2 Why We Need Model-Based Testing

270 Reactive Systems
tester actions enabled and when the observation queue is empty. The default name
of a wait action is "Wait", which can be customized using the waitAction option
of ct. A wait action takes one integer argument that specifies the amount of time
in milliseconds, called an observation time-out. The conformance tester uses the
observation time-out to wait for an observable action to arrive.
The simplest way to define a state independent wait action is to use a finite state
machine (FSM) model program and to compose it with the main model program.
The following FSM, say Wait20, specifies a 20-ms observation time-out:
FSM(0,AcceptingStates(0),Transitions(t(0,Wait(20),0)))
The time-out action is an observable action without arguments. It happens if
the observation queue is empty after executing a wait action. This results in a
conformance failure, unless the time-out action is enabled in the model program.
The default name of the time-out action is "Timeout", which can be customized using
the timeoutAction option of ct.
To see the effect of the wait and time-out actions, execute ct with the above
arguments and add the option /fsm:Wait20 in order to compose the main model
program with Wait20. The following is a possible output from this execution:
TestResult(0, Verdict("Failure"),
"Action symbol ’Timeout’ not enabled in the model",
Trace(
ReqSetup(0, 3),
Wait(20),
Time-Out()
)
)
Reset failed.
The specified observation timeout is too small. After the first test case fails, reset
also fails because the implementation is still busy. (See the defintion of Reset in
Figure 16.5.)
16.5.2 Adaptive strategies
Recall the discussion on strategies from Chapter 12. All the techiques discussed
there apply in the presence of observable actions as well. For example, execution of
ct in the previous section used the default strategy that selects controllable actions
randomly.
With observable actions one can do more. Note that each time an observable
action is consumed in the algorithm in Figure 16.6 the strategy is told to execute that
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