How Far Must Design Go?

Design must define only enough of the system so that it can be implemented unambiguously. What constitutes "enough" varies from project to project and company to company. In some cases, the design may be elaborated to the point that the system can be implemented through a direct and systematic transformation of the design into code.

In other cases, the design resembles a sketch, elaborated only far enough to ensure that the implementer can produce a set of components that satisfy the requirements. The degree of specification therefore varies with the expertise of the implementer, the complexity of the design, and the risk that the design might be misconstrued.

When the design is specified precisely, the code can be tied closely to the design and can be kept synchronized with the design in what we call round-trip engineering, thereby avoiding one transformation step and a potential source of error.

When the degree of completeness and precision in the design is very high ”so high that you can execute the transformation directly by interpreting the design or by rapidly generating from it small amounts of code ”the transformation is almost invisible to the designer, and the design appears to be "executable."