2 Why We Need Model-Based Testing

50 Why We Need Model-Based Analysis
4
Command()
7
1
0
Command()
Timeout()
2
3
Reset()
Message("99.9") TimeoutMsgLate()
Figure 3.10. Portion of controller FSM showing deadlock.
5
Message("999.9")
6
TimeoutMsgLost()
8
19
20
21
NoHandler()
Command()
NoHandler() Command()
For liveness analysis, the tool can search for dead states from which the controller
cannot reach any accepting states where the program’s goals have been achieved.
The most obvious dead states are deadlock states: the program stops, as in the
LostMessageWhenIdle run (Figure 3.10). The deadlock is indicated here by the loop
of alternating Command and NoHandler actions, from which there is no escape. The
supervisor can always command the controller, so the Command action is always
enabled. But the controller can do nothing; recall that NoHandler is a dummy action
that indicates no handlers are enabled. Other dead states belong to livelocks where
the program cycles without making progress. In this FSM, we find the same cycle
of actions with no escape that we found in the InitialOutOfRangeMessage run:
Timeout, Reset, Message, CheckMessage (Figure 3.11).
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