2 Why We Need Model-Based Testing

14 Compositional
Modeling
We have seen many different uses of composition in the previous chapters. In this
chapter we are going to take a closer look at the use of composition as a general
modeling technique to break down larger models into smaller models.
We are going to look at an example that illustrates how this technique can be
applied to real-life complex application-level network protocols. Such protocols are
abundant. Modern software architectures rely heavily on the fact that two parties
that need to communicate, for example a client and a server, or two servers, do so
by using a well-defined (application-level network) protocol.
We then discuss some of the main properties of model program composition and
provide a summary of the various uses of model program composition.
14.1 Modeling protocol features
A real-life protocol can be intrinsically complex. There are several reasons for this.
A protocol typically has multiple layers and depends on or uses other protocols.
In a good protocol design, internal details of the underlying protocols should not
leak out and the layering principle should be maintained. Another reason is that a
protocol typically includes many different features within a single layer. Intuitively,
a feature is a part or an aspect of the overall functionality of the protocol. Features
interact and together define the protocol as a whole.
An important property of protocols is that enough information is present in
messages, usually in message headers, so that it is possible to maintain a consistent
view of the state of the protocol on both sides of the protocol users.
In the protocol model in this chapter, the actions of the model program represent
the messages of the protocol. A run of the model program represents a session where
client and server exchange messages, as shown in Figure 14.18. The steps we follow
in the feature-oriented modeling of a protocol are the following:
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