Software Processes Ian Sommerville 2004 Software Engineering 7
- Slides: 58
Software Processes ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 1
Objectives l l l To introduce software process models To describe three generic process models and when they may be used To describe outline process models for requirements engineering, software development, testing and evolution To explain the Rational Unified Process model To introduce CASE technology to support software process activities ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 2
Topics covered l l l Software process models Process iteration Process activities The Rational Unified Process Computer-aided software engineering ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 3
The software process l A structured set of activities required to develop a software system • • • l Requirement Specification Design Implementation Testing Maintenance A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 4
Deliverables l Requirement Specifications l Design Specifications l Implementation Source & executable Code l Testing & Integration Test Cases & Plan l Maintenance ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 5
Process activities l l Software Requirement specification Software design and implementation Software validation Software evolution ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 6
Software specification l l The process of establishing what services are required and the constraints on the system’s operation and development. Requirements engineering process • • Feasibility study; Requirements elicitation and analysis; Requirements specification; Requirements validation. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 7
The requirements engineering process ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 8
Software design and implementation l l The process of converting the system specification into an executable system. Software design • l Implementation • l Design a software structure that realises the specification; Translate this structure into an executable program; The activities of design and implementation are closely related and may be inter-leaved. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 9
Design process activities l l l Architectural design Abstract specification Interface design Component design Data structure design Algorithm design ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 10
The software design process ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 11
Structured methods l l l Systematic approaches to developing a software design. The design is usually documented as a set of graphical models. Possible models • • • Object model; Sequence model; State transition model; Structural model; Data-flow model. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 12
Programming and debugging l l l 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 13
The debugging process ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 14
Software validation l l 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. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 15
Definition of Validation & Verification l l l Verification • “Am I building the product right? ” • Determine correctness with respect to its specification; Can begin after specification • Objective (if specification is formal) Validation • “Am I building the right product? ” • Predict correspondence; Can begin as soon as project starts • Subjective (since user need is not formal) Asking 2 important questions ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 16
The testing process ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 17
Testing stages l Component or unit testing • • l System testing • l Individual components are tested independently; Components may be functions or objects or coherent groupings of these entities. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 18
Testing phases ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 19
Software evolution l l l Software is inherently flexible and can change. As requirements change through changing business circumstances, the software that supports the business must also evolve and change. Although there has been a demarcation between development and evolution (maintenance) this is increasingly irrelevant as fewer and fewer systems are completely new. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 20
System evolution ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 21
Generic software process models l The waterfall model • l Evolutionary development • l Specification, development and validation are interleaved. Component-based software engineering • l Separate and distinct phases of specification and development. The 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 22
Waterfall model ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 23
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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 24
Waterfall model problems l l l Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. Therefore, this model is only appropriate when the requirements are well-understood and changes will be fairly limited during the design process. Few business systems have stable requirements. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 25
Evolutionary development l Exploratory development • l 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. Throw-away prototyping • Objective is to understand the system requirements. Should start with poorly understood requirements to clarify what is really needed. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 26
Evolutionary development ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 27
Evolutionary development l Problems • • • l Lack of process visibility; Systems are often poorly structured; Special skills (e. g. in languages for rapid prototyping) may be required. Applicability • • • For small or medium-size interactive systems; For parts of large systems (e. g. the user interface); For short-lifetime systems. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 28
Component-based software engineering l l Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems. Process stages • • l Component analysis; Requirements modification; System design with reuse; Development and integration. This approach is becoming increasingly used as component standards have emerged. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 29
Reuse-oriented development ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 30
Process iteration l l 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. Iteration can be applied to any of the generic process models. Two (related) approaches • • Incremental delivery; Spiral development. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 31
Incremental delivery l l 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. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 32
Incremental development ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 33
Incremental development advantages l l 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 34
Spiral development l l 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 35
Spiral model of the software process ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 36
Spiral model sectors l Objective setting • l Risk assessment and reduction • l Risks are assessed and activities put in place to reduce the key risks. Development and validation • l Specific objectives for the phase are identified. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 37
Computer-aided software engineering l l Computer-aided software engineering (CASE) is software to support software development and evolution processes. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 38
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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 39
CASE classification l l Classification helps us understand the different types of CASE tools and their support for process activities. Functional perspective • l Process perspective • l Tools are classified according to their specific function. Tools are classified according to process activities that are supported. Integration perspective • Tools are classified according to their organisation into integrated units. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 40
Functional tool classification ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 41
Activity-based tool classification ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 42
CASE integration l Tools • l Workbenches • l Support individual process tasks such as design consistency checking, text editing, etc. Support a process phase such as specification or design, Normally include a number of integrated tools. Environments • Support all or a substantial part of an entire software process. Normally include several integrated workbenches. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 43
Tools, workbenches, environments ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 44
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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 45
What is “Agility”? 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 l ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 46
An Agile Process l l l Is driven by customer descriptions of what is required (scenarios) Recognizes that plans are short-lived Develops software iteratively with a heavy emphasis on construction activities Delivers multiple ‘software increments’ Adapts as changes occur ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 47
Agile Methodologies (cont’d) l l Agile methods emphasize real-time communication, preferably face-to-face, over written documents. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 48
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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 49
Agile or Plan driven? Low criticality Senior developers High requirements change Small number of developers Culture that thrives on chaos Agile home ground ©Ian Sommerville 2004 High criticality Junior developers Low requirements change Large number of developers Culture that demands order Plan-driven home ground Software Engineering, 7 th edition. Chapter 4 Slide 50
Extreme Programming (XP) l l The most widely used agile process, originally proposed by Kent Beck 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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 51
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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 52
Extreme Programming (XP) ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 53
Scrum l l Originally proposed by Schwaber and Beedle 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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 54
Scrum ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 55
Crystal l l Proposed by Cockburn and Highsmith 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 ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 56
Key points l l l Software processes are the activities involved in producing and evolving a software system. Software process models are abstract representations of these processes. General activities are specification, design and implementation, validation and evolution. Generic process models describe the organisation of software processes. Examples include the waterfall model, evolutionary development and componentbased software engineering. Iterative process models describe the software process as a cycle of activities. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 57
Key points l l l Requirements engineering is the process of developing a software specification. Design and implementation processes transform the specification to an executable program. Validation involves checking that the system meets to its specification and user needs. Evolution is concerned with modifying the system after it is in use. The Rational Unified Process is a generic process model that separates activities from phases. CASE technology supports software process activities. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Chapter 4 Slide 58
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