1. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 1 Software Processes l Coherent sets of activities for specifying, designing, implementing and testing software systems l Objectives To introduce software lifecycle models To describe a number of different lifecycle models and when they may be used To describe outline process models for requirements engineering, software development, testing and evolution

2. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 2 Topics covered l Software lifecycle models l Process iteration l Software specification l Software design and implementation l Software validation l Software evolution

3. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 3 The software process l A structured set of activities required to develop a software system Specification Design Validation Evolution l A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective

4. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 4 Software lifecycle models: Generic software process models l The waterfall model Separate and distinct phases of specification and development l Evolutionary development Specification and development are interleaved l Formal systems development A mathematical system model is formally transformed to an implementation l Reuse-based development The system is assembled from existing components

5. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 5 Waterfall model

6. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 6 Waterfall model problems l Inflexible partitioning of the project into distinct stages l This makes it difficult to respond to changing customer requirements l This model is only appropriate when the requirements are well-understood

7. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 7 Evolutionary development l Exploratory development Objective is to work with customers and to evolve a final system from an initial outline specification. Should start with well-understood requirements l Throw-away prototyping Objective is to understand the system requirements. Should start with poorly understood requirements

8. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 8 Evolutionary development

9. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 9 Evolutionary development l Problems Lack of process visibility Systems are often poorly structured due to lack of proper planning Special skills (e.g. in languages for rapid prototyping) may be required l Applicability For small or medium-size interactive systems For parts of large systems (e.g. the user interface) For short-lifetime systems

10. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 10 Formal systems development l Based on the transformation of a mathematical specification through different representations to an executable program l Transformations are ‘correctness-preserving’ so it is straightforward to show that the program conforms to its specification l Embodied in the ‘Cleanroom’ approach to software development No need to test for defects

11. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 11 Formal systems development

12. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 12 Formal systems development l Problems Need for specialised skills and training to apply the technique Difficult to formally specify some aspects of the system such as the user interface Changes require new transformations and proofs Typically does not scale l Applicability Critical systems (e.g., in terms of safety or security)

13. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 13 Reuse-oriented development l Based on systematic reuse where systems are integrated from existing components or COTS systems l Process stages Component analysis Requirements modification System design with reuse Development and integration l Becoming important but still limited experience with it

14. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 14 Process iteration l System requirements always evolve in the course of a project l Process iteration where earlier stages are reworked is always part of the process for large systems l Iteration can be applied to any of the generic process models l Two (related) approaches Incremental development Spiral development

15. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 15 Incremental development l Development and delivery is broken down into increments l Each increment delivers part of the required functionality l Requirements are prioritised and the highest priority requirements are included in early increments l Once the development of an increment is started, the requirements are frozen Requirements for later increments can continue to evolve

16. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 16 Extreme programming l New approach to development based on the development and delivery of very small increments of functionality l Relies on constant code improvement, user involvement in the development team and pairwise programming l Recently included into ‘agile methods’ since ‘extreme’ had a negative connotation

17. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 17 Incremental development

18. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 18 Incremental development advantages l System functionality is available earlier and customer does not have to wait as long l Early increments act as a prototype to help elicit requirements for later increments l Lower risk of overall project failure l The highest priority system services tend to receive the most testing

19. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 19 Spiral development l Process is represented as a spiral rather than as a sequence of activities with backtracking l Each loop in the spiral represents a phase in the process. l No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required l Risks are explicitly assessed and resolved throughout the process

20. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 20 Spiral model of the software process

21. ©Ian Sommerville 2000 Software Engineering, 6th edition Slide 21 Key points l Software processes are the activities involved in producing and evolving a software system. They are represented in a software process model l General activities are specification, design and implementation, validation and evolution l Lifecycle models describe the organisation of software processes l Iterative process models describe the software process as a cycle of activities