Software maintenance is required for three principal reasons. First, there may be a fault in the software, so that its behavior does not conform to its actual specification. This fault may contradict the specification, or it may demonstrate that the specification is incomplete (or possible inconsistent), so that the user's assumed specification is not sustained. Typically, the fault will be manifested itself in the form of an error when the program has been run, and fault must be removed. This is termed corrective maintenance, though colloquially it is often termed as "bug-fixing." The computing profession abounds with anecdotes of emergency repairs (patching)- these can cause great difficulties for subsequent maintenance work.
Even if a software system is fault-free, the environment in which it operates will often be subject to change. The manufacturer may introduce new versions of the operating system, or remove support for existing facilities. The software may be ported to a new environment, or to a different hardware. Modifications performed as a result of changes to the external environment are categorized as adaptive maintenance.
The third category of maintenance is called perfective maintenance. This is undertaken as a consequence of change in user requirements of the software. For example, a payroll suite may need to be altered to reflect new taxation laws; a real-time power station power station control system may need upgrading to meet new safety standards. A rule-of-thumb or heuristic often used in the industry is that around 10 percent of a software system will change each year because of modifications to the requirements.
Finally, preventive maintenance may be undertaken on a small system to anticipate future problems and make subsequent maintenance easier. For example, a particular part of a large suite may have been found to require sustained corrective maintenance over a period of time. It could be sensible to re-implement this part, using modern software engineering technology, in the expectation that the subsequent errors will be much reduced.
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