1. CS 425/625 Software Engineering Software Evolution
Based on Chapter 21 of the textbook [SE-8] Ian Sommerville,
Software Engineering, 8th Ed., Addison-Wesley, 2006 and on the
“Ch21” PowerPoint presentation available at the book’s web-site
November 12, 2008

2. Outline Introduction Program Evolution Dynamics Software Maintenance
Overview
Prediction
Evolution Processes
Legacy Systems

3. Introduction Software evolves continuously due to demands for changes:.
Software evolves continuously due to demands for changes:
New requirements surface
Existing requirements need be modified
Errors found need be fixed
In some cases 90% of software costs are evolution costs
When the transition from development to evolution is not smooth, changing software after delivery is called software maintenance

4. .Introduction
A spiral model of development and evolution [Fig. 21.1, SE-8]

5. Program evolution dynamics

6. Software Maintenance: Overview….
Software maintenance = the activities of changing the system after it has been delivered
Types of software maintenance:
Corrective maintenance: repair of software faults
Adaptive maintenance: modification of software due to changes in the operating environment (hardware, supporting software)
Perfective maintenance: additions to and/or modifications of system functionality due to organizational or business changes

7. Software Maintenance: .Overview...
Distribution of maintenance effort [Fig. 21.3, SE-8]

8. Software Maintenance: ..Overview..
Software maintenance is a natural continuation of the development process (specification, design, implementation, testing). Hence the term evolution (applied especially when the transition from development is seamless)
Development and maintenance costs vary from application to application
Investing in development leads to reduction of both maintenance costs and overall project costs [slide 9]

9. Software Maintenance: …Overview.
Costs of development and maintenance [Fig. 21.4, SE-8]

10. Software Maintenance: ….Overview
Why maintenance costs are higher than development costs? Factors:
Team stability: development teams break up after delivery
Contractual responsibility: different teams or organizations have the responsibility for maintenance
Staff skills: more experienced software engineers tend to avoid maintenance
Program age and structure: not structured in the first place, the program copes poorly with changes and its structure degrades

11. Software Maintenance: Prediction.
Maintenance prediction [Fig. 21.5, SE-8]

12. Software Maintenance: .Prediction
Generally, more complex the software, more expensive its maintenance
The relationship between a system and its environment is also important. This relationship is characterized by:
Number and complexity of interfaces
Number of inherently volatile requirements
The business process in which the system is used
Factors used to assess maintainability:
Number of requests for corrective maintenance
Average time required for impact analysis
Average time taken to implement a change
Number of outstanding change requests

13. Evolution Processes..
The evolution process: overview [Fig. 21.7, SE-8]

14. .Evolution Processes.
Change implementation [Fig. 21.8, SE-8]

15. ..Evolution Processes Emergency repair [Fig. 21.9, SE-8]. Prompted by:
System faults
Business changes
Environmental changes
all requiring urgent treatment
The dangers of emergency repair:
Software becomes inconsistent
Changes are not reflected in documentation
Software ageing is accelerated by workaround solutions

16. Legacy Systems: Introduction..
Legacy systems: old computer-based systems still in use by organizations
Many of them still business critical
Incorporate many changes made over the years
Many people have been involved in these changes
Replacing legacy systems with new systems is risky, yet keeping them means new changes become more and more expensive

17. Legacy Systems: .Introduction.
Risks of replacing a legacy system:
Specification is difficult because existing documentation is typically incomplete
Changing business processes (now adjusted to the system) may entail high costs
Undocumented, yet important business rules may be embedded in the system; a new system may break these rules
The new system may be delivered late, may cost more than expected, and may not function properly

18. Legacy Systems: ..Introduction
Factors that make changes to legacy systems expensive:
In large systems, different parts were implemented by different teams, without consistent programming style
It is difficult to find personnel who knows the obsolete programming languages used in old systems
In may cases the only documentation is provided by the source code; even this may be missing
It is difficult to understand the system given its ad hoc updating over the years
Data used by the system is difficult to understand and manipulate; it can also be obsolete and/or redundant

19. Legacy system assessment…..
Strategic approaches for dealing with legacy systems:
Scrap the system completely
When business practices have changed and no longer depend significantly on the system (they may be supported by new COTS)
Continue to maintain the system
The system works well, is fairly stable, and users do not request many changes
Transform the system to improve maintainability
When system quality was affected negatively by changes, yet changes are still required
Replace the system with a new one
When obsolete hardware precludes further operation or the new system can be built at reasonable cost

20. .Legacy system assessment….
Assessing legacy systems example [Fig SE-8]

21. ..Legacy system assessment…
Assessment of legacy systems includes:
Business value assessment
Viewpoints:
End-users: look at system’s functionality and performance
Customers: look at the quality of services provided
Business managers: assess the usefulness of the system in terms of business support
IT managers: are concerned with the availability of technical support for the system
Senior managers: interested in system’s contribution to the business goals
Major criteria: system usage, business processes supported, dependability, system outputs

22. …Legacy system assessment..
(2) System quality assessment. Look at all components of the system. Hence:
Environment assessment. Support software & hardware platform (maintenance costs, faults, etc. – slide 23)
Application software assessment. Factors considered as in slide 24 and quantitative data such as:
Number of system change requests
Number of different user interfaces
Volume of data used by the system

23. ….Legacy system assessment.
Factors in environment assessment [Fig SE-8]

24. …..Legacy system assessment
Factors in application software assessment [Fig SE-8]