Therefore, defects in these first products
might not be discovered until much later, after code has been written from them.
The diagram shows how the costs of rework escalate as defects are discovered later
in the project.
It would be better if we could check each product as soon as it is produced. We
would like to check and correct the specification and design as we work down the
left side of the V, before we code the units. Then, soon after the unit tests pass, the
integrated system should just work ??“ with few unpleasant surprises.
Something like this is already being tried at the unit level (down at the point of
the V). In test-driven development, developers write each unit test case before its
code, and execute it as they code. When the code for each unit is completed, it has
already passed its unit tests.
Now it is possible to apply the same principle of immediate feedback to other
development products. Analysis with model programs can check specifications and
designs, much like unit tests check code, so problems can be detected and fixed
immediately (Figure 1.2).
Analyzing and testing models is common in many branches of engineering, where
builders cannot depend on testing the end product in order to expose major defects.
No one plans to crash an airplane or collapse a bridge! Instead, engineers create
mathematical models ??“ such as block diagrams for circuits and control systems, or
10 Describe, Analyze, Test
Unit testing
Model-based component testing
Model-based system testing
Delivery
Code with
unit tests
Design with
modeling and analysis
Speci?¬?cation with
modeling and analysis
Concept
Rework
Rework
Re.
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