2 Why We Need Model-Based Testing

224 Compositional Modeling
1. First, the action vocabulary is identified. Typically the action vocabulary corresponds
to the set of possible value combinations of particular fields in the
message header. Consider for example two fields, one that identifies whether a
message is a Request or a Response, and another field that identifies a particular
operation that is being performed, say a Read or a Write. The actions symbols
could then be ReadRequest, ReadResponse, WriteRequest, WriteResponse.
2. Second, a mapping is defined from concrete messages “on the wire” to abstract
actions in the model program. This mapping may omit all fields of a message
that are not used in any of the features. For example, fields that represent layout
constraints on the message structure may be omitted.
3. Third, a separate model program is written for each feature. The state variables
of the feature model programs should be disjoint, so that independent analysis
of the feature model programs is possible.
4. Finally, some or all of the feature model programs, say M1,M2,...,Mk, can
be composed into a model program M1 × M2 ×···×Mk to validate feature
interactions or to be used as the contract during conformance testing.
14.2 Motivating example: a client/server protocol
We are considering a sample client/server protocol that supports various operations
on shared resources across different machines. Here we are considering a fixed client
and a fixed server. The sample protocol model is called SP here. There are two kinds
of messages, requests and responses. All requests are initiated by the client and all
responses are sent by the server. The feature model programs shown here are realistic
approximations of the corresponding features described in the document. We have
omitted certain details that are not relevant for explaining the ideas. The full protocol
contains over 300 pages of natural language specification, and the complete model
program would all in all comprise between 20 and 30 feature model programs. The
following features are considered here:
• Credits feature is a variation of the sliding window algorithm (Section 14.2.1,
below) that enables the client and the server to maintain a consistent view of
which message IDs are used.
• Cancellation feature enables prior requests from the client to be cancelled later
by the client.
• Commands feature ensures that response messages from the server match with
respect to the particular commands being requested by the client.
• Setup feature is a handshake between the client and the server that activates the
protocol.
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