1. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 1 Software Processes

2. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 2 Objectives l To introduce software process models l To describe three generic process models and when they may be used l To describe outline process models for requirements engineering, software development, testing and evolution l To explain the Rational Unified Process model l To introduce CASE technology to support software process activities

3. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 3 Topics covered l Software process models l Process iteration l Process activities l The Rational Unified Process l Computer-aided software engineering

4. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 4 The software process l A structured set of activities required to develop a software system Requirement Specification Design Implementation Testing Maintenance l A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.

5. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 5 Deliverables l Requirement l Design l Implementation l Testing & Integration l Maintenance Requirement Specifications Design Specifications Source & executable Code Test Cases & Plan

6. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 6 Process activities l Software Requirement specification l Software design and implementation l Software validation l Software evolution

7. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 7 Software specification l The process of establishing what services are required and the constraints on the system’s operation and development. l Requirements engineering process Feasibility study; Requirements elicitation and analysis; Requirements specification; Requirements validation.

8. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 8 The requirements engineering process

9. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 9 Software design and implementation l The process of converting the system specification into an executable system. l Software design Design a software structure that realises the specification; l Implementation Translate this structure into an executable program; l The activities of design and implementation are closely related and may be inter-leaved.

10. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 10 Design process activities l Architectural design l Abstract specification l Interface design l Component design l Data structure design l Algorithm design

11. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 11 The software design process

12. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 12 Structured methods l Systematic approaches to developing a software design. l The design is usually documented as a set of graphical models. l Possible models Object model; Sequence model; State transition model; Structural model; Data-flow model.

13. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 13 Programming and debugging l Translating a design into a program and removing errors from that program. l Programming is a personal activity - there is no generic programming process. l Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process.

14. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 14 The debugging process

15. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 15 Software validation l Verification and validation (V & V) is intended to show that a system conforms to its specification and meets the requirements of the system customer. l Involves checking and review processes and system testing. l 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.

16. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 16 Definition of Validation & Verification l Verification “Am I building the product right?” Determine correctness with respect to its specification; Can begin after specification Objective (if specification is formal) l Validation “Am I building the right product?” Predict correspondence; Can begin as soon as project starts Subjective (since user need is not formal) l Asking 2 important questions

17. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 17 The testing process

18. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 18 Testing stages l Component or unit testing Individual components are tested independently; Components may be functions or objects or coherent groupings of these entities. l System testing Testing of the system as a whole. Testing of emergent properties is particularly important. l Acceptance testing Testing with customer data to check that the system meets the customer’s needs.

19. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 19 Testing phases

20. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 20 Software evolution l Software is inherently flexible and can change. l As requirements change through changing business circumstances, the software that supports the business must also evolve and change. l Although there has been a demarcation between development and evolution (maintenance) this is increasingly irrelevant as fewer and fewer systems are completely new.

21. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 21 System evolution

22. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 22 Generic software process models l The waterfall model Separate and distinct phases of specification and development. l Evolutionary development Specification, development and validation are interleaved. l Component-based software engineering The system is assembled from existing components. l 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.

23. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 23 Waterfall model

24. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 24 Waterfall model phases l Applicability: We use the waterfall model when we have fixed, stable and known requirements. The waterfall model is mostly used for large systems engineering projects where a system is developed at several sites. l The main drawback of the waterfall model is the difficulty of accommodating change after the process is underway. One phase has to be complete before moving onto the next phase.

25. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 25 Waterfall model problems l Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. l Therefore, this model is only appropriate when the requirements are well-understood and changes will be fairly limited during the design process. l Few business systems have stable requirements.

26. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 26 Evolutionary development l Exploratory development Objective is to work with customers and to evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer. l Throw-away prototyping Objective is to understand the system requirements. Should start with poorly understood requirements to clarify what is really needed.

27. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 27 Evolutionary development

28. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 28 Evolutionary development l Problems Lack of process visibility; Systems are often poorly structured; Special skills (e.g. in languages for rapid prototyping) may be required. l Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user interface); For short-lifetime systems.

29. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 29 Component-based software engineering l Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems. l Process stages Component analysis; Requirements modification; System design with reuse; Development and integration. l This approach is becoming increasingly used as component standards have emerged.

30. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 30 Reuse-oriented development

31. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 31 Process iteration l System 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. l Iteration can be applied to any of the generic process models. l Two (related) approaches Incremental delivery; Spiral development.

32. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 32 Incremental delivery l 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. l User requirements are prioritised and the highest priority requirements are included in early increments. l Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.

33. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 33 Incremental development

34. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 34 Incremental development advantages l Customer value can be delivered with each increment so system functionality is available earlier. l Early increments act as a prototype to help elicit requirements for later increments. l Lower risk of overall project failure. l The highest priority system services tend to receive the most testing.

35. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 35 Spiral development l Process is represented as a spiral rather than as a sequence of activities with backtracking. l Each loop in the spiral represents a phase in the process. l No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required. l Risks are explicitly assessed and resolved throughout the process.

36. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 36 Spiral model of the software process

37. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 37 Spiral model sectors l Objective setting Specific objectives for the phase are identified. l Risk assessment and reduction Risks are assessed and activities put in place to reduce the key risks. l Development and validation A development model for the system is chosen which can be any of the generic models. l Planning The project is reviewed and the next phase of the spiral is planned.

38. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 38 Computer-aided software engineering l Computer-aided software engineering (CASE) is software to support software development and evolution processes. l Activity automation Graphical editors for system model development; Data dictionary to manage design entities; Graphical UI builder for user interface construction; Debuggers to support program fault finding; Automated translators to generate new versions of a program.

39. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 39 Case technology l Case technology has led to significant improvements in the software process. However, these are not the order of magnitude improvements that were once predicted Software engineering requires creative thought - this is not readily automated; Software engineering is a team activity and, for large projects, much time is spent in team interactions. CASE technology does not really support these.

40. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 40 CASE classification l Classification helps us understand the different types of CASE tools and their support for process activities. l Functional perspective Tools are classified according to their specific function. l Process perspective Tools are classified according to process activities that are supported. l Integration perspective Tools are classified according to their organisation into integrated units.

41. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 41 Functional tool classification

42. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 42 Activity-based tool classification

43. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 43 CASE integration l Tools Support individual process tasks such as design consistency checking, text editing, etc. l Workbenches Support a process phase such as specification or design, Normally include a number of integrated tools. l Environments Support all or a substantial part of an entire software process. Normally include several integrated workbenches.

44. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 44 Tools, workbenches, environments

45. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 45 The Manifesto for Agile Software Development “We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value: Individuals and interactions over processes and tools Working software over comprehensive documentation Customer collaboration over contract negotiation Responding to change over following a plan That is, while there is value in the items on the right, we value the items on the left more.” Kent Beck et al

46. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 46 What is “Agility”? l Effective (rapid and adaptive) response to change l Effective communication among all stakeholders l Drawing the customer onto the team l Organizing a team so that it is in control of the work performed Yielding … l Rapid, incremental delivery of software

47. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 47 An Agile Process l Is driven by customer descriptions of what is required (scenarios) l Recognizes that plans are short-lived l Develops software iteratively with a heavy emphasis on construction activities l Delivers multiple ‘software increments’ l Adapts as changes occur

48. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 48 Agile Methodologies (cont’d) l Agile methods emphasize real-time communication, preferably face-to-face, over written documents. l Team: include all the people necessary to finish software programmers and their customers, testers, interaction designers, technical writers, and managers. l Most agile methods share iterative development's emphasis on building releasable software in short time periods. Agile methods differ from iterative methods time period is measured in weeks rather than months treat their time period as strict time box. work is performed in a highly collaborative manner.

49. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 49 Agile Methodologies (cont’d) l Reject the notion that we should design for future change don’t “borrow trouble” l Seductive, but Beware: it is not yet widely accepted in industry, and its own proponents admit that it is not always a good choice

50. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 50 Agile or Plan driven? Low criticality Senior developers High requirements change Small number of developers Culture that thrives on chaos High criticality Junior developers Low requirements change Large number of developers Culture that demands order Agile home ground Plan-driven home ground

51. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 51 Extreme Programming (XP) l The most widely used agile process, originally proposed by Kent Beck l XP Planning Begins with the creation of “user stories” Agile team assesses each story and assigns a cost Stories are grouped to for a deliverable increment A commitment is made on delivery date After the first increment “project velocity” is used to help define subsequent delivery dates for other increments

52. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 52 Extreme Programming (XP) l XP Design Follows the KIS principle Encourage the use of CRC cards. For difficult design problems, suggests the creation of “spike solutions”—a design prototype Encourages “refactoring”—an iterative refinement of the internal program design l XP Coding Recommends the construction of a unit test for a store before coding commences (Test driven development). Encourages “pair programming” l XP Testing All unit tests are executed daily “Acceptance tests” are defined by the customer and executed to assess customer visible functionality

53. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 53 Extreme Programming (XP)

54. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 54 Scrum l Originally proposed by Schwaber and Beedle l Scrum—distinguishing features Development work is partitioned into “packets” Testing and documentation are on-going as the product is constructed Work occurs in “sprints” and is derived from a “backlog” of existing requirements Meetings are very short and sometimes conducted without chairs “demos” are delivered to the customer with the time-box allocated

55. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 55 Scrum

56. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 56 Crystal l Proposed by Cockburn and Highsmith l Crystal—distinguishing features Actually a family of process models that allow “maneuverability” based on problem characteristics Face-to-face communication is emphasized Suggests the use of “reflection workshops” to review the work habits of the team

57. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 57 Key points l Software processes are the activities involved in producing and evolving a software system. l Software process models are abstract representations of these processes. l General activities are specification, design and implementation, validation and evolution. l Generic process models describe the organisation of software processes. Examples include the waterfall model, evolutionary development and component- based software engineering. l Iterative process models describe the software process as a cycle of activities.

58. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 4 Slide 58 Key points l Requirements engineering is the process of developing a software specification. l Design and implementation processes transform the specification to an executable program. l Validation involves checking that the system meets to its specification and user needs. l Evolution is concerned with modifying the system after it is in use. l The Rational Unified Process is a generic process model that separates activities from phases. l CASE technology supports software process activities.