Presentation on theme: "Chapter 11 Software Evolution"— Presentation transcript:
1Chapter 11 Software Evolution This chapter is extracted from Sommerville’s slides.Text book chapter 211
2ObjectivesUnderstand that change is inevitable if software systems are to remain usefulLearn different types of software maintenance and factors that effect its costsUnderstand software re-engineering.2
3Overview Software evolution Software maintenance Evolution process System Re-engineering3
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.Software development, therefore, does not stop when a system is delivered but continues throughout the lifetime of the system .
5Importance of Evolution Organizations have huge investments in their software systems.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.
7Evolution vs maintenance If a single organization is responsible for both the initial software development and the evolution evolutionWhen the software is developed externally and the evolutionis the responsibility of the customer’s software development staff maintenanceor an external company for system support and evolution maintenanceMaintenance usually applies to custom software whereas evolution applies to generic software.
8Software MaintenanceModifying 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.
9Types of maintenanceMaintenance to repair software faults (corrective maintenance)Correct coding errors, design errors, or requirement errors.Maintenance to adapt software to a different operating environment (adaptive maintenance)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 (perfective maintenance)Modifying the system to satisfy new requirements.
11Maintenance costs Vary from one application domain to another. (Guimaraes 1983) suggests that the maintenance costs for business application systems are comparable with development costs. For real-time systems, maintenance costs may be up to 4 times higher than development costs.Ageing software can have high support costs (e.g. old languages, compilers etc.).
13Maintenance cost factors Team stabilityMaintenance costs are reduced if the same staff are involved with them for some time.Contractual responsibilityThe developers of a system may have no contractual responsibility for maintenance so there is no incentive to design for future change.Staff skillsMaintenance staff are often inexperienced and have limited domain knowledge.Program age and structureAs programs age, their structure is degraded and they become harder to understand and change.
14Evolution process 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.
18System Re-engineering Re-structuring or re-writing part or all of a legacy system without changing its functionality.Applicable where some but not all sub-systems of a larger system require frequent maintenance.Re-engineering involves adding effort to make them easier to maintain. The system may be re-structured and re-documented.Difference between system re-engineering and new system development (forward engineering) is the starting point.
21Re-engineering process activities Source code translationConvert code to a new language.Reverse engineeringAnalyze the program to understand it;Program structure improvementRestructure automatically for understandability;Program modularizationReorganize the program structure;Data reengineeringClean-up and restructure system data.
22Re-engineering cost factors The quality of the software to be reengineered.The tool support available for reengineering.The extent of the data conversion which is required.The availability of expert staff for reengineering.This can be a problem with old systems based on technology that is no longer widely used.