Descriptors and Handles

We have the concepts feature and dataset (feature collection). We think we need the concept of feature identifier, but some thought will show that we need two such concepts: a feature descriptor and a feature handle. A descriptor is some way of specifying a feature from ``outside the system'', by listing some sufficiently unique combination of attribute values, where the list of attributes will be highly application dependent. If an external agency maintains real world labels, then a single attribute value may be sufficient: but from the software designer's point of view the uniqueness of such labels cannot be entirely relied upon. A feature handle, however, is a concept that is ``inside'' the software system and which is required to have quite tightly defined uniqueness properties which are enforced by the software itself. Feature handles should generally be considered ``opaque'' and it might not even be sensible to think of them as ``values'' at all. Some handles might be string of text which is a query (in any language) which is sufficient to retrive the feature itself.