Datalink Feedback VI: Semantics

[Norman Gray]

Markus, hello.

On 2014 Apr 24, at 08:44, Markus Demleitner <msdemlei at ari.uni-heidelberg.de> wrote:

>> It doesn't mean that everyone has to understand all of the terms.
> 
> ... but it would be very nice if it were easy for a client to
> understand all the terms that are likely in use.  Hence, building in
> a strong incentive to just use the IVOA-defined terms (and maybe we
> can improve the glacier-scale speeds there rather than work around
> them) would, I think, be highly useful.

I think that's very important.

I see no real reason why (an initial version of) the set of terms can't be released at the same time as the WD document.  It can then be adjusted and preened on a largely separate timetable, though it would be very good to have a finalised version ready at the same time as the Datalink standard goes to REC.

> Please, no, let's not say "in preparation".  Either the vocabulary
> comes with datalink, or everyone will be horribly confused.  And
> also, let's avoid having long URLs with 90% of data data constant in
> semantics by default.  Unwieldyness matters even when users shouldn't
> normally be seeing these things.  Can't we say something like:
> 
>  The values of the semantics column are URLs.  If semantics just
>  contains a fragment ("#term"), the fragment refers to
>  http://ivoa.net/rdf/datalink [or whatever].  Clients should
>  understand at least all the values mentioned in that vocabulary,
>  and they should interpret the rdfs:subPropertyOf relations
>  mentioned in there.

That looks good.  It may be worth a sentence saying what 'rdfs:subPropertyOf' means, so that the authors of clients don't feel they have to grok RDF before they can use this.  Something like "Don't worry about the details: think 'subclass of'".

> Added benefit: These things will look like hashtags.

Brilliant!

> Uh... <bambi eyes>... Norman -- since that's probably easiest for you
> among everyone that's reading this: Do you think you could draft
> such a vocabulary, maybe based on Francois' suggestions
> http://www.ivoa.net/pipermail/dal/2014-April/006781.html (plus
> "self", which I suppose would either be top-level or under something
> like "alternative representation")?  As I said, I'm convinced if we
> want this to fly and be understood by the deployers, the vocabulary
> will have to come with datalink.

I'm defenceless in the face of your Bambi eyes, Markus...

In the attachment there is

datalink-terms.csv -- a CSV version of François's suggested structure
make-ontology.py* -- a Python script which takes this as input and generates ...

datalink-core.ttl -- a Turtle version
datalink-core.html -- human-readable (albeit currently very ugly) documentation

datalink-core.rdf -- an RDF/XML-syntax version of the Turtle, principally in order to validate that I've generated syntactically correct Turtle

Makefile -- a Makefile to manage these.

These terms would be <http://www.ivoa.net/rdf/datalink#image>, and so on, but this is obviously easily changeable.  There's a little bit of server-side configuration to do (a few lines in a .htaccess), but according to best practice for this sort of thing, <http://www.ivoa.net/rdf/datalink> would redirect (302, I think) to <http://www.ivoa.net/rdf/datalink-2014-05-01/> (or whatever the date is for a given release).  That would produce the human-readable documentation.

>> Re 1: I think that 'relation' or 'predicate' would be a better name
>> than 'semantics'.  Also, the idea of an RDF 'Property' (aka
>> 'Predicate') is simply that it's the link between a 'subject' and
>> an 'object':
> 
> If we go for a formal RDF interpretation, I'm all for calling the
> thing property (except if that happened to be a reserved word in some
> RDBMS...).


Sounds good.

All the best,

Norman


-- 
Norman Gray  :  http://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK
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