Chapter 2 SW process Models

1Chapter 2 SW Process Models1Assigment12Problem 1:4.05. Disclose to all concerned parties those conflicts of interest that cannot reasonably be avoided or escapedProblem 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.Objectives 3After 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 3Overview4What are SW process models?Why SW process models?The Waterfall ModelEvolutionary ModelsThe CBSE ModelAgile ModelsWhich is best?

4A Software Process isA structured set of activities required to develop a software systemActivityFormulate TeamsAs a team:, answer the following question:

In your previous project, how is the process divided? (according to)Activities?Team work?Documentations?Ad hoc Software Development 7Developing 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.

Activity8As 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).Ad hoc Software Development 9Some 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?9We need ..10A 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.

SW Process Models11The waterfall modelSeparate and distinct phases of specification and development.Evolutionary developmentSpecification, development and validation are interleaved.Component-based software engineeringThe 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.11THE Waterfall model12The Waterfall Model

Niagara Falls, Canada

The Waterfall Model14Oldest model, its been around since 1970.Called Linear Sequential Model.Most widely used model for SW engineeringDocumentation is produced at each stage.

14Phases15Requirements analysis and definitionSystem and software designImplementation and unit testingIntegration and system testingOperation and maintenance15Waterfall model diagram

Evolutionary Models17Evolutionary Models18Evolutionary Models19

19The 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.

The Prototyping Model21Objective 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 gatheringDesign and build SW prototypeEvaluate prototype with customerRefine requirements

21The Prototyping Model22CBSE model23Component Based Software Engineering (CBSE)24Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.Process stagesComponent analysis;Requirements modification;System design with reuse;Development and integration.This approach is becoming increasingly used as component standards have emerged.

24Component Based Software Engineering (CBSE)25

25Process iteration26Process iteration27System 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.Process iteration28The 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.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.

The Incremental Model30

30The Spiral Model31Process 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

31The Spiral Model32

32The Spiral Model33Objective 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.33The Spiral Model34Risk 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)

34Agile Models- Individual Reading Assignment 235Individual Homework AssignmentRead 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.

Evaluation of Models36Activity37Formulate teams

Lets Play a game

LegoPapersDiscussion ( built a plant pot) using empty can and papers

The Waterfall Model 38Inflexible 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.

38The Exploratory Model 39ProblemsLack 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.39The Prototyping Model40AdvantagesUsers 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 structured40Component Based Software Engineering (CBSE)41Advantages: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

41The Incremental Model42Advantages: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.

42The Incremental Model43Disadvantages: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 functionsCan take a long time43The Spiral Model44Advantages: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.

44The Spiral Model45Disadvantages: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

45Which model is best?46The 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

46Models Cost47

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Requirements

Operation & Maintenance

Test & Integration

Code & Unit Test

Design