Models of Software Maintenance (Corrective, Adaptive, Perfective, Preventive)

After delivery, software undergoes changes. Software maintenance is traditionally classified into four types (per IEEE and Pressman/Sommerville):

1. Corrective Maintenance

• What: fixing errors/defects (bugs) discovered in the software after it has been delivered.
• Purpose: restore the software to its intended, correct behaviour.
• Examples: fixing a crash when a specific input is entered; correcting a wrong calculation; patching a security vulnerability.

2. Adaptive Maintenance

• What: modifying the software to respond to changes in its environment — new operating systems, new hardware, new compilers, changed regulations, or new external interfaces.
• Purpose: keep the software working in a changed external environment.
• Examples: porting the application to run on a new Windows/Linux version; updating to support a new database server or browser; changing tax calculations for new government rules.

3. Perfective Maintenance

• What: improving/enhancing the software — adding new features, improving performance, usability, or maintainability — to meet new or changed user needs.
• Purpose: make the software better from the user's perspective (not fixing bugs).
• Examples: adding a new reporting feature; optimizing the response time of a query; redesigning a user interface for better usability.

4. Preventive Maintenance

• What: preventing future problems by making changes that improve the software's maintainability and reliability without changing its external functionality in the short term.
• Purpose: reduce the risk of future failures and lower future maintenance cost (like servicing a machine before it breaks).
• Examples: refactoring complex code; updating comments/documentation; adding comments, restructuring modules, applying better error handling — often using reverse/forward engineering for re-engineering.

Summary table

Type	Trigger	Activity	Example
Corrective	Bugs/defects	Fix errors	Crash fix, wrong output fix
Adaptive	Environment change	Adjust to new environment	Port to new OS, new DB
Perfective	User needs	Enhance/improve	Add features, boost performance
Preventive	Future risk	Re-engineer/harden	Refactor, improve docs, code restructuring

Note: reports on maintenance distribution (Lientz & Swanson) show perfective (~50%) and adaptive (~25%) dominate, while corrective is often overestimated; preventive is the smallest but pays off over the long term.