formal requirement that the properties utypes  
> represent, or the objects with the types the represent, can be  
> present in a description only with a cardinality of zero or one.
>
> That provides in turn an explicit articulation of The Uniqueness  
> Problem: given a data model, is it feasible to generate a usable set  
> of utypes which can reconstruct the data model under this restriction?
>
> It's a pretty severe restriction.  If it weren't there the problem  
> would be solvable almost trivially.
>
>> Since the purpose of utype tags is to simplify manipulation of data
>> model instances by providing a simple keyword-value mechanism, we do
>> not want to parse utypes as this would defeat their whole purpose.
>
> I get the feeling there are multiple accounts of what 'their whole  
> purpose' is.  But that could be just my misunderstanding.
>
> I'm not proposing parsing anything, by the way.  I was very careful  
> to avoid proposing anything in the note I posted, but simply to  
> point out what appear to me to be unanswered but significant  
> questions.  I can see that in some cases (for example utype  
> equality) each of the initial answers is unattractive to someone,  
> but ... answers don't become any less unattractive by failing to ask  
> the question!
>
>> UFI [...] plus we have other tags such as ID and NAME which
>> can provide shorter tags within a controlled context.  Examples of
>> all of the above are already in use in implementations and code  
>> today.
>
> I appreciate it's a complicated issue.  But that's sounding like a  
> large number of distinct elements to nibble at different parts of  
> the problem, which means that keeping things consistent,  
> intelligible, and free from ad-hockery, across multiple applications  
> and disparate projects, could potentially become quite difficult.
>
> Best wishes,
>
> Norman
>
>
> -- 
> Norman Gray  :  http://nxg.me.uk
> Dept Physics and Astronomy, University of Leicester
>