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Chapter 2 Software Process Models

Chapter 2 Software Process Models. 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. Overview.

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Chapter 2 Software Process Models

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  1. Chapter 2Software Process Models Maha Al-Yahya First Semester 1430-1431

  2. 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 Maha Al-Yahya First Semester 1430-1431

  3. Overview • What are SW process models? • Why SW process models? • The Waterfall Model • Evolutionary Models • The CBSE Model • Agile Models • Which is best? Maha Al-Yahya First Semester 1430-1431

  4. A Software Process is A structured set of activities required to develop a software system Maha Al-Yahya First Semester 1430-1431

  5. Ad hoc Software Development • Developing software without planning for each phase, and without specifying tasks, deliverables, or time constraints. • 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. Maha Al-Yahya First Semester 1430-1431

  6. Activity • Formulate Teams • As a team, answer the following question: What do you think are the problems with ad-hoc development? Maha Al-Yahya First Semester 1430-1431

  7. 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? Maha Al-Yahya First Semester 1430-1431

  8. 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. Maha Al-Yahya First Semester 1430-1431

  9. SW Process Models • Waterfall model • Evolutionary models • Component-based development model • Iterative Models Maha Al-Yahya First Semester 1430-1431

  10. THE Waterfall model Maha Al-Yahya First Semester 1430-1431

  11. The Waterfall Model Niagara Falls, Canada Maha Al-Yahya First Semester 1430-1431

  12. 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. Maha Al-Yahya First Semester 1430-1431

  13. Phases • Requirements analysis and definition • System and software design • Implementation and unit testing • Integration and system testing • Operation and maintenance Maha Al-Yahya First Semester 1430-1431

  14. Waterfall model diagram Maha Al-Yahya First Semester 1430-1431

  15. Evolutionary Models Maha Al-Yahya First Semester 1430-1431

  16. Evolutionary Models Maha Al-Yahya First Semester 1430-1431

  17. Evolutionary Development Maha Al-Yahya First Semester 1430-1431

  18. Evolutionary Development • Specification, development, and validation are interleaved rather than separate. • Rapid feedback across activities. Maha Al-Yahya First Semester 1430-1431

  19. 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. Maha Al-Yahya First Semester 1430-1431

  20. The Prototyping Model • 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 Maha Al-Yahya First Semester 1430-1431

  21. The Prototyping Model Maha Al-Yahya First Semester 1430-1431

  22. CBSE model Maha Al-Yahya First Semester 1430-1431

  23. Component Based Software Engineering (CBSE) • Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems. • This approach is becoming increasingly used as component standards have emerged. Maha Al-Yahya First Semester 1430-1431

  24. CBSE Process Stages Maha Al-Yahya First Semester 1430-1431

  25. Iterative models Maha Al-Yahya First Semester 1430-1431

  26. Iterative Models Maha Al-Yahya First Semester 1430-1431

  27. 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. Maha Al-Yahya First Semester 1430-1431

  28. The Incremental Model 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. Maha Al-Yahya First Semester 1430-1431

  29. The Incremental Model Maha Al-Yahya First Semester 1430-1431

  30. The Spiral Model • Defined by Barry Boehm in his 1988 article A Spiral Model of Software Development and Enhancement. • 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. • Suitable for large, expensive and complicated projects Maha Al-Yahya First Semester 1430-1431

  31. The Spiral Model Maha Al-Yahya First Semester 1430-1431

  32. 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. Maha Al-Yahya First Semester 1430-1431

  33. 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) Maha Al-Yahya First Semester 1430-1431

  34. Agile ModelsAssignment 1: Individual Homework • 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. Maha Al-Yahya First Semester 1430-1431

  35. Process ActivitiesReading Please read your book section 4.3 pages 74-82 Maha Al-Yahya First Semester 1430-1431

  36. Evaluation of Models Maha Al-Yahya First Semester 1430-1431

  37. Activity • Formulate teams • Each team chooses a slip from the envelope. • List the advantages, disadvantages, and when to use a certain software process model. • Share your list with the rest of the class. Maha Al-Yahya First Semester 1430-1431

  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. Maha Al-Yahya First Semester 1430-1431

  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. Maha Al-Yahya First Semester 1430-1431

  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 Maha Al-Yahya First Semester 1430-1431

  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 Maha Al-Yahya First Semester 1430-1431

  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. Maha Al-Yahya First Semester 1430-1431

  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 Maha Al-Yahya First Semester 1430-1431

  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. Maha Al-Yahya First Semester 1430-1431

  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 Maha Al-Yahya First Semester 1430-1431

  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 Maha Al-Yahya First Semester 1430-1431

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