S-OGSA has three main aspects: the model (the elements that it is composed of and its interrelationships), the capabilities (the services needed to deal with such components) and the mechanisms (the elements that will enable communication when deploying the architecture in an application).
S-OGSA Model. Although there is no standardized overall model of the Grid and its basic concepts, there is a vocabulary associated with OGSA, and there are project specific models3 19 and capability focused models like the Common Information Model (CIM)20 from the Distributed Management Task Force and the Job Submission Description Language21 from Global Grid Forum. S-OGSA introduces the notion of Semantics into the model of the Grid defining Grid Entities, Knowledge Entities (e.g. ontologies, rules, text), Semantic Bindings between these two for a Grid Entity to become Semantic Grid Entities. Semantic Bindings are (possibly temporary) metadata assertions on Grid entities and are Grid resources with their own identity, manageability features and metadata.
S-OGSA Capabilities. S-OGSA is a mixed economy of these semantically enabled and disabled services. We add to the set of capabilities that Grid middleware should provide to include the Semantic Provisioning Services and Semantically Aware Grid Services (Figure 4).
Semantic Provisioning Services dynamically provision an application with semantic grid entities in the same way a data grid provisions an application with data. The services support the creation, storage, update, removal and access of different forms of Knowledge Entities and Semantic Bindings. Ontology services store and provide access to the conceptual models representing knowledge; reasoning services support computational reasoning with those conceptual models; metadata services store and provide access to semantic bindings and the annotation services generate metadata from different types of information sources, like databases, services and provenance data. These four build on the past work of members of the consortium: a knowledge parser for extracting information from online sources;22 a metadata store;23 and a suite of ontologies and supporting tools to generate semantic descriptions for Grid Services.24
Semantically Aware Grid Services exploit knowledge technologies to deliver their functionality, for example metadata aware authentication of a given identity by a Virtual Organisation manager service or execution of a search request over entries in a semantically enhanced resource catalogue. Sharing this knowledge brings flexibility to components and increases interoperability. OntoGrid is working on a principled re-factoring strategy for legacy Grid Services to quantify the impact on current Grids.
S-OGSA Mechanisms. The model and capabilities are platform independent. To demonstrate the approach in practice, we map the conceptual design to a specific software platform, namely the Globus Toolkit 4, by mapping the semantic bindings to resource properties defined using the Web Service Resource Framework and incorporating S-OGSA entities into the Resource Model of the Common Information Model.
