MIME types in DAL services

[Mark Taylor]

On Mon, 16 Apr 2007, Doug Tody wrote:

> For years now, the query response from cone search and SIAP services
> has come back with a content type of "text/xml".   This is convenient,
> as we can issue simple queries in a Web browser and view the results
> directly, even though the document returned is actually a VOTable.
>
> However, the current generally accepted MIME type for VOTable documents
> appears to be "application/x-votable+xml".  This is probably the most
> accurate and correct content specification, and is best in cases where
> the document is expected to be passed off to some application which
> understands the VOTable format.
>
> Another approach which has been played with a bit is to use
> MIME-type parameterization, e.g., "text/xml;subtype=votable" (the
> form "text/xml;x-votable" has also been suggested but may not be
> legal syntax).  The advantage of this approach is that the base type
> remains "text/xml", and any standard XML tool, such as a browser. will

Doug,

> be able to do something useful with this content (if a browser sees
> "application/x-votable+xml" it will in general have no clue what
> to do and will prompt for what you want to do with the document).

this isn't/shouldn't be true for well-written applications. 
As explained in RFC3023 (section 7), the idea of the *+xml MIME 
convention is so that a specific type (such as VOTable) which 
benefits from special handling can be defined, but that applications 
which do not know about this specific type can fall back to 
standard XML processing.  See Appendix A.6 of RFC3023
for an argument against doing something along the lines of
text/xml;subtype=votable (subtype is in any case not a standard
parameter of application/xml).  Admittedly, I'm not sure how many
applications actually do this +xml media type handling properly.

> The question is what is the best approach for DAL services, most
> notable in the case of VOTable (although parameterization is more
> general).  My suggestion is that we use "application/x-votable+xml",
> possibly with parameterization, as the FORMAT qualifier for datasets
> returned by the service, and continue to use "text/xml", possibly
> with parameterization, for the query response, for consistency with
> existing services and so that we can continue to have the ability to
> view simple queries directly in a browser.

I would therefore argue that application/x-votable+xml for both
query response and datasets is the correct thing to do.  However, if
pragmatic considerations (e.g. most web browsers failing to do the
right thing with it) dictate, I might be persuaded otherwise.

Mark

-- 
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/