An object-based model for dependable real-time distributed systems

A systematic way to introduce fault-tolerant and time-dependent properties into a complex software system is presented. The approach is based on an extended Petri net model, called the G-Net, enhanced with a deterministic timing scheme. In the timed G-Net (TGN) model, exception handling and time-out mechanisms are used to introduce the fault-tolerant and real-time properties. The basic idea is to avoid the timing error propagation from a faulty object to an object that interacts with it via a time-out mechanism, and also to allow an object to treat a timing error by raising an exception and triggering some corrective actions. Using TGNs as the basic building blocks, we further refine the model into the `smart object' (SMO) model. In each SMO, an object can associated with a knowledge structure, which enables the object to dynamically change its behavior to react to changes in the environment. We use examples to show that a simplified real-time distributed air traffic control system can be modeled effectively using the SMO model