Software Evolution  Program evolution dynamics  Software maintenance

Software Evolution Program evolution dynamics Software maintenance Complexity and Process metrics Evolution processes 1

Software change Software change is inevitable – – – – – 2 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.

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. 3

Program 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. 4

Lehman’s laws 5 Law Description Continuing change A program that is used in a real-world environment necessarily must change or become progressively less useful in that environment. Increasing complexity As an evolving program changes, its structure tends to become more complex. Extra resources must be devoted to preserving and simplifying the structure. Large program evolution Program evolution is a self-regulating process. System attributes such as size, time between releases and the number of reported errors is approximately invariant for each system release. Organisational stability Over a programÕs lifetime, its rate of development is approximately constant and independent of the resources devoted to system development. Conservation of familiarity Over the lifetime of a system, the incremental change in each release is approximately constant. Continuing growth The functionality offered by systems has to continually increase to maintain user satisfaction. Declining quality The quality of systems will appear to be declining unless they are adapted to changes in their operational environment. Feedback system Evolution processes incorporate multi-agent, multi-loop feedback systems and you have to treat them as feedback systems to achieve significant product improvement.

Applicability of Lehman’s laws Lehman’s laws seem to be generally applicable to large, tailored systems developed by large organisations. – It is not clear how they should be modified for – – – – 6 Confirmed in more recent work by Lehman on the FEAST project (see further reading on book website). Shrink-wrapped software products; Systems that incorporate a significant number of COTS components; Small organisations; Medium sized systems.

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. 7

Maintenance is inevitable 8 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.

Types of maintenance Maintenance to repair software faults – 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 – 9 Changing a system to correct deficiencies in the way meets its requirements. Modifying the system to satisfy new requirements.

Distribution of maintenance effort Fault repair (17%) Software adaptation (18%) 10 Functionality addition or modification (65%)

Maintenance costs 11 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.).

Development/maintenance costs System 1 System 2 0 50 100 150 Development costs 12 200 250 300 350 400 450 Maintenance costs 500

Maintenance cost factors Team stability – Contractual responsibility – Maintenance staff are often inexperienced and have limited domain knowledge. Program age and structure – 13 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 costs are reduced if the same staff are involved with them for some time. As programs age, their structure is degraded and they become harder to understand and change.

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 – – – 14 Complexity of control structures; Complexity of data structures; Object, method (procedure) and module size.

Process metrics Process measurements may be used to assess maintainability – – – – If 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. any or all of these is increasing, this may indicate a decline in maintainability. 15

Evolution processes 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. 16

Change identification and evolution Change identification process New system 17 Change proposals Software evolution process

The system evolution process Change requests 18 Impact analy sis Release planning Change implementa tion Fault repair Platform adaptation System enhancement System release

Change implementation Proposed changes 19 Requirements analysis Requirements updating Software development

Urgent change requests Urgent changes may have to be implemented without going through all stages of the software engineering process – – – 20 If a serious system fault has to be repaired; If changes to the system’s environment (e.g. an OS upgrade) have unexpected effects; If there are business changes that require a very rapid response (e.g. the release of a competing product).

Emergency repair Change requests 21 Analyse source code Modify source code Deliver modified system