On the complementary nature of event-driven and time-driven models

Event-driven and time-driven models exhibit a complementary nature, in the sense that they reflect the nature of the contrasting requirements that any real-time system must meet - i.e., performing actions "as soon as possible" or "at the right time". Even if each of the models is sufficient to model a system, they should be intermixed to improve the expressiveness. This can be achieved by assuming one model as the primitive, and implementing the other on top of it. The paper introduces the rationale for choosing the time-driven model as the primitive, and discusses why this approach is possibly the most suitable for a wide range of control applications. In conclusion, a unifying model, based on the distinction between the atomic actions performed by reactive agents, the control performed by a time-driven control machine, and the planning performed by an event-driven planning machine, is introduced.