Characterising URL columns

[Mark Taylor]

Dear DAL,

I've got a question about making sense of URL-bearing columns
in generic tables.  I don't think there is currently a good answer
to it, but I thought it was worth checking with the experts.

The question is, is there any way to determine from a generic
table what kind of resource lurks at the end of URLs in
a given column?

For instance, a number of services return tables with columns
containing DataLink URLs, that is results corresponding to
the {links} response described in section 3 of RED-DataLink-1.0,
and which are properly described by the Content-TypeMIME
"application/x-votable+xml;content=datalink".
If the (human or machine) reader of the table in question knows
that the column is supposed to point to DataLink tables,
then she can open the URL with some kind of DataLink client.
But in general I don't know how to tell that a given column
contains DataLink responses, or images, or spectra, or what.

The same thing applies to other content types as well.

In the case of some particular services or datamodels there are
ways to do this, e.g. use of the access_format column in ObsCore
tables.  But I don't know how to mark (or identify as marked)
URL columns by data (MIME?) type in, for instance, a generic TAP
or Cone Search response.

In practice, the best thing may just be to call the column
"datalink_url" or whatever and add some suitably descriptive
human-readable metadata.  Sufficiently unprincipled clients (ahem)
might then even go grubbing through column names to try to guess
the intention.  But is there any more respectable way to do it,
or at least some kind of standard practice that it's a good idea
to follow?

Thanks

Mark

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/