2 Why We Need Model-Based Testing

64 Model Programs
5.3.6 Select the state variables
Now we can select the state variables. This is a bit more difficult because we can’t
simply observe them. We must infer them from the behavior; in particular, from the
sample traces we wrote. (In this example we don’t have access to any source code
or documentation.)
The state variables in a model program serve just three purposes: first, to determine
what runs are possible by selecting the actions that are enabled in each state; second,
to determine the data that appear during runs by storing the arguments and return
values of the actions; and third, to support safety and liveness analyses by describing
the safe states and accepting states.
We select these state variables: The page variable indicates which kind of page is
displayed, Topics or Messages. The style variable indicates the appearance of the
entries on the topics page, WithText or TitlesOnly. The sort variable indicates the
sort order of the topics page when titles only are shown: ByFirst or ByMostRecent.
Stored information that determines the sequencing of actions is called control
state; other stored information is data state. All of our state variables store control
state. For this example we chose a high level of data abstraction where we ignore
the page contents (the titles and messages). Our actions have no arguments or return
values. Therefore this model program has no data state, apart from what is already
stored in the control state.
By limiting the state variables to the control state, we ensure that the model
program is finite. In fact, it is quite small. The state space of a program is the set of
all states that it can reach. Here we have only three state variables, each with only
two values, so at most there can only be 2 × 2 × 2 or eight states in this program’s
state space. The implementation’s state space is “infinite,” because it includes the
topics and messages, which include strings, a data type that has an “infinite” number
of values. Our choice of state variables has finitized the model program. This is an
example of data abstraction.
5.4 Coding the model program
Now that we have selected the actions and state variables, we can code the model
program (Figures 5.4 and 5.5). The modeling library and tools support programs
that are coded in a few particular styles. In order to work with the tools, a model
program must be coded in one of these styles. In this chapter we introduce the style
we use for most of the examples in this book. A complete reference for writing
model programs in this style appears in Appendix A. Other styles are introduced in
Chapter 7, Section 7.3.2.
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