2 Why We Need Model-Based Testing

32
3 Why We Need
Model-Based
Analysis
This chapter demonstrates why we need model-based analysis. We exhibit a program
with design errors that cause safety violations (where the program reaches forbidden
states), deadlocks (where the program seems to stop running and stops responding to
events), and livelocks (where the program cycles endlessly but can’t make progress).
We preview our analysis and visualization techniques and show how they can reveal
the design errors, even before beginning any testing.
3.1 Reactive system
Suppose we are developing a process control program that runs on an embedded
computer connected to sensors, timers, and a supervisor program (Figure 3.1).
The temperature monitor discussed in Chapter 2 could be a component of this
system; here we consider a higher level of integration. This is a reactive system that
responds to events in its environment. In this chapter we consider just one of its
features: the temperature-calibration factor. The controlled process depends on the
temperature. In order to control the process accurately, the control program must
obtain a temperature reading from a sensor and use it to compute the calibration
factor. The calibration factor is then used in subsequent process control computations
(which we do not discuss here).
The temperature in the process can change continuously, so the control program
must sample the temperature often. The control program frequently polls the sensor
(requests a sample). The sensor usually responds with a message that contains the
most recently measured temperature. We distinguish controllable actions that the
program commands from observable actions that originate in the attached equipment.
All that the program can do in regard to observable actions is to wait for them
(and observe them). Polling is a controllable action and message arrival is an observable
action. Observable actions are prefixed with a question mark ? in Figure 3.1.A
reactive system is any system that includes observable actions. Sometimes reactive
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