2 Why We Need Model-Based Testing

Systems with Finite Models 121
This code is inflexible. It is often better to separate the domains from the contract
model program, so the same model program can be configured with different domains
for different scenarios. Figure 7.5 shows how to use features to achieve this.
In the contract model program, code the action method without a domain. Then, in a
feature, code an action method with the same name, but include the domain. Leave
the body of this action method empty. To select that domain, include that feature.
When the combined action executes, the domain from the selected feature is used,
and the body of the action method in the contract model program executes with
a parameter value from that domain. (In this example the feature has no enabling
conditions; the ones in the contract model program are sufficient.) Create different
features that use different domains and include the one that generates the scenario
you want. You must include one of these features; a model program cannot be
explored (is not explorable) if there are parameters without domains.
7.3 Composition
Composition combines separate model programs into a new model program called
the product. The product formed by composing model programs M1 and M2 is
written M1 × M2 or M1∗M2. Composition is performed automatically by the analysis
and testing tools when multiple model programs are named on the command line;
the tools can then analyze or test from the product. You can also do composition
in programs you write by calling the library. Composition can be used to build up
complex model programs by combining simpler ones (as we shall see in Chapter 14).
In this chapter we use it for scenario control.
The next section explains how composition is done. The section after that describes
some additional styles for coding model programs that make it easy to write
scenarios for composition. In Section 7.3.3 we will return to using composition for
scenario control.
7.3.1 Understanding composition
Recall that features are classes in the same model program (the same namespace),
which may use the same state variables and are compiled into the same assembly.
In contrast, we compose separate model programs, which can be in separate namespaces
and can be compiled into separate assemblies. All of the tools accept any
number of model programs; they form the product of all of them. No special coding
is needed to prepare model programs for composition (no attributes, etc.). Any two
(or more) model programs can be composed (although the product is not always
useful, of course).
The product of two or more model programs has all the state variables and all
the actions of each of the composed programs. The key idea in composition is that
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