Chapter 2 SW Process Models

1. Chapter 2 SW Process Models

2. Assigment1 • Problem 1: 4.05. Disclose to all concerned parties those conflicts of interest that cannot reasonably be avoided or escaped • Problem 2: 2.06. Identify, document, collect evidence and report to the client or the employer promptly if, in their opinion, a project is likely to fail, to prove too expensive, to violate intellectual property law, or otherwise to be problematic. • Problem 3: 1.03. Approve software only if they have a well-founded belief that it is safe, meets specifications, passes appropriate tests, and does not diminish quality of life, diminish privacy or harm the environment. The ultimate effect of the work should be to the public good.

3. Objectives After completing this chapter you should be able to: • Describe various SW process models • Identify the advantages and disadvantages of each model • Evaluate the applicability of each model for a certain type of software development

4. Overview • What are SW process models? • Why SW process models? • The Waterfall Model • Evolutionary Models • The CBSE Model • Agile Models • Which is best?

5. A Software Process is A structured set of activities required to develop a software system

6. Activity • Formulate Teams • As a team:, answer the following question: • In your previous project, how is the process divided? (according to) • Activities? • Team work? • Documentations?

7. Ad hoc Software Development • Developing software without planning for each phase, and without specifying tasks, deliverables, time constraints or risks. • Relies entirely on the skills and experience of the individual staff for performing the work. • The software process is constantly changed or modified as the work progresses.

8. Activity • As a team:, answer the following question: What do you think are the problems with ad-hoc development (which you really face it from your previous projects).

9. Ad hoc Software Development • Some problems with ad-hoc development • Difficult to distinguish between tasks and therefore some important tasks may be ignored • Schedules, budgets, functionality, and product quality are generally (inconsistent). • If problems are found late, they are more costly to fix. • How can we avoid these problems?

10. We need .. A Software process Model which is “an abstract representation of a process. It presents a description of a process from some particular perspective.” It provides guidelines to organize how software process activities should be performed and in what order.

11. SW Process Models • The waterfall model • Separate and distinct phases of specification and development. • Evolutionary development • Specification, development and validation are interleaved. • Component-based software engineering • The system is assembled from existing components. • There are many variants of these models e.g. formal development where a waterfall-like process is used but the specification is a formal specification that is refined through several stages to an implementable design.

12. THE Waterfall model

13. The Waterfall Model Niagara Falls, Canada

14. The Waterfall Model • Oldest model, it’s been around since 1970. • Called “Linear Sequential Model”. • Most widely used model for SW engineering • Documentation is produced at each stage.

15. Phases • Requirements analysis and definition • System and software design • Implementation and unit testing • Integration and system testing • Operation and maintenance

16. Waterfall model diagram

17. Evolutionary Models

18. Evolutionary Models

19. Evolutionary Models

20. The Exploratory Model Objective is to work with customers and evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer.

21. The Prototyping Model • Objective is to understand the system requirements. Should start with poorly understood requirements to clarify what is really needed. • When a customer defines a set of general objectives for a software but does not identify detailed input, processing, or output requirement. • It consists of the iterating phases: • Requirements gathering • Design and build SW prototype • Evaluate prototype with customer • Refine requirements

22. The Prototyping Model

23. CBSE model

24. Component Based Software Engineering (CBSE) • Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems. • Process stages • Component analysis; • Requirements modification; • System design with reuse; • Development and integration. • This approach is becoming increasingly used as component standards have emerged.

25. Component Based Software Engineering (CBSE)

26. Process iteration

27. Process iteration • 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. • Iteration can be applied to any of the generic process models.

28. Process iteration

29. The Incremental Model 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. User requirements are prioritised and the highest priority requirements are included in early increments. Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.

30. The Incremental Model

31. The Spiral Model • Process is represented as a spiral rather than as a sequence of activities with backtracking. • Each loop in the spiral represents a phase in the process. • No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required. • Risks are explicitly assessed and resolved throughout the process. • Suitable for large, expensive and complicated projects

32. The Spiral Model

33. The Spiral Model • Objective setting • Specific objectives for the phase are identified. • Risk assessment and reduction • Risks are assessed and activities put in place to reduce the key risks. • Development and validation • A development model for the system is chosen which can be any of the generic models. • Planning • The project is reviewed and the next phase of the spiral is planned.

34. The Spiral Model • Risk driven process model • Different risk patterns can lead to choosing different process models • What is a risk? • Situations or possible events that may cause a project to fail to meet its goal. • Example risks: • Experienced staff leave the project • Hardware which is essential for the system will not be delivered on schedule • (more about risks in Chapter 3)

35. Agile Models- Individual Reading – Assignment 2 • Individual Homework Assignment • Read Chapter 17 of our text book. • Watch this lecture video : http://www.cosmolearning.com/video-lectures/agile-development/ • Search the internet Then : Write a short essay about Agile models (history, description, difference from other models, known methods, evaluation and suitability. The essay should not exceed 1000 words. More details can be found on the course blog.

36. Evaluation of Models

37. Activity • Formulate teams • Lets Play a game  • Lego • Papers • Discussion ( built a plant pot) using empty can and papers

38. The Waterfall Model • Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. • Only appropriate when the requirements are well-understood and changes will be fairly limited during the design process. • The waterfall model is mostly used for large systems engineering projects.

39. The Exploratory Model • Problems • Lack of process visibility; • Systems are often poorly structured; • Applicability • For small or medium-size interactive systems; • For parts of large systems (e.g. the user interface); • For short-lifetime systems.

40. The Prototyping Model • Advantages • Users get a feel for the actual system • Developers get to build something immediately • Specifications can be developed incrementally • Disadvantages • The developer may make implementation compromises in order to get a prototype working quickly. • The process in not visible (few documents that reflect every version of the system) • Systems poorly structured

41. Component Based Software Engineering (CBSE) • Advantages: • Reduce amount of software to be developed • Reduce costs and risks • Faster delivery • Disadvantages: • Requirements compromises, system does not meet real needs of users • Control over system evolution is lost

42. The Incremental Model Advantages: • Customer value can be delivered with each increment so system functionality is available earlier. • Early increments act as a prototype to help elicit requirements for later increments. • Lower risk of overall project failure. • The highest priority system services tend to receive the most testing.

43. The Incremental Model Disadvantages: • Increments should be relatively small (20,000 lines of code) • Can be difficult to map the customer’s requirements onto increments of the right size • Hard to identify common functions • Can take a long time

44. The Spiral Model Advantages: • Risks are explicitly assessed and resolved throughout the process. • Software engineers can start working on the project earlier rather than wading through a lengthy early design process.

45. The Spiral Model Disadvantages: • Requires highly skilled people in risk analysis and planning • Requires more time, and is more expensive • Estimates of budget and time are harder to judge at the beginning of the project since the requirements evolve through the process

46. Which model is best? • The choice of a model depends on the project circumstances and requirements. • A combination of models is used sometimes get the benefits of more than one model. • Criteria for evaluating models: • Risk management • Quality / cost control • Visibility of progress • Early system functionality • Customer involvement and feedback

47. Models Cost