Software Engineering: The Four Types Of Modifiability

The skill with which we can apply engineering methods and tools will depend on the degree to which we have a clear and precise view of our objectives. In the realm of software engineering our objectives will always be stated in terms of desired properties of the resultant software. Four properties that are sufficiently general to be accepted as goals for the entire discipline of software engineering are modifiability, efficiency, reliability, and understandability. The goal of modifiability is historically the most difficult goal to master. Modifiability implies controlled change, in which some parts or aspects remain the same while others are altered, all in-such a way that a desired new result is obtained. The characterization of "sameness"

In general, different approaches are necessary to satisfy these different types of modifiability. Modifiability requires not only the ability to have an adaptable, evolutionary design, employ standardized software building-blocks, tune for performance, etc., but also the more subtle ability to maintain project schedules and budgets by allowing dummy test modules to be used as drivers before later parts of a system are prepared, etc. The variety of ways modifiability affects software engineering is one of the reasons for giving it a primary role -in our discussion of software engineering. A much-abused goal is efficiency, usually because in an excess of zeal it is prematurely permitted a high priority in engineering trade-offs. Blatant inefficiency cannot, of course, be tolerated, but usually efficiency questions are best treated within the context of other issues. For example, achieving a high degree of modifiability can provide the basis for meeting efficiency goals during the tuning phase of software

Unlike efficiency, which frequently is prematurely applied, -reliability is more often considered too late, or not at all, in most software development efforts. Reliability can only be built in from the start; it cannot be added on at the end. Hence, reliability has a pervasive and crucial effect on software engineering practices. The final goal which should exert a strong influence in all aspects of software engineering is understandability. It depends, of course, on the intended audience: technical, management, or user. Note in particular that understandability is not merely a property of legibility. Much more importantly, the entire conceptual structure is involved. Also, in any given circumstance an acceptable level of understandability either is or is not present. There is no middle ground. Although understandability is, in a sense, a prerequisite to reliability and modifiability, it is also important as a goal in itself because it draws attention to an important barrier to understandability-complexity. Management of complexity is a crucial aspect of software engineering methods, and the need to manage complexity arises from the goal of