Sommerville-Software-Engineering-10ed

280 Chapter 9 ■ Software evolution
Key Points
■
■
■
■
■
■
■
■
■
Software development and evolution can be thought of as an integrated, iterative process that
can be represented using a spiral model.
For custom systems, the costs of software maintenance usually exceed the software development
costs.
The process of software evolution is driven by requests for changes and includes change impact
analysis, release planning, and change implementation.
Legacy systems are older software systems, developed using obsolete software and hardware
technologies, that remain useful for a business.
It is often cheaper and less risky to maintain a legacy system than to develop a replacement system
using modern technology.
The business value of a legacy system and the quality of the application software and its environment
should be assessed to determine whether a system should be replaced, transformed,
or maintained.
There are three types of software maintenance, namely, bug fixing, modifying software to work
in a new environment, and implementing new or changed requirements.
Software reengineering is concerned with restructuring and redocumenting software to make it
easier to understand and change.
Refactoring, making small program changes that preserve functionality, can be thought of as
preventative maintenance.
Further Reading
Working Effectively with Legacy Code. Solid practical advice on the problems and difficulties of
dealing with legacy systems. (M. Feathers, 2004, John Wiley & Sons).
“The Economics of Software Maintenance in the 21st Century.” This article is a general introduction
to maintenance and a comprehensive discussion of maintenance costs. Jones discusses the factors
that affect maintenance costs and suggests that almost 75% of the software workforce are involved
in software maintenance activities. (C. Jones, 2006) http://www.compaid.com/caiinternet/ezine/
capersjones-maintenance.pdf
“You Can’t Be Agile in Maintenance?” In spite of the title, this blog post argues that agile techniques
are appropriate for maintenance and discusses which techniques as suggested in XP can be effective.
(J. Bird, 2011) http://swreflections.blogspot.co.uk/2011/10/you-cant-be-agile-in-maintenance.html
“Software Reengineering and Testing Considerations.” This is an excellent summary white
paper of maintenance issues from a major Indian software company. (Y. Kumar and Dipti, 2012)
http://www.infosys.com/engineering-services/white-papers/Documents/software-re-engineeringprocesses.pdf