Software Life-Cycle Models

1. Software Life-Cycle Models • Life-cycle model (formerly, process model) • The steps through which the product progresses • Requirements phase • Specification phase • Design phase • Implementation phase • Integration phase • Maintenance phase • Retirement

2. Overview • Build-and-fix model • Waterfall model • Rapid prototyping model • Incremental model • Extreme programming • Synchronize-and-stabilize model • Spiral model • V model • Object-oriented life-cycle models • Comparison of life-cycle models

3. Build and Fix Model • Product is constructed with no specifications

4. Build and Fix Model • Problems • No specifications • No design • Totally unsatisfactory • Need life-cycle model • “Game plan” • Phases • Milestones

5. Build and Fix Model • Disadvantages • Cost: changes are made in the later phases of software development life cycle • Maintenance is difficult

6. Waterfall Model • Requirements determined and checked by the client and SQA group. • Specifications drawn • Specification checked by the SQA group and shown to the client. • Sign off the specification document, • Software project management plan - checked by SQA group

7. Waterfall Model • Design -“how to do”. If some problems in specification (Incomplete, contradictory and ambiguous.) • Incomplete-: some features of the product have been omitted. • Contradictory-: two or more statements in the specification document differ. • Ambiguous-: the specification document has more than one possible interpretation.

8. Waterfall Model • Implementation: If flaws-- Feedback loops permits modification to be made to design document, specification document and requirement if needed. • Documentation : No phase without documentation Also approved by SQA group.

9. Waterfall Model • Acceptance Testing: when the product is completed it is given to the client for testing. Deliverables at this stage include the user manual and other documentation listed in the contract. When the client agrees that that the product is as per the specification document, the product is installed on the client’s machine. • Installation • Maintenance- changes made after the installation of the product are handled in this phase.

10. Waterfall Model (contd) • Characterized by • Feedback loops • Documentation-driven • Advantages • Enforce the documentation after every phase • Products of each phase checked by SQA. • Testing is Inherent in every phase • Dynamic model-feedback loops • Maintenance easier

11. Waterfall Model (contd) • Disadvantages • Specification documents are long, detailed and very boring to read. • Client is inexperienced in reading software specification document because the specification document is written in the style client won’t understand. • The first time the client see a working product is only after the entire product has been coded.

12. Rapid Prototyping Model • Build a rapid prototype • The client and future users interact and experiment with it. • Once the client is satisfied, the developer can draw up specification document. • The feedback loops are not required in this model as the working prototype has been validated by the client, it is reasonable to expect that the resulting specification document will be correct. • .

13. Rapid Prototyping Model • In the rapid prototyping model, the fact that a preliminary working model has been built tends to lessen the need to repair the design during or after the implementation.

14. Rapid Prototyping Model • Rapid • Construct the prototype as rapidly as possible • The developers should develop the rapid prototype as rapidly as possible the speed up the software development process. • Use of rapid prototype - determine what the client’s real needs are; • Rapid prototype implementation is discarded • Internal structure of rapid prototype is not relevant. • Prototype is modified rapidly to reflect client need.

15. Waterfall and Rapid Prototyping Models • Waterfall model • Many successes • Client needs • Rapid prototyping model • Not proved • Has own problems • Solution • Rapid prototyping for requirements phase • Waterfall for rest of life cycle

16. Rapid Prototyping Model • Advantages • As compared to waterfall model – • Development of the process is linear preceding from rapid prototype to delivered product. • Feedback loops are less likely to be (needed) in this case.

17. Rapid Prototyping Model • Disadvantages • If user cannot be involved throughout the life cycle this model is not useful. • Development time may not be reduced if reusable components are not available. • Highly specialized and skilled developers are expected and such developers may not be available easily. • Client expects changes to made as rapidly as the rapid prototype

18. Incremental Model • The product is designed, implemented, integrated and tested as a series of incremental builds, where a build consist of code pieces from various modules interacting to provide a specific functional capability.

19. Incremental Model • At each stage- a new build is coded and then integrated and tested as a whole. • Break up the target product into builds subject to constraint that each build is integrated into existing software, the resulting product must be testable.

20. Incremental Model • If a product has too many builds, then at each stage, considerable time is spent in the integration testing of only a small amount of additional functionality. On the other hand, if a product has too few build then the incremental model degenerated into build and fix model. Analysis – • In waterfall and rapid prototyping model – • deliver to client a complete product • There is project delivery date • Incremented Model – It deliver an operational quality product at each stage

21. Incremental Model (contd) • Advantages • Gradual introduction of the product via this model provides time for the client to adjust to the new product. • Change and Adoptions are natural

22. Incremental Model (contd) • Disadvantages • Each additional build has to be incorporated into existing structure without destroying what has been made till date. • Addition of new build should be simple and straightforward. • Incremental model does not distinguish between developing a product and maintaining it. • Begin with a design that support entire product • View product as a sequence of builds, each independent of next.

23. Incremental Model (contd) • More risky concurrent version—pieces may not fit

24. Extreme Programming • Somewhat controversial new approach • Software development team determines the various features (Stories) • Estimate duration and cost of each feature • Client select features for next build using cost benefit analysis • Each build is divided into tasks • Test cases for task are drawn up first • Pair programming (working with a partner on one screen) • Continuous integration of tasks

25. Extreme Programming • Tasks are integrated into the current version of the product • Test cases used for the tasks are retained and utilized in all further integration testing

26. Unusual Features of XP • Computers are put in center of large room lined with small cubicles • Client representative is always present • No individual can work overtime for 2 successive weeks • No specialization. All members of XP team work on specifications, design, code and testing • There is no overall design is modified while the product is being built. The design is modified while product is being built. This procedure is known as refactoring

27. Evaluating XP • XP has had some successes on small and medium sized projects • Good when requirements are vague or changing • Too soon to evaluate XP

28. Synchronize-and Stabilize Model • Microsoft’s life-cycle model • Requirements analysis—interview potential customers and extract a list of features with priorities set up by the customers • Draw up specifications • Divide project into 3 or 4 builds • First build consists of the most critical features, the second build consists of the next most critical features and so on. • Each build is carried out by small teams working in parallel

29. Synchronize-and Stabilize Model (contd) • At the end of the day all the teams—put together the partially completed components and synchronize (test and debug) • At the end of the build—stabilize (freeze build i.e. any remaining faults that have been detected are fixed). • Repeated synchronization steps ensure that the components always work together • Developers get early insights into operation of product and can modify the requirements

30. Spiral Model • Simplified form • Waterfall model plus risk analysis • Precede each phase by • Alternatives • Risk analysis • Follow each phase by • Evaluation • Planning of next phase

31. Simplified Spiral Model • If risks cannot be resolved, project is immediately terminated • Prototypes can be effectively used to provide information about certain classes of risks

32. Full Spiral Model • Represents cumulative cost to date and progress through the spiral • Each cycle of the spiral corresponds to a phase • Phase begins by determining objectives of that phase, alternatives for achieving those objectives, and constraints imposed on these alternatives • Strategy is analyzed from view point of risk • If all risks are successfully resolved development starts • Then the results of the phase are evaluated

33. Full Spiral Model (contd)

34. Analysis of Spiral Model • Strengths • Incorporation of software quality • Easy to judge how much to test as risks for too much and too low testing are analyzed • No distinction between development, maintenance i.e. maintenance is treated same way as development • Weaknesses • For large-scale software only. In case of contract all risk analysis should be made before the contract is signed. • Skilled developers are required for analyzing and detecting potential risks • For internal (in-house) software only

35. Object-Oriented Life-Cycle Models • Need for iteration within and between phases • Fountain model • Recursive/parallel life cycle • Round-trip gestalt • Unified software development process • All incorporate some form of • Iteration • Parallelism • Incremental development

36. Fountain Model • Features • Overlap (parallelism) • Arrows (iteration) • Smaller maintenance circle

37. V Shaped Software Life cycle Model • There is a shift in testing activities from validation to verification where we want to review/inspect every activity of software development life cycle. • These verification activities are treated as error preventive exercises and are applied at requirements analysis and specification phase, high level design, detailed design phase, implementation phase. • We not only want to improve the quality of end products of all phases but also want to design test cases and test plans during these phases. • V shaped model is the modified form of waterfall model with a special focus on testing activities at every phase.

38. V Shaped Software Life cycle Model A Requirements Analysis and Specification Acceptance Testing B System testing High level design C Development Part Unit and Integration testing Detailed design Testing Part Implementation A: Acceptance test cases design and planning B: System test cases design and planning C: Unit and integration test cases design and planning

39. V Shaped Software Life cycle Model • The model brings the quality into the development of our products • The encouragement of writing test cases and test plans in the early phases of software development life cycle is the real strength of this model. • We require more resources to implement this model as compared to waterfall model. • LIMITATIONS • This model suffers from many disadvantages of waterfall model like non availability of working version of the product until late in the life cycle, difficult to accommodate any change etc. this model has also limited applications in today’s iterative software processes

40. Conclusions • Different life-cycle models • Each with own strengths • Each with own weaknesses • Criteria for deciding on a model include • The organization • Its management • Skills of the employees • The nature of the product • Best suggestion • “Mix-and-match” life-cycle model

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