What is a cloud application precisely? In this paper, we formulate a computing cloud as a kind of graph, a computing resource such as services or intellectual property access rights as an attribute of a graph node, and the use of a resource as a predicate on an edge of the graph. We also propose to model cloud computation semantically as a set of paths in a subgraph of the cloud such that every edge contains a predicate that is evaluated to be true. Finally, we present algorithms to compose cloud computations and a family of model-based testing criteria to support the testing of cloud applications.