LINK VOTable element for DATALINK

[Pierre Fernique]

Hi Markus and other datalinkers,

I would like to catch your attention about the new usage of VOTable LINK 
element that your are discussing for DATALINK implementation.

For no DAL members:
- The last message : 
http://www.ivoa.net/pipermail/dal/2014-February/006641.html
- The full thread : 
http://www.ivoa.net/pipermail/dal/2014-February/thread.html => subject: 
param-link-to-field (was: Datalink data access services) 
<http://www.ivoa.net/pipermail/dal/2014-February/006641.html>

And the Markus' example :
    <FIELD name="obs_publisher_did" ID="datalinkID"
        utype="obscore:Curation.PublisherDID"
      <PARAM name="ID" arraysize="*" datatype="char" 
ucd="meta.id;meta.main" value="">
         <DESCRIPTION>The pubisher DID of the dataset of 
interest</DESCRIPTION>
*<LINK content-role="ddl:id-source" value="#datalinkID"/> *
      </PARAM>

If I correctly catch the idea, the LINK element would be used for 
referencing other FIELD elements in order to be able to build a query 
(an URL ?) to a data provider. In this context, the content-role 
attribute would be used for tagging the "type" of referenced fields in 
order to be able to build correctly the query.

My purpose was not to discuss about this idea, but*I just want to bring 
your attention on the fact that the LINK element is already used since 
the begining of the VOTable standard* for building on the fly URL for 
documentation or access to additional data (quite similar issue in 
fact). It is implemented by VizieR, NED, Simbad and some other VOTable 
providers (LEDA, IMCCE,NED,...) since the begining of the VO (and even 
before with Astrores precursor of VOTable). It is extremely useful, 
notably for Aladin users to access full records, images, spectra, and 
other additional data.

Here an example from IMCCE skybot VOTable output for providing an access 
to the Astorb VizieR catalog corresponding entry:

<FIELD ID="name" name="Name" ucd="meta.id;meta.main" datatype="char" 
arraysize="32" width="32">
    <DESCRIPTION>Solar system object name</DESCRIPTION>
*<LINK 
href="http://vizier.u-strasbg.fr/cgi-bin/VizieR-5?-source=B/astorb/astorb&Name===${Name}"/>*
</FIELD>

Full result here:
http://vo.imcce.fr/webservices/skybot/skybotconesearch_query.php?-ep=1991-10-17T10%3A59%3A59&-ra=83.624166&-dec=21.9444&-size=36.76,36.76&-mime=votable&-out=basic&-loc=500&-search=Asteroids+and+Planets&-filter=120+arcsec


Last year, *Norman has suggested to extend LINK usage for setting SKOS 
concepts* based on PURL url. Even it was strange to see the LINK 
attribute used for type specification rather than utype, it was decided 
to support both purposes simultaneously.
*In order to avoid to clash with the existing link usage, the VOTable 
1.3 specification (section 3.5) precised the content-role LINK attribute 
: "doc" for real link usage (the default usage), "type" for SKOS concepts.*

*So my question: is this third emerging extension of LINK usage not 
clash with the two previous usages.*
For instance, until now, the "value" attribute can be used for 
alternative link text. And in this third usage, the "value" attribute is 
used as a kind of "ref" attribute.

Also, I wonder if the already existing URL substitution mechanism 
described in VOTable Annex A1 will not solve a part of the DATALINK 
discussion without inventing something else.


Best regards
Pierre Fernique

PS. I post my mail both on DAL and Apps mailing lists as VOTable 
standard is fully concerned.


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