[DALI] error documents

[Patrick Dowler]

One topic came up in the recent DALI session that we decided to consult a 
wider audience on: error documents. In the past, it has been standard practice 
that error documents from DAL services were always VOTable documents with a
special INFO element showing the error state, plus whatever extra description 
the service could provide.

It was pointed out that the practices in some GWS specs (eg VOSpace) of using 
text/plain and specified could be alot easier for clients to consume and, if 
they had a UI, display to the user.

I would like to change the language in the DALI spec to allow this. We would 
keep the description of how to specify errors in VOTable as-is but also allow 
non-VOTable responses if that made more sense in the specific service spec. The 
language would be something like 

"if the service normally returns results in VOTable format, errors should also 
be expressed with VOTable error documents"

"if a service does not use VOTable, it may use text/plain error documents" 

Specific services would decide, so services that support multiple response 
formats (eg TAP) and that already require VOTable support could still specify 
that all errors are VOTable documents.

The use case we envisioned was a service that did some data operations (eg 
image mosaic service). In this case, input and output might be FITS files but 
the client would need VOTable and XML parser support just to handle error 
messages. That seemed asking too much to everyone at the DALI session.

So, this is just relaxing the language a bit to permit text/plain error 
documents in specific DAL services. Comments?

-- 

Patrick Dowler
Tel/Tél: (250) 363-0044
Canadian Astronomy Data Centre
National Research Council Canada
5071 West Saanich Road
Victoria, BC V9E 2M7

Centre canadien de donnees astronomiques
Conseil national de recherches Canada
5071, chemin West Saanich
Victoria (C.-B.) V9E 2M7