We propose a composable, loosely coupled Service-Oriented Architecture for middleware-independent Grid job management. The architecture is designed for use in federated Grid environments and aims to decouple Grid applications from Grid middlewares and other infrastructure components. The notion of an ecosystem of Grid infrastructure components is extended, and Grid job management software design is discussed in this context. Nonintrusive integration models and abstraction of Grid middleware functionality through hierarchical aggregation of autonomous Grid job management services are emphasized, and service composition techniques facilitating this process are explored. Earlier efforts in Service-Oriented Architecture design are extended upon, and implications of these are discussed throughout the paper. A proof-of-concept implementation of the proposed architecture is presented along with a technical evaluation of the performance of the prototype, and a details of architecture implementation are discussed along with trade-offs introduced by the service composition techniques used.