Discovering Data Collections Within Services Version 1.1

This note proposes a general mechanism through which data collections exposed by services aggregating several of them can be registered and efficiently discovered in the Virtual Observatory Registry. We discuss the problem setting - in short, that the Registry must support both VO-wide discovery of services by type ("service enumeration") and discovery by data collection ("data discovery") - and some attempts for a solution that were found to be unsatisfactory. Based on this, a scheme employing special identifiers to annotate simple capabilities associated with data collections is described. We also detail the implications for both standard authors and Registry clients, possible shortcomings of the scheme, as well as measures required to ensure a smooth transition to the new scheme.