2 Why We Need Model-Based Testing

Systems with Finite Models 73
exceptions are thrown. Therefore, certain sequences of method calls are forbidden.
If the client attempts to connect before the server is listening on its socket, the client
throws an exception. If one partner attempts to receive before the other partner
sends, the receive method will block and never return. The model program must not
generate runs like these.
The runs that the implementation can execute depend on its configuration. If
the client and server run in separate threads, then one partner could call its receive
method before the other partner sends. A different model program would be needed
to model that configuration. Selecting the configuration can be an important step in
preliminary analysis.
The challenges in this example are to write a model program that generates all
the runs we want but none that are forbidden, and that generates enough different
temperatures to expose the defect while keeping the model finite.
Features
We will model all of the features of the implementation client and server classes
(Chapter 2, Section 2.4). Usually, a model program only represents some of the
features of the implementation. But for this example we selected a very small
implementation, so we could show the complete implentation and write a complete
model program.
Actions
We plan to generate test runs that call the methods of the implementation, so there
must be an action in the model for each public method in the implementation. The
client and server actions are distinct; the client’s send is a different action from the
server’s send, so we have actions ClientSend and ServerSend, and so on.
Traces
We write some sample traces to make sure that the actions we chose can express the
behaviors we wish to model. There are many examples in Chapter 2. For example,
the run discussed in Section 2.8, which is coded in the test method in Figure 2.10,
is expressed by this trace:
ServerSocket();
ServerBind();
ServerListen();
ClientSocket();
ClientConnect();
ServerAccept();
ClientSend();
ServerReceive();
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