2 Why We Need Model-Based Testing

230 Compositional Modeling
FSM(0, AcceptingStates(0),
Transitions(t(0,ReqSetup(0,2),0),t(0,ReqSetup(1,2),0),
t(0,ResSetup(0,0),0),t(0,ResSetup(0,1),0),t(0,ResSetup(0,2),0),
t(0,ResSetup(1,0),0),t(0,ResSetup(1,1),0),t(0,ResSetup(1,2),0)),
Vocabulary("ReqWork", "ResWork"))
Figure 14.4. Another scenario,CreditsScenario2, for the Credits model program. Message IDs are 0
and 1. Requests ask for two credits. Responses grant up to two credits.
ReqSetup(0, 2)
0 1
ResSetup(0, 2)
ResSetup(0, 1)
2
ReqSetup(1, 2)
ReqSetup(1, 2)
3 5
4
ResSetup(1, 2)
ResSetup(1, 0)
ResSetup(1, 1)
ResSetup(1, 1)
ResSetup(1, 2)
Figure 14.5. Composition of the scenario in Figure 14.4 with the amended credits model program.
Let us consider another scenario that allows two message IDs to be used and
assumes that the client always requests two credits, whereas the server grants at
most two credits (see Figure 14.4). The full exploration of the composition of this
scenario with the amended credits model program is shown in Figure 14.5.
14.2.2 Message parameters
It is useful to identify those parameter domains of actions that have finite domains
that can be derived from the model program state. These domains can be used
to make the model program explorable. Recall that for a model program to be
explorable, all action parameters must have finite domains. Looking at the credits
model program for example, we can see that all responses from the server must
have message identifiers present in the requests map. Also, all granted credits from
the server must be between zero and the maximum credit requested by the client.
We add these derived parameter domains as another feature model program to SP
(see Figure 14.6).
Notice that this feature has no state variables of its own. It is a dependent feature
of the credits feature model program. We could also have annotated the credits
feature model program in Figure 14.1 directly, but it is useful to keep the dependent
feature separate for readability and for later use in compositions. In some cases the
dependent feature is not needed and can be omitted.
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