1. ECE 453 – CS 447 – SE 465 Software Testing & Quality Assurance Lecture 25 Instructor Paulo Alencar

2. Overview Software Maintenance and Evolution Definitions
Evolution Dynamics
Software Maintenance

3. Terminology Software Evolution Software Maintenance
a continuous change from a lesser, simpler, or worse state to a higher or better state.
Software Maintenance
consists of the activities required to keep a software system operational and responsive after it is accepted and placed into production.

4. Objective
To explain why change is inevitable if software systems are to remain useful
To discuss software maintenance and maintenance cost factors
To describe the processes involved in software evolution
To discuss an approach to assessing evolution strategies for legacy systems

5. 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.
A key problem for organisations is implementing and managing change to their existing software systems.

6. Importance of Evolution
Organizations have huge investments in their software systems - they are critical business assets.
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.

7. Spiral Model of Evolution

8. Evolution Dynamics
Program evolution dynamics is the study of the processes of system change.
After major empirical studies, Lehman and Belady proposed that there were a number of ‘laws’ which applied to all systems as they evolved.
There are sensible observations rather than laws. They are applicable to large systems developed by large organisations. Perhaps less applicable in other cases.

9. Lehman’s Laws The Law of Continuing Change (1974)
The Law of Increasing Complexity (1974)
The Law of Self Regulation (1974)
The Law of Conservation of Organizational Stability (1980)
The Law of Conservation of Familiarity (1980)
The Law of Continuing Growth (1980)
The Law of Declining Quality (1996)
The Feedback System Law (1996)
Source: Lehman, M., et al, “Metrics and Laws of Software Evolution—The Nineties View,” Proceedings of the 4th International Software Metrics Symposium (METRICS '97), IEEE, 1997.

10. Lehman’s Laws Law Description Continuing change Increasing complexity
A program must change or become progressively less useful
Increasing complexity
As program changes, its structure becomes more complex Extra resources are required.
Large program evolution
System attributes, (eg, size, time between releases) is ~invariant for each system release.
Organizational stability
A program’s rate of development is ~constant.
Conservation of familiarity
The incremental change in each release is ~constant.
Continuing growth
The functionality has to continually increase to maintain user satisfaction.
Declining quality
Quality of systems appear to be declining unless adapted to changing environments
Feedback system
Evolution processes involve feedback systems for product improvement

11. Applicability of Lehman’s Laws
Lehman’s laws seem to be generally applicable to large, tailored systems developed by large organisations.
Confirmed in more recent work by Lehman on the FEAST project
It is not clear how they should be modified for
Shrink-wrapped software products;
Systems that incorporate a significant number of COTS components;
Small organisations;
Medium sized systems.

12. Software Maintenance
‘When the transition from development to evolution is not seamless, the process of changing the software after delivery is often called software maintenance’.
(Sommerville, 2004)
Maintenance involves extra process activities:
Program understanding

13. 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.

14. Maintenance is Inevitable
The system requirements are likely to change while the system is being developed because the environment is changing. Therefore a delivered system won't meet its requirements!
Systems are tightly coupled with their environment. When a system is installed in an environment it changes that environment and therefore changes the system requirements.
Systems MUST be maintained therefore if they are to remain useful in an environment.

15. Types of Maintenance Maintenance to repair software faults
Changing a system to correct deficiencies in the way meets its requirements.
Maintenance to adapt software to a different operating environment
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
Modifying the system to satisfy new requirements.
Maintenance to detect and correct latent faults before they become effective (preventive).

16. Types of Maintenance Corrective Maintenance Adaptive Maintenance
Perfective Maintenance
(Preventative Maintenance - Pressman)
Sommerville (2004) avoids the use of these names because their definitions are uncertain
We will work with the following definitions

17. Corrective Maintenance
Focuses on fixing defects, ie, fault repair
It is a reactive process
i.e. defects generally need to be corrected either immediately or in the near future
Defects
refer to the system not performing as originally intended or as specified in the requirements
17% of maintenance is correcting faults

18. Adaptive Maintenance
It includes all changes to meet the evolving needs of the user and the environment
i.e. modifications to a system to cope with a changing environment, eg changed hardware, operating system
18% of maintenance is change to adapt to a new environment
It includes work to enhance a system’s functionality
i.e. relates to changing a system when requirements change due to organisational or business changes
The scale of this maintenance is often greater than for other types of maintenance
65% of maintenance is new requirements

19. Perfective Maintenance
It includes all efforts to improve the quality of the software
It includes restructuring code, creating and updating documentation, improving reliability or efficiency, improving performance
Maintenance work in the above categories is often performed concurrently

20. Differences: S/W Development and Maintenance
Constraints of an existing system
Changes must conform or be compatible with an existing architecture, design and code constraints
Shorter time frames
Development spans 1 or more years Maintenance spans hours or up to 6 months
Available test data
Development creates all test data from scratch Maintenance uses existing test data with regression testing creating new data for the changes

21. Distribution of Maintenance Effort

22. Relative Costs of Maintenance
3% Requirements definition
3% Preliminary design
5% Detailed design
7% Implementation
15% Testing
67% Operations and Maintenance
The majority of a software budget in large companies is devoted to maintaining systems
Sommerville (2004) states that 90% of software costs are evolution costs

23. Maintenance Costs
Usually greater than development costs (2* to 100* depending on the application).
Affected by both technical and non-technical factors.
Increases as software is maintained. Maintenance corrupts the software structure so makes further maintenance more difficult.
Ageing software can have high support costs (e.g. old languages, compilers etc.).

24. Development/Maintenance Costs

25. 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.

26. Maintenance Prediction
Maintenance prediction is concerned with assessing which parts of the system may cause problems and have high maintenance costs
Change acceptance depends on the maintainability of the components affected by the change;
Implementing changes degrades the system and reduces its maintainability;
Maintenance costs depend on the number of changes and costs of change depend on maintainability.

27. Maintenance Prediction

28. Change Prediction
Predicting the number of changes requires an understanding of the relationships between a system and its environment.
Tightly coupled systems require changes whenever the environment is changed.
Factors influencing this relationship are
Number and complexity of system interfaces;
Number of inherently volatile system requirements;
The business processes where the system is used.

29. Complexity Metrics
Predictions of maintainability can be made by assessing the complexity of system components.
Studies have shown that most maintenance effort is spent on a relatively small number of system components.
Complexity depends on
Complexity of control structures;
Complexity of data structures;
Object, method (procedure) and module size.

30. Process Metrics
Process measurements may be used to assess maintainability
Number of requests for corrective maintenance;
Average time required for impact analysis;
Average time taken to implement a change request;
Number of outstanding change requests.
If any or all of these is increasing, this may indicate a decline in maintainability.