mmerville 2000 Software Engineering, 6th edition. Chapter 1 Slide Software Processes (Chapter 3)
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 1
Software Processes (Chapter 3)
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 2
Objectives To introduce the software process model concept To describe different process models and their
pros and cons To outline process models for requirements
engineering, software development, testing, and evolution
To introduce CASE technology to support software process activities
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 3
The Software Process A structured set of activities required to develop a
software system. These activities include:• Specification
• Design
• Implementation
• Validation
• Evolution
A software process model is an abstract representation of a process.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 4
Generic Software Process Models The waterfall model
• Separate and distinct 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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 5
Waterfall ModelRequirements
definition
System andsoftware design
Implementationand unit testing
Integration andsystem testing
Operation andmaintenance
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 6
Waterfall Model Pros and Cons
Pros
Cons
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 7
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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 8
Evolutionary Development
ValidationFinal
version
DevelopmentIntermediate
versions
SpecificationInitial
version
Outlinedescription
Concurrentactivities
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 9
Evolutionary Development Pros and Cons
Pros
Cons
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 10
Throw-away Prototyping Pros and Cons
Pros
Cons
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 11
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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 12
Reuse-oriented Development
Requirementsspecification
Componentanalysis
Developmentand integration
System designwith reuse
Requirementsmodification
Systemvalidation
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 13
Reuse-oriented Development Pros and Cons
Pros
Cons
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 14
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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 15
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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 16
Incremental Development
Valida teincrement
Develop systemincrement
Design systemarchitecture
Integrateincrement
Valida tesystem
Define outline requirements
Assign requirements to increments
System incomplete
Finalsystem
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 17
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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 18
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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 19
Spiral model of the software process
Riskanalysis
Riskanalysis
Riskanalysis
Riskanalysis Proto-
type 1
Prototype 2Prototype 3
Opera-tionalprotoype
Concept ofOperation
Simulations, models, benchmarks
S/Wrequirements
Requirementvalidation
DesignV&V
Productdesign Detailed
design
CodeUnit test
IntegrationtestAcceptance
testService Develop, verifynext-level product
Evaluate alternativesidentify, resolve risks
Determine objectivesalternatives and
constraints
Plan next phase
Integrationand test plan
Developmentplan
Requirements planLife-cycle plan
REVIEW
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 20
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
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 21
Requirements Engineering
The process of establishing what services are required of the system the constraints on the system’s operation and
development
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 22
The Requirements Engineering Process
Feasibilitystudy
Requirementselicitation and
analysisRequirementsspecification
Requirementsvalidation
Feasibilityreport
Systemmodels
User and systemrequirements
Requirementsdocument
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 23
Software Design and Implementation
The process of converting the system specification into an executable system.
Software design• Design a software structure that realizes the specification
Implementation• Translate this structure into an executable program
The activities of design and implementation are closely related and may be inter-leaved.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 24
The Software Design Process
Architecturaldesign
Abstractspecification
Interfacedesign
Componentdesign
Datastructuredesign
Algorithmdesign
Systemarchitecture
Softwarespecification
Interfacespecification
Componentspecification
Datastructure
specification
Algorithmspecification
Requirementsspecification
Design activities
Design products
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 25
Design Methods
Systematic approaches to developing a software design
The design is usually documented as a set of graphical models.
Possible models• Data-flow model
• Entity-relation-attribute model
• Structural model
• Object models
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 26
Programming and Debugging 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 2000 Software Engineering, 6th edition. Chapter 1 Slide 27
Software Validation 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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 28
The Testing Process
Sub-systemtesting
Moduletesting
Unittesting
Systemtesting
Acceptancetesting
Componenttesting
Integration testing Usertesting
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 29
Testing Stages Unit testing
• Individual components are tested
Module testing• Related collections of dependent components are tested
Sub-system testing• Modules are integrated into sub-systems and tested. The focus here
should be on interface testing
System testing• Testing of the system as a whole. Testing of emergent properties
Acceptance testing• Testing with customer data to check that it is acceptable
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 30
Testing Phases
Requirementsspecification
Systemspecification
Systemdesign
Detaileddesign
Module andunit codeand tess
Sub-systemintegrationtest plan
Systemintegrationtest plan
Acceptancetest plan
ServiceAcceptance
testSystem
integration testSub-system
integration test
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 31
Software Evolution 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.
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 32
System Evolution
Assess existingsystems
Define systemrequirements
Propose systemchanges
Modifysystems
Newsystem
Existingsystems
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 33
Automated Process Support (CASE)
Computer-aided software engineering (CASE) is software used 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 2000 Software Engineering, 6th edition. Chapter 1 Slide 34
Case Technology Case technology has led to significant improvements
in the software process, though not the order of magnitude improvements that were once predicted.• Software engineering requires creative thought - this is not readily
automatable.
• 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 2000 Software Engineering, 6th edition. Chapter 1 Slide 35
CASE ClassificationClassification helps us understand the different types of
CASE tools and their support for process activities. Functional perspective
• Tools are classified according to their specific function
Process perspective• Tools are classified according to process activities that are supported
Integration perspective• Tools are classified according to their organization into integrated units
©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 36
Functional Tool Classification
Activity-based classification
Reengineering tools
Testing tools
Debugging tools
Program analysis tools
Language-processingtools
Method support tools
Prototyping tools
Configurationmanagement tools
Change management tools
Documentation tools
Editing tools
Planning tools
Specification Design Implementation Verificationand
Validation