Chapter 2 SW process Models

Chapter 2 Software Process Models1Maha Al-Yahya First Semester 1430-14311Objectives After completing this chapter you should be able to:Describe various SW process modelsIdentify the advantages and disadvantages of each modelEvaluate the applicability of each model for a certain type of software development 2Maha Al-Yahya First Semester 1430-14312OverviewWhat are SW process models?Why SW process models?The Waterfall ModelEvolutionary ModelsThe CBSE ModelAgile ModelsWhich is best?

3Maha Al-Yahya First Semester 1430-14313A Software Process isA structured set of activities required to develop a software system4Maha Al-Yahya First Semester 1430-1431Ad 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.

5Maha Al-Yahya First Semester 1430-1431ActivityFormulate TeamsAs a team, answer the following question:

What do you think are the problems with ad-hoc development?6Maha Al-Yahya First Semester 1430-1431Ad hoc Software Development Some problems with ad-hoc developmentDifficult to distinguish between tasks and therefore some important tasks may be ignoredSchedules, 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?7Maha Al-Yahya First Semester 1430-14317We need ..A Software process Model which isan 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.

8Maha Al-Yahya First Semester 1430-1431SW Process ModelsWaterfall modelEvolutionary modelsComponent-based development modelIterative Models9Maha Al-Yahya First Semester 1430-14319THE Waterfall model10Maha Al-Yahya First Semester 1430-1431The Waterfall Model

Niagara Falls, Canada11Maha Al-Yahya First Semester 1430-1431The Waterfall ModelOldest model, its been around since 1970.Called Linear Sequential Model.Most widely used model for SW engineeringDocumentation is produced at each stage.

12Maha Al-Yahya First Semester 1430-143112PhasesRequirements analysis and definitionSystem and software designImplementation and unit testingIntegration and system testingOperation and maintenance13Maha Al-Yahya First Semester 1430-143113Waterfall model diagram

14Maha Al-Yahya First Semester 1430-1431Evolutionary Models15Maha Al-Yahya First Semester 1430-1431Evolutionary Models16Maha Al-Yahya First Semester 1430-1431Evolutionary Development

17Maha Al-Yahya First Semester 1430-143117Evolutionary DevelopmentSpecification, development, and validation are interleaved rather than separate.Rapid feedback across activities.

Maha Al-Yahya First Semester 1430-143118The Exploratory ModelObjective 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.

19Maha Al-Yahya First Semester 1430-1431The Prototyping ModelWhen 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 gatheringDesign and build SW prototypeEvaluate prototype with customerRefine requirements

20Maha Al-Yahya First Semester 1430-143120The Prototyping Model21Maha Al-Yahya First Semester 1430-1431CBSE model22Maha Al-Yahya First Semester 1430-1431Component 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.

23Maha Al-Yahya First Semester 1430-143123CBSE Process Stages

24Maha Al-Yahya First Semester 1430-143124Iterative models25Maha Al-Yahya First Semester 1430-1431Iterative Models26Maha Al-Yahya First Semester 1430-1431The Incremental ModelRather 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.

27Maha Al-Yahya First Semester 1430-1431The Incremental ModelUser 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 1430-143128The Incremental Model

29Maha Al-Yahya First Semester 1430-143129The Spiral ModelDefined 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

30Maha Al-Yahya First Semester 1430-143130The Spiral Model

31Maha Al-Yahya First Semester 1430-143131The Spiral ModelObjective settingSpecific objectives for the phase are identified.Risk assessment and reductionRisks are assessed and activities put in place to reduce the key risks.Development and validationA development model for the system is chosen which can be any of the generic models.PlanningThe project is reviewed and the next phase of the spiral is planned.32Maha Al-Yahya First Semester 1430-143132The Spiral ModelRisk driven process modelDifferent risk patterns can lead to choosing different process modelsWhat is a risk?Situations or possible events that may cause a project to fail to meet its goal. Example risks: Experienced staff leave the projectHardware which is essential for the system will not be delivered on schedule(more about risks in Chapter 3)

33Maha Al-Yahya First Semester 1430-143133Agile ModelsAssignment 1: Individual HomeworkWrite 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.

34Maha Al-Yahya First Semester 1430-1431Process ActivitiesReadingPlease read your book section 4.3 pages 74-82Maha Al-Yahya First Semester 1430-143135Evaluation of Models36Maha Al-Yahya First Semester 1430-1431ActivityFormulate teamsEach 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.

37Maha Al-Yahya First Semester 1430-1431The 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.

38Maha Al-Yahya First Semester 1430-143138The Exploratory Model ProblemsLack of process visibility;Systems are often poorly structured;ApplicabilityFor small or medium-size interactive systems;For parts of large systems (e.g. the user interface);For short-lifetime systems.39Maha Al-Yahya First Semester 1430-143139The Prototyping ModelAdvantagesUsers get a feel for the actual systemDevelopers get to build something immediatelySpecifications can be developed incrementallyDisadvantagesThe 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 structured40Maha Al-Yahya First Semester 1430-143140Component Based Software Engineering (CBSE)Advantages:Reduce amount of software to be developedReduce costs and risksFaster deliveryDisadvantages:Requirements compromises, system does not meet real needs of usersControl over system evolution is lost

41Maha Al-Yahya First Semester 1430-143141The Incremental ModelAdvantages: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.

42Maha Al-Yahya First Semester 1430-143142The Incremental ModelDisadvantages:Increments should be relatively small (20,000 lines of code)Can be difficult to map the customers requirements onto increments of the right sizeHard to identify common functions43Maha Al-Yahya First Semester 1430-143143The Spiral ModelAdvantages: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.

44Maha Al-Yahya First Semester 1430-143144The Spiral ModelDisadvantages:Requires highly skilled people in risk analysis and planningRequires 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

45Maha Al-Yahya First Semester 1430-143145Which 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 managementQuality / cost controlVisibility of progressEarly system functionalityCustomer involvement and feedback

46Maha Al-Yahya First Semester 1430-143146Requirements

Operation & Maintenance

Test & Integration

Code & Unit Test

Design