Need for Software Maintenance and Categories of Maintenance

Software maintenance is the modification of software after delivery to correct faults, improve performance or adapt it to a changed environment.

Need for maintenance

1. Defects remain after delivery: testing cannot find every fault.
2. Environment changes: new operating systems, hardware, databases, laws or tax rules.
3. New user requirements: users ask for new features or improvements once they use the system.
4. Prevent decay: repeated changes degrade the structure; software must be restructured to stay maintainable.
5. Long life of software: maintenance is typically the largest part (often 60% or more) of total life-cycle cost.

Types of maintenance

Type	Purpose	Trigger
Corrective	Fix errors/bugs found after delivery	Defects
Adaptive	Modify software to work in a changed environment	OS, hardware, platform, regulation changes
Perfective	Add or improve features and performance	New or changed user requirements
Preventive	Improve structure/readability to avoid future problems	Reduce future maintenance cost

Mapping the given changes

Cause of change	Maintenance category	Example
Defects	Corrective	Fixing a wrong tax calculation
Operating-environment changes	Adaptive	Porting the app to a new OS version or database
New user requirements	Perfective	Adding a report or a new payment option

Preventive maintenance (code cleanup, refactoring, updating documentation) applies when there is no immediate trigger but the code is becoming hard to change.

Typical effort split

Perfective is usually the largest share (about 50%), then adaptive (about 25%), corrective (about 20%) and preventive (about 5%).