Maintenance is what happens after a software product has been delivered.Discussions of software methodology tend to focus on the development phase.so do introductory programming courses. But it is widely estimated that seventy percentage of the cost of software is devoted to maintenance. No study of software quality can be satisfactory if it neglects this aspect.what is not acceptable is to have the knowledge of the exact length of the data plastered all across the program, so that changing that length will cause program changes of a magnitude out of proportion with the conceptual size of the specification change.
A minute’s reflection shows this term to be a misnomer.A software product does not wear out from repeated usage, and thus need not be “maintained” the way a car or a TV set does. In fact, the word is used by software people to describe some noble and some not so noble activities. The noble part is modification.As the specifications of computer systems change, reflecting changes in the external world, so must the systems themselves. The less noble part is late debugging.Removing errors that should never have been there in the first place.
More than two-fifths of the cost is devoted to user-requested extensions and modifications. This is what was called above the noble part of maintenance, which is also the inevitable part. The unanswered question is how much of the overall effort the industry could spare if it built its software from the start with more concern for extendibility. We may legitimately expect object technology to help.
This is inevitable since the data must eventually be accessed for internal handling. But with traditional design techniques this knowledge is spread out over too many parts of the system, causing unjustifiably large program changes if some of the physical structure changes as it inevitably will.In other words, if postal codes go from five to nine digits,or dates require one more digit, it is reasonable to expect that a program manipulating the codes or the dates will need to be adapted.
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