2 Why We Need Model-Based Testing

Systems with Finite Models 145
Here is an example from Figure 8.4, the case where DoAction handles the server’s
Bind action. Here the socket s and the host and port have been declared and assigned
in the stepper.
case("ServerBind"):
s.Bind(host,port);
return null;
Here is a more complex case, where DoAction handles the server’s Send action.
This code extracts the argument from the action term using the indexer expression
a[0], casts it to double, and passes it to the implementation. The modeling library
provides action arguments through a C# indexer, which accesses elements by subscripts.
(Appendix A.3 explains this and other facilities for working with action
terms.)
case("ServerSend"):
// s.Send sends a double, not a string
s.Send((double) action[0]);
return null;
The most complex cases occur where there is a split action (that has outputs).
The test runner passes DoAction a term that represents the start action. DoAction
invokes the corresponding method in the implementation. When that method returns,
DoAction collects the output, constructs a term that represents the finish action using
Action.Create, and returns the finish action to the test runner. Here is an example,
the case where DoAction handles the client’s Receive action:
case("ClientReceive_Start"):
// c.Receive returns a double, not a string
return Action.Create("ClientReceive_Finish", c.Receive());
The DoAction code could also read implementation state variables, and include
their values in the term for the finish action. To make use of this information, the
corresponding action method of the model would have to return the values of model
state variables in out parameters.
The test runner indicates a test failure if the finish action returned by DoAction
does not match the finish action generated by the model program (e.g., if the
implementation returns a different value than the model program computed). That
is, how the model program acts as a test oracle.
The test runner also indicates a test failure if DoAction throws an exception. For
this reason, DoAction usually does not handle exceptions thrown by the implementation.
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