Software Evolution

Software must change to remain useful. Most software budget in large organizations goes to evolving existing systems, not building new ones.

• Business and technology changes drive evolution.
• Development and maintenance phases are increasingly blurred.
• Most systems evolve. Complete rewrites are rare.

Life-Cycle Stages

• Evolution
  System in operational use. New requirements proposed and implemented.
• Servicing
  Only bug fixes and environment adaptations. No new functionality.
• Phase-out
  Software may still be used but no further changes are made.

Types of Maintenance

• Fault repair
  Correcting deficiencies in how the system meets its requirements.
• Adaptive maintenance
  Changing the system to operate in a different environment.
• Perfective maintenance
  Modifying the system to satisfy new or changed requirements.

Reengineering

Restructures or rewrites part or all of a legacy system without changing its functionality.

• Applied after maintenance costs have risen.
• Uses automated tools.
• Lower risk and cost than full redevelopment.

Refactoring

Improves code structure continuously throughout development and evolution. Slows structural degradation without adding functionality.

Common bad smells that prompt refactoring:

• Duplicate code.
• Long methods.
• Switch statements.
• Data clumping.
• Speculative generality.

Legacy System Strategy