Opportunistic Service Composition in Dynamic Ad Hoc Environments

Mobile devices capture, process, and exchange sensory data about their operating environment making them attractive service providers for pervasive computing. Composing services from different devices supports context-aware applications for smart spaces. However, carrier mobility and participation autonomy cause frequent changes to the network and service topology and impose a high failure probability on composites. Decentralised composition solutions assign service providers at runtime to provide for flexibility and to avoid a single point of failure. However, existing solutions rely on a pre-established service overlay network and employ a conservative allocation strategy which limits their applicability for highly dynamic environments. This paper presents a novel service composition protocol that allocates and invokes service providers opportunistically to minimise the impact of topology changes and to reduce failure. The protocol supports service sequences and parallel service flows. Automated model checking verifies that the protocol does not deadlock and that it terminates in a valid end state after having allocated the correct number of service providers for all required sub-services. The results of the simulation-based evaluation demonstrate that the opportunistic approach generally reduces composition failure. At the same time, the evaluation reveals the protocol's limits with regard to composite complexity, network density, and service demand.