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Software Life Cycle Models/Processes

Software Life Cycle Models/Processes

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Software Life Cycle Models/Processes

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  1. Software Life Cycle Models/Processes CMSC 345 Software EngineeringFall 05 Section 0301

  2. Objectives • To introduce the general phases of the software development life cycle • To introduce the software process model concept • To describe different generic process models and their pros and cons

  3. The Software Process • A structured set of activities required to develop a software system. These activities include: • Requirements (Specification) • Design • Implementation (Coding) • Testing (Validation) • Maintenance (Evolution) • A software process model is an abstract representation of a process.

  4. Requirements The process of establishing • what services are required of the system • the constraints on the system’s operation and development Requirements are the what part

  5. A Generic Requirements Process

  6. Software Design The process of converting the system specification (requirements) into a software structure that realizes that specification Design is the How part

  7. A Generic Software Design Process

  8. Implementation • Translating a design into a program and removing errors from that program • Programming is a personal activity - there is no generic programming process. • Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process. • The activities of design and implementation are closely related and generally are interleaved.

  9. Testing • Verification and validation is intended to show that a system conforms to its specification and meets the requirements of the system customer. • Involves checking and review processes and system testing • System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.

  10. A Generic Testing Process

  11. Generic Testing Planning

  12. System Maintenance • Software is inherently flexible and can change (as opposed to hardware). • In the past, there has been a demarcation between development and evolution (maintenance). This is increasingly irrelevant as fewer and fewer systems are completely new. • Software engineering should be thought of as an evolutionary process where software is continually changed over its lifetime in response to customer needs.

  13. System Evolution

  14. Software Process Models • The Waterfall model • Separate, non-overlapping phases of specification and development • Evolutionary development • Specification and development are interleaved • Reuse-based development • The system is assembled from some (most likely) or all existing components

  15. Waterfall Model

  16. Waterfall Model Pros and Cons Pros Cons

  17. Evolutionary Development Two general types: • Exploratory development • Objective is to work with the customers to evolve a final system from an initial outline specification. Process starts with the well-understood requirements. • Prototyping (throw-away?) • Objective is to understand the system requirements. Process starts with the poorly understood requirements.

  18. Evolutionary Development

  19. Evolutionary Development Pros and Cons Pros Cons

  20. Prototyping Pros and Cons Pros Cons

  21. Question Should a prototype be evolved into the final system or thrown away?

  22. Reuse-oriented Development • Based on systematic reuse where systems are integrated from existing components or COTS (commercial-off-the-shelf) systems • This approach is becoming more important, but there is still limited experience with it.

  23. Reuse-oriented Development

  24. Reuse-oriented Development Pros and Cons Pros Cons

  25. 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, especially for large systems. • Iteration can be applied to any of the generic process models. • Examples of two iterative approaches: • Incremental development • Spiral development

  26. Incremental Development • 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 prioritized 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.

  27. Incremental Development

  28. Incremental Development Advantages • Customers do not have to wait until the entire system is delivered until they can gain value from it. • 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.

  29. Spiral Development • 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.

  30. Spiral Model of the Software Process

  31. Spiral Model Sectors • 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

  32. Modified Spiral

  33. Efficient Lots of “just in time” Adjust to circumstances (Re)Plan regularly Lots of person-to-person comm. Adaptively cut fat in the process Hacking Giant Energy Up Front (GEUF) Only XP (XP is one alternate) Plan-less People sitting in isolation rigid adherence Good agile developmentIs / DoesIsn’t / Doesn’t Agility *can* Be heavier or lighter, depending on circumstances Use various requirements techniques (e.g., use cases, stories, features) In agile development we value following the principles over following specific practices !

  34. (Never forget this!) Lighter-agile vs. Heavier-agile : Light is good, but has limits. # people needed Heavy methodology more people Light methodology fewer people Problem Size

  35. eXtreme programmingPrinciples

  36. Contents • Agile methodologies • What is XP? • Risk: the basic problem and how XP address risks • Basic principles • Basic activities • Practices

  37. Agile alliance • February 2001 – group of 17 people • • Agile word • The manifesto • Principles and recommendations

  38. Methodology • Software development methodology – the set of conventions the team adopts • Methodology can be lightened • By getting running software out for examination sooner • By increasing the richness of the communication channels • Light delivers more quickly and with reduced cost

  39. Light methodologies • XP • Adaptive Software Development • Scrum • Crystal • Feature Driven Development • DSDM (Dynamic System Development Method)

  40. Why “extreme” XP takes commonsense principles and practices to extreme levels. • If code reviews are good, we'll review code all the time (pair programming). • If testing is good, everybody will test all the time (unit testing), even the customers (functional testing).

  41. Why “extreme”-2 • If design is good, we'll make it part of everybody's daily business (refactoring). • If simplicity is good, we'll always leave the system with the simplest design that supports its current functionality (the simplest thing that could possibly work). • If architecture is important, everybody will work defining and refining the architecture all the time (metaphor).

  42. Why “extreme”-3 • If integration testing is important, then we'll integrate and test several times a day (continuous integration). • If short iterations are good, we'll make the iterations really, really short—seconds and minutes and hours, not weeks and months and years (the Planning Game).

  43. XP promises • To reduce project risk, • To improve responsiveness to business changes, • To improve productivity throughout the life of a system, • To add fun to building software in teams All at the same time.

  44. What Is XP? XP is a • lightweight, • efficient, • low-risk, • flexible, • predictable, • scientific, and • fun way to develop software.

  45. Innovation of XP • Putting all proven practices (for the implementation strategy and for the management strategy) under one umbrella • Making sure they are practiced as thoroughly as possible. • Making sure the practices support each other to the greatest possible degree.

  46. Risk: The Basic Problem • Schedule slips • Project canceled • System goes sour • Defect rate • Business misunderstood • Business changes • False feature rich • Staff turnover

  47. How does XP address risks? • Schedule slips • short Release cycles - a few months at most, • Iterations of customer requested features - one- to four-week. • Tasks - one- to three-day • implementing the highest priority features first

  48. How does XP address risks?-2 • Project canceled • XP asks the customer to choose the smallest release that makes the most business sense. • System goes sour • comprehensive suite of tests, which are run and re-run after every change. • XP always keeps the system in prime condition.

  49. How does XP address risks?-3 • Defect rate - XP tests from the perspective of • programmers - writing tests function-by function • customers - writing tests program-feature-by-program-feature. • Business misunderstood • customer to be an integral part of the team.

  50. How does XP address risks?-4 • Business changes • XP shortens the release cycle, so there is less change during the development of a single release. • False feature rich • only the highest priority tasks are addressed. • Staff turnover • programmers accept responsibility for estimating and completing their own work • human contact among the team