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.