SIAv2: FORMAT?

[alberto micol]

Dear DAL,

I have a question regarding the FORMAT input parameter described in SIAv2.0:
	2.1.17 FORMAT

	The FORMAT parameter specifies response format(s)  (the access_format column from the ObsCore [7] data model). This column describes the format of the response from the access_url (see 3.1.3 <http://www.ivoa.net/documents/SIA/20151223/REC-SIA-2.0-20151223.html#bkmRefHeading200962138146524>) so the values could be data file types (e.g. application/fits) or they could be the DataLink [8] MIME type. 

It is unclear to me how to read that.
I have two possibilities in mind:

1) The FORMAT input param is used to add a constraint onto the access_format, e.g.:
    ...&FORMAT=application/fits

    gets translated into a constraint like: 

    SELECT ... WHERE access_format = 'application/fits

but that does not seem to have much sense, as some data providers might have decided that access_format in the ObsCore table is always the datalink mime type, delegating and delaying the specification of the correct mime type to datalink. 
In such case, the result of the above query will always be an empty set.

2) a different way of reading 2.1.17 is that FORMAT determines which access_format will be used in output, that is...

if ...&FORMAT=application/fits

then the returned votable will contain:

access_format=application/fits
access_url=the URL to directly download the FITS file (and not a datalink)
while if FORMAT=application/x-votable+xml;content=datalink

then the access_format will be "application/x-votable+xml;content=datalink”
and the access_url will return the datalink link.


Could you please help me understand the 2.1.17 specification?
I would go for interpretation (2) above, but I see that some of you have implemented (1) instead...

Many thanks in advance,
Alberto

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