1. Chapter 11 Software Evolution
This chapter is extracted from Sommerville’s slides.
Text book chapter 21 1

2. Objectives
Understand that change is inevitable if software systems are to remain useful
Learn different types of software maintenance and factors that effect its costs
Understand software re-engineering. 2

3. Overview Software evolution Software maintenance Evolution process
System Re-engineering 3

4. Software change Software change is inevitable
New requirements emerge when the software is used;
The business environment changes;
Errors must be repaired;
New computers and equipment is added to the system;
The performance or reliability of the system may have to be improved.
Software development, therefore, does not stop when a system is delivered but continues throughout the lifetime of the system .

5. Importance of Evolution
Organizations have huge investments in their software systems.
To maintain the value of these assets to the business, they must be changed and updated.
The majority of the software budget in large companies is devoted to evolving existing software rather than developing new software.

6. Spiral model of evolution

7. Evolution vs maintenance
If a single organization is responsible for both the initial software development and the evolution  evolution
When the software is developed externally and the evolution
is the responsibility of the customer’s software development staff  maintenance
or an external company for system support and evolution  maintenance
Maintenance usually applies to custom software whereas evolution applies to generic software.

8. Software Maintenance
Modifying a program after it has been put into use.
Maintenance does not normally involve major changes to the system’s architecture.
Changes are implemented by modifying existing components and adding new components to the system.

9. Types of maintenance
Maintenance to repair software faults (corrective maintenance)
Correct coding errors, design errors, or requirement errors.
Maintenance to adapt software to a different operating environment (adaptive maintenance)
Changing a system so that it operates in a different environment (computer, OS, etc.) from its initial implementation.
Maintenance to add to or modify the system’s functionality (perfective maintenance)
Modifying the system to satisfy new requirements.

10. Distribution of maintenance efforts

11. Maintenance costs Vary from one application domain to another.
(Guimaraes 1983) suggests that the maintenance costs for business application systems are comparable with development costs. For real-time systems, maintenance costs may be up to 4 times higher than development costs.
Ageing software can have high support costs (e.g. old languages, compilers etc.).

12. Development and maintenance costs

13. Maintenance cost factors
Team stability
Maintenance costs are reduced if the same staff are involved with them for some time.
Contractual responsibility
The developers of a system may have no contractual responsibility for maintenance so there is no incentive to design for future change.
Staff skills
Maintenance staff are often inexperienced and have limited domain knowledge.
Program age and structure
As programs age, their structure is degraded and they become harder to understand and change.

14. Evolution process Evolution processes depend on
The type of software being maintained;
The development processes used;
The skills and experience of the people involved.
Proposals for change are the driver for system evolution. Change identification and evolution continue throughout the system lifetime.

15. Change identification and evolution

16. The system evolution process

17. Change implementation

18. System Re-engineering
Re-structuring or re-writing part or all of a legacy system without changing its functionality.
Applicable where some but not all sub-systems of a larger system require frequent maintenance.
Re-engineering involves adding effort to make them easier to maintain. The system may be re-structured and re-documented.
Difference between system re-engineering and new system development (forward engineering) is the starting point.

19. Forward and re-engineering

20. The re-engineering process

21. Re-engineering process activities
Source code translation
Convert code to a new language.
Reverse engineering
Analyze the program to understand it;
Program structure improvement
Restructure automatically for understandability;
Program modularization
Reorganize the program structure;
Data reengineering
Clean-up and restructure system data.

22. Re-engineering cost factors
The quality of the software to be reengineered.
The tool support available for reengineering.
The extent of the data conversion which is required.
The availability of expert staff for reengineering.
This can be a problem with old systems based on technology that is no longer widely used.