0"), but the client only fails when it receives the message after the first
message with four characters. At least two messages must be sent and received to
expose the defect (where the first message contains more than four characters). Each
of the test cases in the first test suite only send and receive one message, so they
cannot expose the defect.
The single test case in the second test suite does expose the defect, because
two messages are sent and received, and the first message contains more than four
characters.
148 Testing Closed Systems
0 1
A()
2
C()
D()
B()
Figure 8.6. Transition coverage versus path coverage.
The number of messages sent by each test case is a consequence of the traversal
algorithm used by the offline test generator tool, otg. The postman tour algorithm
achieves full transition coverage: every transition in the FSM is taken at least once.
The postman tour is minimal: the total number of transitions in the test suite is the
minumum needed to achieve transition coverage; the algorithm avoids traversing
the same transition more than once, if that is possible. This explains the shapes of
the test suites displayed in Figures 8.1 and 8.2. (In the second test suite, it is pure
luck that the longer message happens to be sent first.)
These examples confirm that minimal transition coverage is not sufficient to
expose some defects.
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