Chapter 2 Software Processes

Transcription

1 Chapter 2 Software Processes Chapter 2 Software Processes Slide 1

2 Topics covered Software processes and process models Generic models: Waterfall Incremental development Reuse-oriented software engineering Basic process activities: Specification Development Validation Evolution (cont'd) Chapter 2 Software Processes Slide 2

3 Topics covered (cont d) Coping with change Software prototyping: Uses of prototypes Classifying prototypes User Interface prototyping (Mills ) Incremental Delivery (cf. Incremental development) Boehm s spiral model The Rational Unified Process (an example of a modern software development method) Chapter 2 Software Processes Slide 3

4 Software processes and process models Chapter 2 Software Processes Slide 4

5 The software process [struhk-cherd] adjective; having a clearly defined structure or organization. A process is a structured set of activities required to develop a software system. Many different software processes, but all involve: Specification defining what the system should do; Design & implementation; Validation checking that it does what the customer wants (and Verification checking that it does what is specified); Evolution changing the system in response to changing customer needs. A process MODEL is an abstract representation of a process. It presents a description of a process from some particular perspective Models should be as simple as possible, but no simpler. A. Einstein Chapter 2 Software Processes Slide 5

6 The software process [struhk-cherd] adjective; having a clearly defined structure or organization. A process is a structured set of activities required to develop a software system. Many different software processes, but all involve: Specification defining what the system should do; Design & implementation; Validation checking that it does what the customer wants (and Verification checking that it does what is specified); Evolution changing the system in response to changing customer needs. A process MODEL is an abstract representation of a process. It presents a description of a process from some particular perspective Models should be as simple as possible, but no simpler. A. Einstein Chapter 2 Software Processes Slide 6

7 Plan-driven and agile processes Plan-driven processes: all of the process activities are planned in advance and progress is measured against this plan. Agile processes: planning is incremental and it is easier to change the process to reflect changing customer requirements. In practice, most practical processes include elements of both plan-driven and agile approaches. There are no right or wrong software processes! Chapter 2 Software Processes Slide 7

8 Generic models Chapter 2 Software Processes Slide 8

9 Generic software process models The Waterfall Model plan-driven model; separate and distinct phases of specification and development. not iterative Incremental Development specification, development, & validation are interleaved; may be plan-driven or agile. Reuse-Based Development e.g., componentbased SE : the system is assembled from existing components; may be plan-driven or agile In practice, most large systems are developed using a process that incorporates elements from all of these models. (And, at no additional cost: Boehm s Spiral Model.) Chapter 2 Software Processes Slide 9

10 Waterfall model (W. Royce) Requirements definition System and software design Implementation and unit testing Integr ation and system testing Operation and maintenance Chapter 2 Software Processes Slide 10

11 Waterfall model (cont d) There are separate phases in the waterfall model: Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance Model is mostly used for large systems engineering projects where a system is developed at several sites. In such circumstances, its plan-driven nature helps in coordinating the work. Chapter 2 Software Processes Slide 11

12 Waterfall model problems Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. Thus, more appropriate when requirements are wellunderstood to begin with. This applies to relatively few systems In general, the drawback of the waterfall model is the difficulty of accommodating change after the process is underway. Chapter 2 Software Processes Slide 12

13 Incremental development Concurr ent activities Specification Initial version Outline description Development Intermediate versions Validation Final version Chapter 2 Software Processes Slide 13

14 Incremental development benefits Reduces cost of accommodating changing customer requirements - amount of analysis and documentation that has to be re-done is much less than is required with the waterfall model. Easier to get customer feedback - customers can comment on demonstrations of the software and see how much has been implemented. More rapid delivery & deployment of useful software to the customer is possible - customers may be able to use and gain value from the software earlier than is possible with a waterfall process. Chapter 2 Software Processes Slide 14

15 Incremental development problems Lack of process visibility - lack of document deliverables to measure progress. If systems are developed quickly, it is not cost-effective to produce documents that reflect every version of the system. System structure tends to degrade as new increments are added - unless time and money is spent on refactoring to improve the software, regular change tends to corrupt its structure. Incorporating further software changes becomes increasingly difficult and costly. Chapter 2 Software Processes Slide 15

16 Reuse-oriented software engineering Based on systematic (as opposed to serendipitous) reuse - systems are integrated from existing components or COTS (Commercial-Off-The-Shelf) systems. Process stages Component analysis (Assessing what s available.) Requirements modification (Why is this needed?) System design with reuse Development and integration Reuse is now the standard approach for building many types of business systems. (cont'd) Chapter 2 Software Processes Slide 16

17 Reuse-oriented software engineering (what s available?) Requirements specification Component analysis Requirements modification System design with reuse Development and integration System validation (cont'd) Chapter 2 Software Processes Slide 17

18 Types of reusable software components Web services that are developed according to service standards and which are available for remote invocation. Collections of objects that are developed as a package to be integrated within a component framework such as.net or J2EE (Java EE 6). Stand-alone software systems (COTS) that are configured for use in a particular environment. Chapter 2 Software Processes Slide 18

19 Basic process activities Chapter 2 Software Processes Slide 19

20 Process activities Actual software processes are interleaved or sequences of technical, collaborative and managerial activities with the overall goal of specifying, designing, implementing and testing a software system. The four basic process activities of specification, development, validation (& verification), and evolution are organized differently in different development processes. In the waterfall model, they are organized in sequence; in incremental development they are interleaved. Chapter 2 Software Processes Slide 20

21 Software specification / RE The process of establishing what services are required and the constraints on the system s operation and development. Requirements Engineering (RE) process: Feasibility (technical and otherwise) study Requirements elicitation and analysis Requirements specification (documentation) Requirements validation (cont'd) Chapter 2 Software Processes Slide 21

22 The requirements engineering process (waterfall perspective) Feasibility study Feasibility report Requirements elicitation and analysis System models Requir ements specification User and system requirements Requirements validation Requirements document Chapter 2 Software Processes Slide 22

23 Software design and implementation The process of producing an executable system based on the specification (waterfall perspective) Software design design a software structure that realizes the specification. Implementation translate this structure into an executable program. Note: the activities of specification, design, and implementation are closely related and may be interleaved (as with incremental development). Chapter 2 Software Processes Slide 23

24 Design activities Architectural design: identify overall structure of system, principal components (sometimes called sub-systems or modules), their relationships, and how they are distributed. Interface design: define the interfaces between system components. Component design: design how each system component will operate. Database (data structure) design: design the system data structures and how these are to be represented in a database. Chapter 2 Software Processes Slide 24

25 Software verification & validation Verification and validation (V&V) determines whether or not a system (1) conforms to its specification and (2) meets the requirements of the customer. Involves checking processes (e.g., inspections/reviews, formal verification) and program (machine-based) testing. Testing is the most commonly used V&V activity and involves executing program elements with test cases that are derived from analyzing specifications and/or program logic. Chapter 2 Software Processes Slide 25

26 Testing stages Development or component testing Individual components are tested independently; Components may be functions or objects or coherent groupings of these entities. System testing Testing of the system as a whole. Testing of emergent properties is particularly important. Acceptance testing Testing with customer data to check that the system meets the customer s needs/desires. Chapter 2 Software Processes Slide 26

27 Software evolution ( maintenance ) Software is inherently flexible and subject to change. As requirements change through changing business/environmental circumstances, the software must also evolve and change. The distinction between development and evolution has become increasingly irrelevant as fewer and fewer systems are completely new. (cont'd) Chapter 2 Software Processes Slide 27

28 Software evolution ( maintenance ) Define system requirements Assess existing systems Propose system changes Modify systems e.g., change requests Existing systems New system Chapter 2 Software Processes Slide 28

29 Coping with change Chapter 2 Software Processes Slide 29

30 Inevitability of change Change is inevitable in all large software projects. Business/environmental changes lead to new and changed system requirements. New technologies open up new possibilities for improving implementations. Changing platforms require application changes. Change costs include both re-work (redoing already completed work -- e.g., re-analyzing requirements) and the cost of implementing new functionality. Chapter 2 Software Processes Slide 30

31 Reducing the costs of re-work Change avoidance: includes process activities that can stimulate user/stakeholder anticipation of needs/requirements before development begins. E.g., a prototype may allow users/stakeholders to better envision how the system would actually be used. Change tolerance: the development process and/or product may be designed to reduce change costs. If the systems is delivered in increments (e.g., via Mills Incremental Delivery), re-work may only be required in those increments that have already been developed. Information hiding techniques (isolating potentially changeable design decisions) may be employed in product design. Chapter 2 Software Processes Slide 31

32 Software prototyping Chapter 2 Software Processes Slide 32

33 What is prototyping? An iterative process emphasizing: Rapid development Concreteness and evaluative use (a real system is developed and presented to real users for hands-on evaluation) Consideration of alternatives Feedback Modification Chapter 2 Software Processes Slide 33

34 General Prototyping process What to include & what NOT to include. Establish prototype objectives Define prototype functionality Develop prototype Evaluate prototype Prototyping plan Outline definition Executable prototype Evaluation report Chapter 2 Software Processes Slide 34

35 Uses of prototypes Principal use is to help customers and developers better understand system requirements. Experimentation stimulates anticipation of how a system could/may actually be used. Attempting to use functions together to accomplish some task often reveals subtle requirements problems. (cont d) Chapter 2 Software Processes Slide 35

36 Uses of prototypes (cont d) Other potential uses: 1. Evaluating proposed solutions for feasibility (= Experimental Prototyping ) 2. Develop and evaluate User Interface designs 3. Back-to-back testing 4. Training users before system delivery Prototyping is most often undertaken as a risk reduction activity. Chapter 2 Software Processes Slide 36

37 Classifying prototypes By purpose: Throw-away prototyping to elicit and validate requirements Experimental prototyping to evaluate proposed solutions for feasibility, performance, etc. horizontal vs. vertical (breadth vs. depth) mockups vs. breadboards (form vs. function) Wizard of Oz prototyping (Turing test reversed) Chapter 2 Software Processes Slide 37

38 Throw-away prototyping Outline requirements Develop prototype Elicit/validate REQMTS Evaluate prototype Specify system Reusable components Develop software Validate system Delivered software system Chapter 2 Software Processes Slide 38

39 Classifying prototypes By purpose: Throw-away prototyping to elicit and validate requirements Experimental prototyping to evaluate proposed solutions for feasibility, performance, etc. horizontal vs. vertical (breadth vs. depth) mockups vs. breadboards (form vs. function) Wizard of Oz prototyping (Turing test reversed) Chapter 2 Software Processes Slide 39

40 Classifying prototypes By purpose: Throw-away prototyping to elicit and validate requirements Experimental prototyping to evaluate proposed solutions for feasibility, performance, etc. horizontal vs. vertical (breadth vs. depth) mockups vs. breadboards (form vs. function) Wizard of Oz prototyping (Turing test reversed) Chapter 2 Software Processes Slide 40

41 high Vertical prototype F i d e l i t y low points of comparable effort Horizontal prototype few Number of features many

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44 Classifying prototypes By purpose: Throw-away prototyping to elicit and validate requirements Experimental prototyping to evaluate proposed solutions for feasibility, performance, etc. horizontal vs. vertical (breadth vs. depth) mockups vs. breadboards (form vs. function) Wizard of Oz prototyping (Turing test reversed) Chapter 2 Software Processes Slide 44

45 Quin Tech Self-service check-in and baggage drop-off design The design was tested through a full-scale mock-up. Chapter 2 Software Processes Slide 45

46 Electronic circuit on a breadboard (REUK.co.uk) There is no need to solder anything, and the components can be moved around and the circuit modified thousands of times without damaging parts. Chapter 2 Software Processes Slide 46

47 Classifying prototypes By purpose: Throw-away prototyping to elicit and validate requirements Experimental prototyping to evaluate proposed solutions for feasibility, performance, etc. horizontal vs. vertical (breadth vs. depth) mockups vs. breadboards (form vs. function) Wizard of Oz prototyping (Turing test reversed?) Chapter 2 Software Processes Slide 47

48 The Turing Test (Alan Turing, 1950) Chapter 2 Software Processes Slide 48

49 The Wizard of Oz exposed The truth is the Wizard was an illusion created by a man hidden behind a curtain. Chapter 2 Software Processes Slide 49

50 Prototyping versus simulation What s the difference between prototyping and simulation? Chapter 2 Software Processes Slide 50

51 Throw-away prototype delivery? Developers may be pressurized to deliver a throwaway prototype as the final system. This is problematic... It may be impossible to meet non-functional requirements with the prototype. The prototype is almost certainly undocumented. The system (prototype) may be poorly structured and therefore difficult to maintain. Normal organizational quality standards may not have been applied. Chapter 2 Software Processes Slide 51

52 No, no, no! I won t deliver the prototype to you! User Mgmt Developer Air Tank Pressurizing the Developer

53 Implementation techniques Various techniques may be used to implement prototypes: Dynamic, high-level languages Database programming (RAD) Component and application assembly These are NOT mutually exclusive they are often used together. Visual programming is also an inherent part of most prototype development systems. Chapter 2 Software Processes Slide 53

54 User interface prototyping User interface development consumes an increasing part of overall system development costs. It is usually impossible to pre-specify the look and feel of a complex user interface in an effective way. Thus, prototyping is essential. (cont d) Chapter 2 Software Processes Slide 54

55 User interface prototyping (cont d) Aim is to allow users to gain direct experience with the interface. Without this, it is almost impossible to judge usability. Often a two-stage process: paper prototypes are developed initially, followed by a series of increasingly sophisticated automated prototypes. Chapter 2 Software Processes Slide 55

56 Paper prototyping Step through scenarios using sketches of the interface. Use storyboards to present a series of interactions with the system. Paper prototyping is a cost-effective way of obtaining user reactions to an interface design proposal. Chapter 2 Software Processes Slide 56

57 (Mills ) Incremental Delivery (not to be confused with incremental development ) Chapter 2 Software Processes Slide 57

58 Incremental Delivery Rather than deliver the system as a single unit, the development and delivery is broken down into increments, each of which incorporates 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, its requirements are frozen while requirements for later increments can continue to evolve. (Compromise between Waterfall & Incremental Development) (cont d) Chapter 2 Software Processes Slide 58

59 Incremental Delivery Define outline requirements Assign requirements to increments Design system architecture Develop system increment Valida te increment System incomplete Integrate increment Valida te system Final system (cont'd) Chapter 2 Software Processes Slide 59

60 Incremental development versus Incremental Delivery Incremental development Develop the system in increments that are made available for customer evaluation & feedback (but not usually for actual work in the customer s own environment) before proceeding to the development of the next increment; Normal approach used in agile methods (but may also be used in plan-driven development); Evaluation undertaken by user/customer (or a proxy). (Mills ) Incremental Delivery Deploy increments for actual work-place use by end-users; Permits more realistic evaluation of practical usefulness; Difficult to carry out for replacement systems as increments provide less functionality than the system being replaced. Chapter 2 Software Processes Slide 60

61 Advantages of Incremental Delivery Useful functionality is delivered with each increment, so customers derive value early. Early increments assist in eliciting requirements for later increments. Lower risk of overall project failure. The highest priority system services tend to receive the most testing. (They're subject to more validation steps.) (cont'd) Chapter 2 Software Processes Slide 61

62 Potential problems with Incremental Delivery Requirements may NOT be partitionable into usable, stand-alone increments. (e.g., consider a compiler) Most systems require a set of basic facilities that are used by different parts of the system. But since requirements are not defined in detail until an increment is to be implemented, it can be hard to identify common facilities that are needed by all increments to be useful. The essence of an iterative process is that the specification is developed in conjunction with the software. But this conflicts with the procurement model of many organizations, where the complete specification is part of the system development contract (also a problem with incremental development). Chapter 2 Software Processes Slide 62

63 Spiral development model Chapter 2 Software Processes Slide 63

64 Boehm s spiral development model Process is represented as a spiral rather than a sequence of activities. 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. Explicitly incorporates risk assessment and resolution throughout the process. (cont'd) Chapter 2 Software Processes Slide 64

65 Boehm s spiral development model Chapter 2 Software Processes Slide 65

66 Boehm s spiral development model Determine objectives alternatives and constraints Plan next phase REVIEW Requirements plan Life-cycle plan Development plan Integration and test plan Risk analysis Risk analysis Risk analysis Prototype 2 Risk analysis Prototype 1 Concept of Operation S/W requirements Requirement validation Design V&V Service Acceptance test Prototype 3 Operational protoype Simulations, models, benchmarks Product design Integr ation test Evaluate alternatives identify, resolve risks Code Unit test Detailed design Develop, verify next-level product Chapter 2 Software Processes Slide 66

67 Spiral model quadrants Objective Setting specific objectives for the phase are identified. Project risks and alternative strategies 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. Chapter 2 Software Processes Slide 67

68 Spiral model usage The model has been very influential in helping people think about iteration in software processes and introducing the riskdriven approach to development. In practice, however, the model is rarely used as published for practical software development. Chapter 2 Software Processes Slide 68

69 RUP Chapter 2 Software Processes Slide 69

70 The Rational Unified Process A modern process model derived from the work on the UML and its associated process. A hybrid process model that brings together aspects of the generic process models discussed previously it represents a new generation of generic processes. Normally described from 3 perspectives A dynamic perspective that shows phases over time; A static perspective that shows process activities; A practice perspective that suggests good practices. Chapter 2 Software Processes Slide 70

71 RUP phase model in-phase iteration cross-phase iteration Phase iteration Inception Elaboration Construction Transition cf. Waterfall Model: RUP phases are more closely related to business rather than technical concerns. Chapter 2 Software Processes Slide 71

72 RUP phases Inception Establish the business case for the system. Elaboration Develop an understanding of the problem domain and the system architecture. Construction System design, programming and testing. Transition Deploy the system in its operating environment. Chapter 2 Software Processes Slide 72

73 Static workflows (process activities) in RUP Workflow Business modelling Requirements Analysis and design Implementation Description The business processes are modelled using business use cases. Actors who interact with the system are identified and use cases are developed to model the system requirements. A design model is created and documented using architectural models, component models, object models and sequence models. The components in the system are implemented and structured into implementation sub-systems. Automatic code generation from design models helps accelerate this process. Chapter 2 Software Processes Slide 73

74 RUP good practices Develop software iteratively: plan increments based on customer priorities and deliver highest priority increments first. Manage requirements: explicitly document and keep track of changes. Use component-based architectures: organize system architecture as a set of reusable components. (cont'd) Chapter 2 Software Processes Slide 74

75 RUP good practices (cont d) Visually model software: using graphical UML models to present static and dynamic views. Verify software quality: ensure software meets organizational quality standards. Control changes to software: using change management system and configuration management tools. Chapter 2 Software Processes Slide 75

76 Key points Software processes are the activities involved in producing and evolving a software system. They are represented abstractly by software process models. Generic models are very general and represent different approaches to development. Examples are the waterfall model, incremental development, and reuse-oriented development. (cont'd) Chapter 2 Software Processes Slide 76

77 Key points (cont d) Requirements engineering is the process of establishing what services are required and the constraints on the system s operation and development. Design and implementation processes produce an executable system based on the specification. V&V involves checking that the system meets its specification and satisfies user needs. Evolution is concerned with modifying the system after it is placed in use. Software must evolve to remain useful. (cont'd) Chapter 2 Software Processes Slide 77

78 Key points (cont d) Processes should include activities to cope with change. This may involve a prototyping phase that helps avoid poor decisions on requirements and design. Throw-away prototyping is used to explore requirements and design options. Rapid development of prototypes is essential. This usually requires leaving out functionality or relaxing non-functional constraints. (cont'd) Chapter 2 Software Processes Slide 78

79 Key points (cont d) Prototyping may be essential for parts of the system such as the user interface which cannot be effectively pre-specified. Users must be involved in prototype evaluation. The Rational Unified Process is a modern generic process model that is organized into phases (inception, elaboration, construction and transition) but separates activities (requirements, analysis and design, etc.) from these phases. Chapter 2 Software Processes Slide 79

80 Chapter 2 Software Processes Chapter 2 Software Processes Slide 80