Service-computing is the computing paradigm that utilizes services as building blocks for developing applications or solutions. Because simplicity of services, applications, and their interaction are critical for low cost and high productivity of service-computing, this paper tries to explore a proper bound of the simplicity. By proving that any Turing machine is equivalent to the interaction product of three generalized finite automata, we show that the services can be as simple as generalized finite automata, and their interaction can be as simple as interaction product. Since generalized finite automata are equivalent to finite automata which are intuitively very simple, and interaction product is an interaction mechanism imposing few constraints on its components, this result is generally helpful in distributed-system designs for achieving low cost and high productivity.