58.

Implementations Are Not Specifications

Like it or not, a great deal of industrial software is implemented without a discernible specification. If and when the software is completed, the implementation is then presented as the specification. Whatever behavior the software exhibits is said to be the specified behavior. Some poor souls might argue that this is a good approach, since it doesn't bog the developers down with working out some sort of formal plan that is bound to change anyway. But, while it is true that project specifications often change, an implementation makes for a lousy specification in several respects:

• Many of the choices made in an implementation are arbitrary. Thus, a team that wishes to implement the system on another platform has nothing to go on but the existing implementation. The developers will have to wade through numerous implementation details to determine the behavior that the implementation entails. It is much easier to determine such behavior when it is specified at a higher level of abstraction.

• You cannot define a bug. If an implementation is literally taken as its own specification, then, as in the case where there is no specification at all, it is impossible to identify any behavior as a bug!

• Initial developers have no model of behavior. Obviously, an implementation cannot serve as the specification for the initial developers, since no such implementation yet exists. These developers must rely on some model of behavior for the system they're creating. But the source of this model should then serve as the software's specification.

This last point sheds some light on what sort of a specification a developer might use with reasonable cost. While it's true that developers must have some mental model of the feature they are implementing, they needn't have a mental model of the entire application.

In other words, specifications can be developed a piece at a time. Not only does this make them more tractable, but it also allows them to be modified more efficiently as the customers' needs change.