ch21 - Software evolution Ian Sommerville 2004 Software...

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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 1 Software evolution
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 2 Objectives 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
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 3 Topics covered Program evolution dynamics Software maintenance Evolution processes Legacy system evolution
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 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. A key problem for organisations is implementing and managing change to their existing software systems.
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 5 Importance of evolution Organisations 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.
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 6 Spiral model of evolution
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 7 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. Program evolution dynamics
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 8 Lehman’s laws 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
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ch21 - Software evolution Ian Sommerville 2004 Software...

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