2 Why We Need Model-Based Testing

216 Testing Systems with Complex State
steps. When that lower limit is reached, the algorithm tries to reach an accepting
state without exceeding an upper limit maxSteps on the number of actions. The
InvokeStepper method uses a worker thread to invoke the stepper to do the given
action in the implementation. This thread calls the DoAction method in the stepper
(see Figure 12.4, for example). If the invocation does not return within a time limit
given by the timeout option of ct, then the test run fails.
12.5.3 Classification of actions
Actions are generally divided into controllable and observable ones. The option
observableAction of ct is used to declare that an action symbol is observable.
Moreover, controllable actions can be classified as being internal, using the
internalAction option of ct. Furthermore, controllable actions can also be marked
as being cleanup actions, using the cleanupAction option of ct.
Controllable and observable actions. In a closed system it is generally assumed
that all actions are controlled by the tester; that is, all action symbols are assumed to
be controllable. Controllable action symbols are also called tester action symbols.
A tester action or a controllable action is an action that has a controllable action
symbol. An exception to this general rule are finish action symbols that are typically
classified as being observable because they are returned by the stepper. An
action with an observable action symbol is called an observable action. The general
treatment of observable actions is deferred to Chapter 16.
In the case of the bag example the action symbols Lookup Finish and Count
Finish are observable because the corresponding actions are returned by the stepper.
Notice, however, that even if the finish actions are considered as controllable
actions, the algorithm in Figure 12.7 will work in exactly the same way, because
the strategy cannot be in a state where SelecAction is called and a finish action is
enabled, unless the stepper is faulty and returns null when a finish action should be
returned.
Internal actions. Tester actions are generally divided into two categories: the actions
that are related to an operation that the IUT must perform, and the actions
that are not related to any such operation. The latter case arises when the action is
used for making internal choices in the model or scenario. Such actions are called
internal. An internal action is not executed in the stepper, which can be seen in the
algorithm in Figure 12.7. A typical case where internal actions arise are in selecting
a test case in a test suite. Suppose, for example, that you have the following two test
cases for the bag example. The first test case is to add one "b" and then delete one
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