lec 9 -Software Evolution.pptx

Software Evolution Software change Legacy systems Software maintenance Exam tips  Software change Software change is inevitable – New requirements emerge when the software is used; – The business environment changes; – Errors must be repaired; – Ne...

Software Evolution Software change Legacy systems Software maintenance Exam tips  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 all organizations is implementing and managing change to their existing software systems 2  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 changing and evolving existing software rather than developing new software. 3  Evolution processes Software 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. – Should be linked with components that are affected by the change, thus allowing the cost and impact of the change to be estimated. Change identification and evolution continues throughout the system lifetime. 4 Change identification and evolution processes 5  Change implementation Iteration of the development process where the revisions to the system are designed, implemented and tested. A critical difference is that the first stage of change implementation may involve program understanding, especially if the original system developers are not responsible for the change implementation. During the program understanding phase, you have to understand how the program is structured, how it delivers functionality and how the proposed change might affect the program. 6  Urgent change requests Urgent changes may have to be implemented without going through all stages of the software engineering process – If a serious system fault has to be repaired to allow normal operation to continue; – 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). 7 The emergency repair process 8  Agile methods and evolution Agile methods are based on incremental development so the transition from development to evolution is a seamless one. – Evolution is simply a continuation of the development process based on frequent system releases. Automated regression testing is particularly valuable when changes are made to a system. Changes may be expressed as additional user stories. 9  Handover problems Where the development team has used an agile approach but the evolution team is unfamiliar with agile methods and prefer a plan-based approach. – The evolution team may expect detailed documentation to support evolution and this is not produced in agile processes. Where a plan-based approach has been used for development but the evolution team prefers to use agile methods. – The evolution team may have to start from scratch developing automated tests and the code in the system may not have been refactored and simplified as is expected in agile development. 10 Legacy systems 11  Legacy systems Legacy systems are older systems that rely on languages and technology that are no longer used for new systems development. Legacy software may be dependent on older hardware, such as mainframe computers and may have associated legacy processes and procedures. Legacy systems are not just software systems but are broader socio-technical systems that include hardware, software, libraries and other supporting software and business processes. 12  Legacy system components System hardware Legacy systems may have been written for hardware that is no longer available. Support software The legacy system may rely on a range of support software, which may be obsolete or unsupported. Application software The application system that provides the business services is usually made up of a number of application programs. Application data These are data that are processed by the application system. They may be inconsistent, duplicated or held in different databases. Business processes These are processes that are used in the business to achieve some business objective. Business processes may be designed around a legacy system and constrained by the functionality that it provides. Business policies and rules These are definitions of how the business should be carried out and constraints on the business. Use of the legacy application system may be embedded in these policies and rules. 13 Legacy system layers 14  Legacy system replacement Legacy system replacement is risky and expensive so businesses continue to use these systems System replacement is risky for a number of reasons – Lack of complete system specification – Tight integration of system and business processes – Undocumented business rules embedded in the legacy system – New software development may be late and/or over budget 15  Legacy system change Legacy systems are expensive to change for a number of reasons: – No consistent programming style – Use of obsolete programming languages with few people available with these language skills – Inadequate system documentation – System structure degradation – Program optimizations may make them hard to understand – Data errors, duplication and inconsistency 16  Legacy system management 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. The strategy chosen should depend on the system quality and its business value. 17  Legacy system categories Low quality, low business value – These systems should be scrapped. 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. High-quality, low-business value – Replace with COTS, scrap completely or maintain. High-quality, high business value – Continue in operation using normal system maintenance. 18  Business value assessment Assessment should take different viewpoints into account – System end-users; – Business customers; – Line managers; – IT managers; – Senior managers. Interview different stakeholders and collate results. 19 Software maintenance 20  Software maintenance Modifying a program after it has been put into use. The term is mostly used for changing custom software. Generic software products are said to evolve to create new versions. 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. 21  Types of maintenance Fault repairs – Changing a system to fix bugs/vulnerabilities and correct deficiencies in the way meets its requirements. Environmental adaptation – 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. Functionality addition and modification – Modifying the system to satisfy new requirements. 22 Maintenance effort distribution 23  Maintenance costs 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.). 24  Maintenance costs It is usually more expensive to add new features to a system during maintenance than it is to add the same features during development – A new team has to understand the programs being maintained – Separating maintenance and development means there is no incentive for the development team to write maintainable software – Program maintenance work is unpopular Maintenance staff are often inexperienced and have limited domain knowledge. – As programs age, their structure degrades and they become harder to change 25  Exam tips Try to understand the big picture Apply most concepts using scenarios 26  Chapters Chapter 1- Introduction Chapter 2 – Software Processes Chapter 3 – Agile Software Development Chapter 4 – Requirements Engineering Chapter 5 – System modeling Chapter 6 – Software Evolution 27 End 28