[vos-d] VOS to RDF mapping

[Karsten Otto]

Let me contribute one more animal to the  vobject graph language  
zoo... In my own framework I currently map VOS to RDF (Resource  
Description Format). The mapping is somewhat ad-hoc, but works nicely  
for me. I'd appreciate comments and suggestions!

             Mapping VOS graphs to RDF is relatively easy, since VOS  
already
             uses URIs to identify objects. For other purposes, such  
as child
             relations and object types, VOS does not use URIs, but  
instead
             utilizes strings conforming to an XML qname notation.  
For mapping
             purposes, this can be represented by URIs of the form
             http://interreality.org/otd/*schema*/*name*.
             For some child relations, the actual name is not deemed  
important,
             and VOS uses a randomly generated name without namespace  
part.
             In a way, this represents an "anonymous" relation, which
             unfortunately cannot be expressed in RDF; instead the  
mapping
             http://interreality.org/otd/_/*name* is used.

             Thus, VOS objects map directly to RDF resources, using  
the normal
             rdf:type property to indicate types, which are mapped  
from their
             native qname form to URIs as described above. Child  
relations
             work in a similar way, using mapped contextual names as RDF
             properties. Since the order of child relations matters  
in VOS,
             there must be an additional [rdf:_*n*] property for each.
             Together, these two RDF properties reflect the  
contextual name and
             position of a VOS child relation.

             VOS uses the special object type property:Property to
             represent literal values. In addition to the actual  
value, each VOS
             Property has a certain length, and usually also an  
associated data
             type. VOS Properties are represented as RDF resources as  
usual for
             VOS objects. The property rdf:value then contains the  
actual
             value, while vos:length indicates its length, and  
vos:datatype
             the respective data type string. Since VOS Properties  
can contain
             several megabytes of binary data, the value is only  
reflected
             if it has a "reasonable" size (currently 256 bytes).