Chapter 7 - Road junction

Abstract

This chapter presents a case study of a traffic light control system. The chapter addresses a real-time engineering level problem, where synchronization and timing are significant aspects of the requirements. The case study examines the details of states, interactions and system testing. In addition to the core unified modeling language (UML) features, concepts and elements introduced in the proposal for a “profile for schedulability, performance, and time” are used. The development of a set of components from which the traffic light system can be constructed with the help of a class diagram is described. The chapter provides a brief summary of the aspects of the profile used in the case study, especially RTtimer and RTtimeout. It shows how the formal definition of the key elements is built up using the UML extension mechanisms. The key features of Timer are its duration, which determines the period after which it sends a Timeout and the isPeriodic flag, which makes it send repeatedly each time the duration elapses. A sequence diagram shows how the objects and actors need to interact to allow the “junction” to function. A possible statechart for a TrafficLight is presented to show the internal behavior of a system's components. To see an overall view of this behavior, the collaboration diagram (external view) and the statechart (internal view) can be combined, which gives a full model. Combined statechart and collaboration diagrams generate the overall behavior generated by the logic of the statecharts of the individual objects in the system. Building on this the overall state transition diagrams of the system and the resulting event sequences can be derived to match against those required. Adding timings to the model allows checking the response times either deterministically or as bounds or averages.