Figure 2.

A Resource Description Graph (RDG) Sample. Actors--providers, clients, ontologies, and semantic search engines (Discovery Server)--meet in a semantic middle layer. The middle layer is instantiated as OWL documents, thereby enabling explicit descriptions amenable to formal reasoning. 1) Providers of data and services describe and publish their offerings; 2) these descriptions are available for semantic searching; 3) all actors use and reuse publicly available and extensible ontologies; 4) clients engage resources either directly or via a Discovery Server. Abbreviations: RDG: Resource Description Graph; SDK: Software Development Kit; KB: Knowledge Base.