1. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 1 Software evolution

2. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 2 Objectives l To explain why change is inevitable if software systems are to remain useful l To discuss software maintenance and maintenance cost factors l To describe the processes involved in software evolution l To discuss an approach to assessing evolution strategies for legacy systems

3. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 3 Topics covered l Program evolution dynamics l Software maintenance costs l Evolution processes l System re-engineering l Legacy system evolution

4. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 4 Software change l 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. l A key problem for organisations is implementing and managing change to their existing software systems.

5. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 5 l Repair software faults Changing a system to correct deficiencies in the way meets its requirements. l 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. l Add to or modify the system’s functionality Modifying the system to satisfy new requirements. l Improve the program structure and system performance Modifying the system without changing functional behavior. Types of changes Maintenance Evolution Reengineering

6. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 6 Software evolution l No standard definition. l Generally, software evolution refers to the study and management of the process of making changes to software over time. Software evolution comprises: Development activities Maintenance activities Reengineering activities l Sometimes, software evolution is used to refer to the maintenance activity of adding new functionality to existing software.

7. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 7 Importance of evolution l Organisations have huge investments in their software systems - they are critical business assets. l To maintain the value of these assets to the business, they must be changed and updated. l The majority of the software budget in large companies is devoted to evolving existing software rather than developing new software.

8. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 8 Spiral model of evolution

9. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 9 l Program evolution dynamics is the study of the processes of system change. l After major empirical studies, Lehman and Belady proposed that there were a number of ‘laws’ which applied to all systems as they evolved. l These are sensible observations rather than laws. They are applicable to large systems developed by large organisations. Perhaps less applicable in other cases. Program evolution dynamics

10. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 10 Lehman’s laws

11. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 11 Applicability of Lehman’s laws l 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 (http://www.doc.ic.ac.uk/~mml/feast2/). l 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. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 12 Topics covered l Program evolution dynamics l Software maintenance costs l Evolution processes l System re-engineering l Legacy system evolution

13. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 13 l Modifying a program after it has been put into use. l Maintenance does not normally involve major changes to the system’s architecture. l Changes are implemented by modifying existing components and adding new components to the system. Software maintenance

14. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 14 l 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! l 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. l Systems MUST be maintained therefore if they are to remain useful in an environment. Maintenance is inevitable

15. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 15 Distribution of maintenance effort

16. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 16 l Usually greater than development costs (2* to 100* depending on the application). l Affected by both technical and non-technical factors. l Increases as software changes accumulate. Changes corrupt the software structure making further maintenance more difficult. l Ageing software can have high support costs (e.g. old languages, compilers etc.). Maintenance costs

17. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 17 Development/maintenance costs

18. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 18 l Team stability Maintenance costs are reduced if the same staff are involved with them for some time. l Contractual responsibility The developers of a system may have no contractual responsibility for maintenance so there is no incentive to design for future change. l Staff skills Maintenance staff are often inexperienced and have limited domain knowledge. l Program age and structure As programs age, their structure is degraded and they become harder to understand and change. Maintenance cost factors

19. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 19 Maintenance prediction l Maintenance prediction is concerned with assessing which parts of the system are likely to change and have high maintenance costs l Change and maintainability are interrelated Allowing change 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. l This is an example software evolution as a feedback system.

20. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 20 Maintenance prediction Predicting maintainability Predicting system changes Predicting maintenance costs What will be the lifetime maintenance costs of this system? What will be the costs of maintaining this system over the next year? What parts of the system will be the most expensive to maintain? How many change requests can be expected? What parts of the system are most likely to be affected by change requests?

21. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 21 Change prediction l Predicting the number of changes requires and understanding of the relationships between a system and its environment (customer environment, technological environment). l Tightly coupled systems require changes whenever the environment is changed. l 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.

22. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 22 Complexity metrics l Predictions of maintainability can be made by assessing the complexity of system components. l Studies have shown that most maintenance effort is spent on a relatively small number of complex components. l Complexity depends on Complexity of control structures; Complexity of data structures; Object, method (procedure) and module size.

23. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 23 Change-related process metrics l Change-related 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. l If any or all of these is increasing, this may indicate a decline in maintainability.

24. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 24 Topics covered l Program evolution dynamics l Software maintenance costs l Evolution processes l System re-engineering l Legacy system evolution

25. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 25 Evolution processes l Evolution processes depend on The type of software being maintained; The development processes used; The skills and experience of the people involved. l Proposals for change are the driver for system evolution. Change identification and evolution continue throughout the system lifetime.

26. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 26 Change identification and evolution

27. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 27 The system evolution process Release planning Change implementation System release Impact analysis Change requests Platform adaptation System enhancement Fault repair

28. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 28 Change implementation Requirements updating Software development Requirements analysis Proposed changes

29. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 29 Urgent change requests l Urgent changes may have to be implemented without going through all stages of the evolution process 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).

30. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 30 Emergency repair Modify & test source code Deliver modified system Anal yse sour ce code Change requests

31. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 31 Topics covered l Program evolution dynamics l Software maintenance costs l Evolution processes l System re-engineering l Legacy system evolution

32. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 32 System re-engineering l Re-structuring or re-writing part or all of a legacy system without changing its functionality. l To facilitate continued evolution, systems may need to be re-engineered. l Re-engineering is applicable where some but not all sub-systems of a larger system require frequent maintenance. l Re-engineering involves adding effort to make them easier to maintain. The system may be re- structured and re-documented.

33. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 33 Advantages of reengineering l Reduced risk There is a high risk in new software development. There may be development problems, staffing problems and specification problems. l Reduced cost The cost of re-engineering is often significantly less than the costs of developing new software.

34. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 34 Forward and re-engineering Understanding and transformation Existing software system Re-engineered system Design and implementation System specification New system Software re-engineering Forward engineering

35. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 35 The re-engineering process

36. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 36 Reengineering process activities l Source code translation Convert code to a new language. l Reverse engineering Analyse the program to understand it; l Program structure improvement Restructure automatically for understandability; l Program modularisation Reorganise the program structure; l Data reengineering Clean-up and restructure system data.

37. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 37 Re-engineering approaches Automated restructuring with manual changes AUtomated source code conversion Restructuring plus architectural changes Automated program restructuring Program and data restructuring Increased cost

38. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 38 Reengineering cost factors l The quality of the software to be reengineered. l The tool support available for reengineering. l The extent of the data conversion which is required. l The availability of expert staff for reengineering. This can be a problem with old systems based on technology that is no longer widely used.

39. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 39 Topics covered l Program evolution dynamics l Software maintenance costs l Evolution processes l System re-engineering l Legacy system evolution

40. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 40 Legacy system evolution l Organisations that rely on legacy systems must choose a strategy for evolving these systems Scrap the system completely and modify business processes so that it is no longer required; Continue maintaining the system; Transform the system by re-engineering to improve its maintainability; Replace the system with a new system. l The strategy chosen should depend on the system quality and its business value.

41. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 41 System quality and business value 1 2 34 5 6 7 8 9 10 System quality High business value Low quality High business value High quality Low business value Low quality Low business value High quality Business value

42. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 42 Legacy system categories l Low quality, low business value These systems should be scrapped. l Low quality, high business value These make an important business contribution but are expensive to maintain. Should be re-engineered or replaced if a suitable system is available. l High quality, low business value Replace with COTS, scrap completely or maintain. l High quality, high business value Continue in operation using normal system maintenance.

43. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 43 Business value assessment l Assessment should take different viewpoints into account System end-users; Business customers; Line managers; IT managers; Senior managers. l Interview different stakeholders and collate results.

44. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 44 System quality assessment l Business process assessment How well does the business process support the current goals of the business? l Environment assessment How effective is the system’s environment and how expensive is it to maintain? l Application assessment What is the quality of the application software system?

45. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 45 Business process assessment l Use a viewpoint-oriented approach and seek answers from system stakeholders Is there a defined process model and is it followed? Do different parts of the organisation use different processes for the same function? How has the process been adapted? What are the relationships with other business processes and are these necessary? Is the process effectively supported by the legacy application software? l Example - a travel ordering system may have a low business value because of the widespread use of web-based ordering.

46. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 46 Environment assessment 1

47. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 47 Environment assessment 2

48. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 48 Application assessment 1

49. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 49 Application assessment 2

50. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 50 System measurement l You may collect quantitative data to make an assessment of the quality of the application system The number of system change requests; The number of different user interfaces used by the system; The volume of data used by the system.

51. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 51 Key points l Software development and evolution should be a single iterative process. l Lehman’s Laws describe a number of insights into system evolution. l Four types of software changes are bug fixing, adapting software for a new environment, implementing new requirements, and re- engineering. l For custom systems, maintenance costs usually exceed development costs.

52. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 21 Slide 52 Key points l The process of evolution is driven by requests for changes from system stakeholders. l Software re-engineering is concerned with re- structuring and re-documenting software to make it easier to change. l The business value of a legacy system and its quality should determine the evolution strategy that is used.