2 Why We Need Model-Based Testing

4
Systems with Finite Models 113
Command()
7
1
0
Command()
Timeout()
2
3
Reset()
Message("99.9") TimeoutMsgLate()
5
Message("999.9")
Figure 6.14. Reactive system: interactive exploration showing path to deadlock.
6
TimeoutMsgLost()
8
19
20
21
NoHandler()
Command()
NoHandler() Command()
We also reproduce the LostMessageWhenIdle run, where the program reaches
a deadlocks (Figure 6.14). The deadlock is indicated by the loop of alternating
Command and NoHandler actions, from which there is no escape. The deadlock here
is a loop rather than a dead end. The supervisor can always command the controller,
so the Command action is always enabled. But the controller can do nothing; recall
that NoHandler is a dummy action that indicates no handlers are enabled.
These examples confirm that model-based analysis reveals the design errors we
originally discovered by executing the implementation.
In Chapter 7 we will discuss another kind of analysis. Instead of searching for
unsafe states or dead states, that analysis checks temporal properties defined by
sequences of actions.
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