A possible GrADS HFA class heirarchy.
Topology is a virtual class.
Realizable topologies include mesh, ring, star, graph, etc. The resource object represents a physical resource, most likely a machine but possibly bandwidth on a network, etc. A resource set is an unstructured bag of resources. A virtual machine is a set of resources and the topology that defines how these resources are related. For example, the minimum amount of memory required to execute the program, the desired execution time, or the limited set of machines on which a user has accounts.