2 Why We Need Model-Based Testing

132 Structuring Model Programs with Features and Composition
this way: the calibrate action should not follow the arrival of a message containing
an out-of-range temperature. And, the liveness requirement is: the calibrate action
should follow the arrival of a command. A property expressed in this way, by
describing the ordering of actions, is a temporal property.
Scenario model programs can express temporal properties, and composition can
check them. Compose the contract model program with the scenario model program
that expresses the property. Examine the projection of the product onto the scenario.
If the projection is complete (it contains all of the transitions of the original scenario
model program), then the contract model program can execute that scenario, and
you will be able to find the runs of the scenario in the product. If the projection is
incomplete (if it is missing some transitions), then the contract model program cannot
execute that scenario, and you will not find any runs of the scenario in the product.
Notice how this use of composition differs from scenario control, where we write
a scenario that we want the contract model program to execute, that we assume it
can execute. For analysis, we write a scenario that the contract model program may
not be able to execute; that is what we want to find out.
Let’s look at some examples. First, we consider an example where the scenario
model program describes exactly one run. The analysis tells us whether the contract
model program can execute that run; it provides the same information as simulation
(Chapter 5, Section 5.5) or interactive exploration (Chapter 6, Section 6.2.3), but
may be easier to set up and repeat.
To make this easy to understand, we return to the simple news reader example. Let
us see what happens when the contract model program cannot execute the scenario.
We choose the run that we simulated in Chapter 5:
ShowTitles(); SortByFirst(); SortByFirst(); ShowText();
We encode this run as a scenario in an FSM text file with these contents:
FSM(0, AcceptingStates(), Transitions(
t(0,ShowTitles(),1), t(1,SortByFirst(),2), t(2,SortByFirst(),3),
t(3,ShowText(),4)),
Vocabulary("ShowTitles","ShowText","SelectMessages","SelectTopics",
"SortByFirst","SortByMostRecent"))
This FSM text file contains an entry at the end that defines the vocabulary of the
scenario machine, the names of all its actions. If this entry is omitted, the vocabulary
is understood to be the actions that appear in the transitions. Here we need to list the
vocabulary because it includes actions that do not appear in any of the transitions
of this particular scenario. When we use composition to check whether the contract
model program can execute a particular run, we must include the whole vocabulary
of the contract model program in the scenario model program. In the product, any
actions of the contract model program that do not appear in the scenario vocabulary
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