Time Triggered vs. Event Triggered - Towards Predictably Flexible Real-Time Systems

Time triggered real-time systems have been shown to be appropriate for a variety of critical applications. They provide verifiable timing behavior and allow distribution, complex application structures, and general requirements. Their benefits are, however, limited in changing environments and for applications with not completely known characteristics. In these scenarios, event triggered systems provide appropriate mechanisms and better efficiency. Standard practices confront designers with an exclusive choice of one paradigm over the other, and consequently between determinism and flexibility, instead of allowing a customized selection.In this talk, we will discuss tradeoffs between determinism and flexibility and show how event triggered methods can be combined with time triggered systems to enhance flexibility. We will present algorithms to extend purely offline scheduling approaches, overcoming a number of their limitations and increasing flexibility. We will define requirements on online scheduling mechanisms to allow for their integration. With the example of slot shifting and constant bandwidth server, we will demonstrate the combination of two seemingly adverse methods, to lay a basis for predictably flexible real-time systems.