2 Why We Need Model-Based Testing

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6 Exploring and
Analyzing Finite
Model Programs
In this chapter we introduce exploration, our primary technique for analyzing model
programs. Exploration generates a finite state machine (FSM) from a model program.
The FSM can then be used for visualization, analysis, and offline test generation.
In this chapter we show how model-based analysis reveals the design errors in
the reactive system we discussed in Chapter 3, by exploring the model program we
developed in Chapter 5, Section 5.7. We explain and demonstrate safety analysis
that identifies unsafe (forbidden) states and liveness analysis that identifies dead
states from which goals cannot be reached. Dead states indicate deadlocks (where
the program seems to stop running and stops responding to events) or livelocks
(where the program keeps running but can’t make progress).
In this chapter we also introduce the mpv (Model Program Viewer) tool for
visualization and analysis, and explain how to use the modeling library features that
support analysis.
6.1 Finite state machines
In this section we motivate and explain FSMs.
Simulation is the most limited and labor-intensive model-based analysis technique
because it only considers one run at a time (Chapter 5, Section 5.5). To
perform more thorough analyses – to detect the design errors discussed in Chapter
3, for example – we need to consider many different runs. The obvious way
would be to code a large number of runs, but this is not a practical solution. In order
to get good coverage of program behaviors, we would usually have code a great many
runs, and some would be very long. But the collection of runs would be redundant.
The same sequences of actions would appear again and again in different runs, and
even within a single run. We use a better representation of behaviors that removes
the redundancy, representing a great many runs in a compact way: the FSM. Exploration
automatically generates an FSM from a model program.
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