2 Why We Need Model-Based Testing

138 Testing Closed Systems
Instead of explaining the traversal algorithm here, we simply exhibit the results. 2
Figure 8.1 shows the test suite generated by otg from the client/server contract model
program developed in Chapter 5, Section 5.6, whose true FSM is shown in Chapter 7,
Figure 7.1. This test suite was generated and stored in the file ContractTest.txt by
the command:
otg /r:ClientServer.dll ClientServer.Factory.Create ˆ
/file:ContractTest.txt
This test suite is itself an FSM; in fact, Figure 8.1 was created by this mpv commmand:
mpv /testSuite:ContractTest.txt
It is clear from Figure 8.1 that the test suite describes a collection of six sequences.
Each sequence is a test case. The ct tool can interpret the contents of the test suite file,
and, when harnessed to the implementation, can execute and check all the test cases.
The test suite in Figure 8.1 describes many similar runs because it was generated
from the true FSM of the contract model program, without using any scenario
control. It is likely that a smaller test suite would be as effective in detecting defects,
for the reasons explained in Chapter 7, Section 7.1.
We can generate a smaller test suite by using scenario control. Let us generate a
test suite from the client/server contract model program composed with the scenario
machine developed in Chapter 7, Section 7.3.3. The FSM of the composition is
shown in Figure 7.15. This command generates the test suite.
otg /r:ClientServer.dll ClientServer.Factory.Create ˆ
/fsm:Scenario.txt ˆ
/file:ScenarioTest.txt
Figure 8.2 shows the generated test suite. There is just a single run, because all
the states and transitions in the FSM of the composition can be visited by traversing
a single path from the initial state to the accepting state, including each of the loops
in the middle of the FSM (Figure 7.15 in Chapter 7).
Other traversal algorithms are possible. In addition to the postman tour, modelbased
offline test generators sometimes provide random traversals, traversals that
cover the shortest path to an accepting state (or some other interesting state), or
traversals that achieve transition coverage but with an upper limit on the length of
each test case (which generates more runs, but shorter runs, than does the postman
tour). At the time of this writing, the otg tool provides none of these, but the modeling
library could be used to write a custom offline test generator.
2 The traversal algorithm is explained in several references cited in Further Reading (Chapter 9).
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