2.6 Software maintenance  (Page 3/10)

Majority of maintenance costs

Maintenance consumes a major share of software life cycle financial resources. A common perception of software maintenance is that it merely fixes faults. However, studies and surveys over the years have indicated that the majority, over 80%, of the software maintenance effort is used for non-corrective actions. Jones describes the way in which software maintenance managers often group enhancements and corrections together in their management reports. This inclusion of enhancement requests with problem reports contributes to some of the misconceptions regarding the high cost of corrections. Understanding the categories of software maintenance helps to understand the structure of software maintenance costs. Also, understanding the factors that influence the maintainability of a system can help to contain costs. Pfleeger presents some of the technical and non-technical factors affecting software maintenance costs, as follows:

• Application type
• Software novelty
• Software maintenance staff availability
• Software life span
• Hardware characteristics
• Quality of software design, construction, documentation and testing

Evolution of software

Lehman first addressed software maintenance and evolution of systems in 1969. Over a period of twenty years, his research led to the formulation of eight “Laws of Evolution”. Key findings include the fact that maintenance is evolutionary developments, and that maintenance decisions are aided by understanding what happens to systems (and software) over time. Others state that maintenance is continued development, except that there is an extra input (or constraint)–existing large software is never complete and continues to evolve. As it evolves, it grows more complex unless some action is taken to reduce this complexity.

Since software demonstrates regular behavior and trends, these can be measured. Attempts to develop predictive models to estimate maintenance effort have been made, and, as a result, useful management tools have been developed.

Categories of maintenance

Maintenance consists of four parts:

• Corrective maintenance: Reactive modification of a software product performed after delivery to correct discovered problems. It deals with fixing bugs in the code.
• Adaptive maintenance: Modification of a software product performed after delivery to keep a software product usable in a changed or changing environment. It deals with adapting the software to new environments.
• Perfective maintenance: Modification of a software product after delivery to improve performance or maintainability. It deals with updating the software according to changes in user requirements.
• Preventive maintenance: Modification of a software product after delivery to detect and correct latent faults in the software product before they become effective faults. It deals with updating documentation and making the software more maintainable.

All changes to the system can be characterized by these four types of maintenance. Corrective maintenance is ‘traditional maintenance’ while the other types are considered as ‘software evolution’.