2 Why We Need Model-Based Testing

Systems with Finite Models 63
conditions, which allow the sequence of method calls to be chosen at run time by
the tool that executes the model program, or by the environment where the model
program runs.
5.3.4 Select the actions
It is often easier to select the actions first, because we can identify them by simply
observing the behavior. Recall that we choose a high level of behavioral abstraction
where each action is the selection of an option that causes a new screen to appear.
We select these actions:
The SelectMessages and SelectTopics actions select those pages; these are the
actions that occur when users click on a topic title or the group name, respectively.
ShowTitles and ShowText select the details shown on the topics page. SortByFirst
and SortByMostRecent select the sort order for the topics page when the titles only
are shown.
The name of an action is called an action symbol. The entire collection of action
symbols used by a model program is called its action vocabulary or just its
vocabulary.
5.3.5 Write sample traces
Now that we have identified the actions, we can describe the behavior. Drawing on
our observations and understanding of how the system is supposed to behave, we
write down some traces that the system can execute and that the model program
must also be able to execute. Each trace is simply a sequence of (some of) the actions
from the vocabulary we identified. Here is a sample trace:
ShowTitles(); SortByFirst(); SortByMostRecent(); ShowText();
Not all sequences are allowed, because not all actions are available or enabled in
each state. It is also useful to write down some traces that the model program must
not be able to execute. For example:
ShowTitles(); SortByFirst(); SortByFirst(); ShowText();
The system cannot execute this trace because there are two SortByFirst actions in
succession. After the first, this action is no longer enabled.
It is helpful to refer to these sample traces as we continue the preliminary analysis
and then code the model program. It can also be helpful at this stage to make a table
or a diagram that summarizes many traces, as we show in Chapter 6.
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