The emerging IVOA standards distinguish a few types of resource according to the different metadata that are registered: e.g. Organization, Service, TabularSkyService. This structural analysis leaves us short of a few terms that are useful when describing how the registry is used. If the usage of resources is considered, we discover some extra types.

Let a ScienceResource be something that could be use in an experiment and referred to in a paper: e.g. 'the early data release of the Sloan Digital Sky Survey' or 'the INT Wide-Field Survey reduced images' or 'version 2 of the HyperZ application'. These things exist independently of the VObs and may be used via the VObs.

Let a VObsService be some service that provides access to a ScienceResource. Specific types of service like TabularSkyService are specializations of VObsService. In practice we would expect to have a specialized type for each variety of service understood by the VObs, so registration of a basic VObsService wuld be rare.

A ScienceResource is not itself a VObsService. Metadata describing a ScienceResource do not say how to call it as a service. Instead, a caller has to ask the registry for the VObsServices corresponding to a given ScienceResource.

IVOA registry WG already has VObsService; it's called just Service in the WG discussion but I'll stick with the longer name here as it makes a better wiki-word. The WG currently has no type equivalent to ScienceResource. It might be worth creating such a type in order to illustrate the way the hierarchy works. There's currently no WG equivalent to DataSet that is independent of the service types. I think we need to create this type.

-- GuyRixon - 18 May 2004