Use case analysis for handling data cubes in VO

[Markus Dolensky]

Two comments on DAL aspects of Mireille's and Francois' proposal:

1. mime types

 > AccessAndViewsProductType:
 > cube, image, spectrum, catalog, metadata
 > Different from the DataSetType of the DataSEt itself, because it
 > is the  Type of the Specific view. It includes also catalog, as a
 > result of source extraction and metadata to allow the retrieval of
 > full metadata descriptions such as characterisation ...

Let's just use recognized values like application/fits, application/xml, 
image/png etc. Unregistered types can be flagged with a "x-" as in 
application/x-votable+xml.

Pseudo mime-types such as spectrum/fits, however, where invented ad hoc and 
under pressure to get a demo working. If I recall properly we used it to 
associate a node (image,spectrum,etc.) in an IDHA tree with a plug-in. This 
mechanism should no longer be needed(?).


2. direct retrieval

 >    As mentionned before 2 options could be envisionned for retrieval:
 >     By default (or with rerieval= false) this query will retrieve an
 > SIA/SSA query response. But with a retrieval=true parameter it will
 > instead retrieve directly the view.

At some stage there was an SSA-flag called DirectAccess. The reason why I 
removed it again from the draft (to my regret) was that somebody pointed out a 
conflict with the requirement to support error responses which are implemented 
through an <INFO> element in the query response in the same way as in SIA V1.0.

An example:
Suppose an SIA or SSA cutout server consumes an invalid query. Encoding the 
error diagnostics in a document might not be practical when the expected result 
is of type FORMAT=image/jpeg.


Always requiring a VOTable query result resolved the issue. Maybe it puts too 
much emphasis on the error response handling instead of other functionality? 
Obviously one could argue that an error document is not needed at all, but 
defining the error handling at a different level like HTTP might confuse network 
problems with usage issues...

- Markus