2 Why We Need Model-Based Testing

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Systems with Finite Models 133
Figure 7.16. Newsreader scenario FSM for analysis by composition.
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Figure 7.17. News reader scenario FSM, incomplete projection after composition.
might interleave anywhere among the actions of the scenario. Defining the scenario
vocabulary as we do here prevents these unwanted interleavings from confusing the
analysis.
Figure 7.16 shows the FSM for this scenario. We compose this with the contract
model program (whose FSM is shown in Chapter 6, Figure 6.2). Figure 7.17 shows
the projection of the product onto the scenario. The projection is incomplete; it only
includes the first two transitions of the scenario. This shows that the contract model
program cannot execute this scenario, because the third transition of the scenario is
not enabled in the contract model program. It was at this point that the simulation
failed (Chapter 5, Section 5.5). The FSM of the product shows the same thing, but
the product sometimes contains additional transitions that make it harder to interpret,
so we recommend inspecting the projection for this analysis.
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