1. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 1 Software Processes

2. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 2 Objective l To introduce three generic process models and when they may be used

3. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 3 Topics covered l Software process models l Process iteration l Process activities l Computer-aided software engineering

4. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 4 The software process l A structured set of activities required to develop a software system Specification; Design; Validation; Evolution.

5. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 5 Characteristics l prescribes major activities l utilizes resources, subject to constraints such as schedule, to produce intermediate and final results l constraints and controls apply to activities, resources, and products l constraints on activities: time, budget, tools l controls on activities: config. mgmt, reports

6. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 6 Generic software process models l A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective. l Process should be: Visible: Activities should provide clear indications of progress (deadlines/milestones) Understandable: Activities and their order of execution are well-defined Supportable: Automated support for activities is available Usable: Process is acceptable to and usable by developers

7. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 7 l The waterfall model Separate and distinct phases of specification and development. l Evolutionary development Specification, development and validation are interleaved. l There are many variants of these

8. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 8 Waterfall model

9. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 9 Waterfall model phases l Requirements analysis and definition l System and software design l Implementation and unit testing l Integration and system testing l Operation and maintenance l The main drawback of the waterfall model is the difficulty of accommodating change.

10. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 10 Waterfall model problems l Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. l Requirements must be well-understood up front. l Few business systems have stable requirements. l The waterfall model is mostly used for large systems engineering projects where a system is developed at several sites.

11. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 11 Waterfall Methods and Variants l Variants V-shaped model: relationship between types of tests and phases before testing Prototyping variant: requirements and design prototypes

12. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 12 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 and add new features as proposed by the customer. l Throw-away prototyping Objective is to understand the system requirements. Should start with poorly understood requirements to clarify what is really needed. Good for conceptual development.

13. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 13 Evolutionary development

14. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 14 Prototyping Process Model l Process: quick design, build prototype, evaluate, refine prototype, … engineer product l Forms of prototypes varies depending on forms of analysis Paper spec of SW from functional analysis or requirements gathering through FAST Coded prototype (not fully functional!) Preliminary user manual Story boards

15. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 15 Evolutionary development l Problems Lack of process visibility; Systems are often poorly structured; Special skills (e.g. in languages for rapid prototyping) may be required. Customer sees prototype, then wants a few fixes quick and delivery Implementation compromises made to get prototype done quickly/forgets compromises and become part of the system

16. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 16 Evolutionary development l Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user interface); For short-lifetime systems.

17. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 17 Process iteration l System requirements ALWAYS evolve in the course of a project so 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 delivery; Spiral development.

18. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 18 Incremental/Iterative Development Iterative: put in all features, but they are limited Incremental: functionality developed in pieces

19. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 19 Incremental delivery l Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality. l User 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 though requirements for later increments can continue to evolve.

20. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 20 Incremental development

21. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 21 Incremental development advantages l Customer value can be delivered with each increment so system functionality is available earlier. 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.

22. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 22 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.

23. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 23 Spiral model of the software process

24. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 24 Spiral model sectors l Objective setting Specific objectives for the phase are identified. l Risk assessment and reduction Risks are assessed and activities put in place to reduce the key risks. l Development and validation A development model for the system is chosen which can be any of the generic models. l Planning The project is reviewed and the next phase of the spiral is planned.

25. ©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 4 Slide 25 Process activities l Software specification l Software design and implementation l Software validation l Software evolution