2 Why We Need Model-Based Testing

226 Compositional Modeling
a lot of implementation freedom in the way that the server can grant credits and in
the way that the client can ask for new credits or use the available message IDs.
This is an example of a sliding window protocol. Here the pool of identifiers is
the “window” whose contents change or “slide” as time progresses. However, the
pool is not necessarily a consecutive interval of numbers because the client does not
have to pick the numbers from the pool in any particular order.
This feature is defined uniformly for all of the commands that are communicated
between the server and the client, except for Cancel.
State variables
The model program of the credits feature is shown in Figure 14.1. The model program
has three state variables: window is the set of all message IDs that the client may use
to send new requests to the server, requests is a map containing all the outstanding
credit requests with message IDs as keys, and maxId is the largest ID that has been
granted by the server. In the initial state of the model program the only possible
message ID is 0, the maximum ID is also 0, and there are no pending requests.
Actions
The action vocabulary of the credits model program consists of the action symbols
ReqSetup, ReqWork, ResSetup, and ResWork. The action symbol Cancel is not in the
vocabulary.
In order to use features and composition most effectively, we treat the relation between
actions and methods more freely here than in previous examples. The actions
(ResSetup etc.) which were defined at the beginning of Section 14.2 are distinct
from the methods (Res etc.) in the code in Figure 14.1; they have different names
and different parameters. It is the actions that appear in the traces (Figure 14.18
etc.), not the methods.
Each action is associated with its method (ResSetup with Res etc.) by labeling
the method with an Action attribute whose argument indicates the action names and
parameters, following the rules given in Appendix A. Two or more actions may use
the same update rule (that is why actions and methods have different names here). For
example, the setup response and the work response actions ResSetup and ResWork
both use the update rule Res. Furthermore, these actions have three parameters but
the update rule only has two. An underscore in the attribute indicates that the
action’s third parameter is not used by this update rule. These same two actions are
associated with a different update rule (also named Res) in the cancellation model
program (Figure 14.7). In that rule, it is the action’s second parameter which is not
used.
In the product model program which is formed by composition, the actions of
the separate model programs are combined. When a combined action executes, all
of its update rules execute and all of its parameters are used. It is the actions (the
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