2 Why We Need Model-Based Testing

8 Testing Closed
Systems
In this chapter we introduce model-based testing. We test the client/server implementation
we described in Chapter 2, and reveal the defect we discussed there. We
generate the test suite and check the results with the model program we developed in
Chapter 5, Section 5.6. We introduce our test generator tool otg and our test runner
tool ct, and show how to write a test harness that connects an implementation to the
ct tool.
In this chapter we describe offline test generation, where test suites are generated
before running the tests. Offline test generation works best with finite model programs
that can be explored completely. In Chapter 12 we introduce on-the-fly testing,
where the test case is generated as the test executes. On-the-fly testing is an attractive
alternative for “infinite” model programs that cannot be explored completely.
In this chapter we test closed systems where the tester controls every action.
In Chapter 16 we test reactive systems, where some actions are invoked by the
environment.
8.1 Offline test generation
The offline test generator otg explores a model program to create a finite state
machine (FSM), traverses paths through the FSM, and saves the paths in a file so
they can be used later. The paths define a collection of program runs. The collection
is called a test suite; each run in the collection is a test case. Later, the implementation
can execute the test suite, under the control of a test runner. 1
The otg tool has one traversal algorithm built in. The algorithm eliminates all
paths to dead states, then traverses the remaining paths using a postman tour that
covers the entire graph of the FSM in the minimum number of steps, visiting each
state and taking each transition at least once.
1 A complete command reference for otg appears in Appendix B.2.
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