A graph-based Particle Swarm Optimisation approach to QoS-aware web service composition and selection

Web services are network-accessible modules that perform specific tasks and can be integrated into Web service compositions to accomplish more complex objectives. Due to the fast-growing number of Web services and the well-defined nature of their interfaces, the field of automated Web service composition is quickly expanding. The use of Particle Swarm Optimisation composition techniques that take Quality of Service (QoS) properties into account is well-established in the field. However, the commonly utilised approach is to optimise a preselected Web service composition workflow, which requires domain expertise and prior knowledge and thus may lead to the loss of better solutions that require different workflow configurations. This paper presents a graph-based PSO technique which simultaneously determines an optimal workflow and near-optimal Web services to be included in the composition based on their QoS properties, as well as a greedy-based PSO technique which follows the commonly utilised approach. The comparison of the two techniques shows that despite requiring more execution time, the graph-based approach provides equivalent or better solutions than the greedy-based approach, depending on the workflow preselected by the greedy-based PSO. These results demonstrate that under certain circumstances, the graph-based approach is capable of producing solutions whose fitness surpasses that of the solutions obtained by employing the greedy-based approach.