The ongoing convergence of grid computing and web services has
inspired a number of studies on the use of SOAP-based web services for scientific computing. These studies have exposed several performance problems in using SOAP-based communication; to eliminate these bottlenecks, extensions to the SOAP standard and sophisticated implementation strategies have been proposed. In this paper, we will describe the ASP system, a simulation testbed based on web services for simulating multi-physics, coupled fluid/thermal/mechanical/fracture problems. The system is organized as a collection of geographically-distributed software components in which each component provides a web service, and uses standard SOAP-based web service protocols to interact with other components. There are a number of advantages to organizing a system in this way, which we discuss. We have analyzed the performance of our system for several applications and a number of problem sizes and have found that the overhead for using SOAP-based web services is small and tends to decrease as the problem size increases. Our results suggest that the previously identified potential bottlenecks may not be major issues in practice, and that a standards-compliant implementation like ours can delivery excellent scalable performance even on tightly-coupled problems, provided web services are used judiciously.