2 Why We Need Model-Based Testing

212 Testing Systems with Complex State
switch (a.FunctionSymbol.Name)
{
case ("CheckView") :
Set modelView =
(Set)CompoundValue.InterpretTerm(a.Arguments[0]);
Set implView = new Set(bag.table.Keys);
if (!modelView.Equals(implView))
throw new Exception("Inconsistent views of state: model:" +
modelView + " iut:" + implView);
return null;
case ...
}
Scenarios. Probes are usually checked more frequently than other actions. For
the bag model, we can express this using an FSM scenario called ProbeScenario.
It requires that each Add action and each Delete action must be followed by a
CheckView action. In the same scenario we also disable all lookup and count actions.
FSM(0,AcceptingStates(0),
Transitions(t(0,Add(),1),t(0,Delete(),1),t(1,CheckView(),0)),
Vocabulary("Lookup_Start","Count_Start"))
The following additional FSM scenario called AddDeleteScenario requires that
all Add actions happen before all Delete actions:
FSM(0,AcceptingStates(0,1),
Transitions(t(0,Add(),0),t(0,Delete(),1),t(1,Delete(),1)))
In general, it may be useful to have a whole collection of FSM scenarios stored
in separate files and experiment with various combinations of them using the fsm
option of ct, without having to create factory methods for all of the combinations.
Running ct. Let us add another factory method CreateModelWithProbe to the
Factory class in BagModel that constrains the bag model by including the ElementRestriction
and the Probe features.
public static ModelProgram CreateModelWithProbe()
{
return new LibraryModelProgram(typeof(Contract).Assembly,
"BagModel", new Set("ElementRestriction","Probe"));
}
Figure 12.6 shows a partially explored part of the state machine of CreateModelWithProbe
composed with ProbeScenario and AddDeleteScenario, using mpv.
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