2 Why We Need Model-Based Testing

190 Analyzing Systems with Complex State
grouping, then the branching is eliminated. Instead, we explore an action only if it
leads to a state with a Count value that has not previously been seen. The idea in this
example is that the size of the stack is the interesting property and that the particular
choice of elements may be ignored for the purposes of analysis.
The mpv tool supports state grouping over state property state-property with the
/group:state-property switch. More than one /group switch may be supplied. If
grouping is specified then each state must produce a new value for at least one of
the given state properties in order to be included in the exploration.
11.3 Sampling
Up to this point we have discussed pruning techniques that eliminate some transitions
from an exhaustive search. In effect, we are “cutting away” (pruning) choices.
Another category of techniques is sampling. Here we focus on selecting a desirable
path to explore, rather than excluding undesired transitions as with pruning.
Exploration via sampling is a stochastic algorithm because it combines random and
directed aspects.
We present exploration via sampling in Chapter 12 in the context of testing.
However, stochastic techniques may be appropriate for design analysis in some
cases where the state space is large.
11.4 Exercises
1. Use the Domain attributes and StateFilter attributes in a feature that makes the
revision control system a finite model program. Which state variables must be
mentioned in your state filter expressions? Which action parameters must be
given finite domains?
2. Use a Domain attribute in a feature to limit the number of users of the revision
control system to two. Then, use state grouping to create two properties that represent
the state of each client. Can you use these properties with state groupings
to eliminate some interleavings? You will also need a property that represents
the state of the repository.
3. Create a scenario for the state repository system that shows all of the ways that
Revert can affect the behavior of the system for one user.
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