RIVIER CS 608 - Software evolution

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Software evolutionObjectivesTopics coveredSoftware changeImportance of evolutionSpiral model of evolutionProgram evolution dynamicsLehman’s lawsApplicability of Lehman’s lawsSoftware maintenanceMaintenance is inevitableTypes of maintenanceDistribution of maintenance effortMaintenance costsDevelopment/maintenance costsMaintenance cost factorsMaintenance predictionSlide 18Change predictionComplexity metricsProcess metricsEvolution processesChange identification and evolutionThe system evolution processChange implementationUrgent change requestsEmergency repairSystem re-engineeringAdvantages of reengineeringForward and re-engineeringThe re-engineering processReengineering process activitiesRe-engineering approachesReengineering cost factorsLegacy system evolutionSystem quality and business valueLegacy system categoriesBusiness value assessmentSystem quality assessmentBusiness process assessmentEnvironment assessment 1Environment assessment 2Application assessment 1Application assessment 2System measurementKey pointsSlide 47©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 1Software evolution©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 2Objectives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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 3Topics coveredProgram evolution dynamicsSoftware maintenanceEvolution processesLegacy system evolution©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 4Software 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 5Importance 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 6Spiral model of evolutionSpecificationImplementionValidationOperationStartRelease 1Release 2Release 3©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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 8Lehman’s lawsLaw DescriptionContinuing change A program that is used in a real-world environment necessarilymust change or become progressively less useful in thatenvironment.Increasing complexity As an evolving program changes, its structure tends to becomemore complex. Extra resources must be devoted to preservingand simplifying the structure.Large program evolution Program evolution is a self-regulating process. Systemattributes such as size, time between releases and the number ofreported errors is approximately invariant for each systemrelease.Organisational stability Over a program’s lifetime, its rate of development isapproximately constant and independent of the resourcesdevoted to system development.Conservation offamiliarityOver the lifetime of a system, the incremental change in eachrelease is approximately constant.Continuing growth The functionality offered by systems has to continually increaseto maintain user satisfaction.Declining quality The quality of systems will appear to be declining unless theyare adapted to changes in their operational environment.Feedback system Evolution processes incorporate multi-agent, multi-loopfeedback systems and you have to treat them as feedbacksystems to achieve significant product improvement.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 9Applicability 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 (see further reading on book website).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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 10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.Software maintenance©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 11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.Maintenance is inevitable©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 21 Slide 12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


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RIVIER CS 608 - Software evolution

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