l coherent sets of activities for specifying, designing

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 1 Slide 1

Software Processes

l Coherent sets of activities forspecifying, designing, implementingand testing software systems

Objectives

l To introduce software process models

l To describe a number of different process modelsand when they may be used

l To describe outline process models forrequirements engineering, software development,testing and evolution

l To introduce CASE technology to supportsoftware process activities

Topics covered

l Software process models

l Process iteration

l Software specification

l Software design and implementation

l Software validation

l Software evolution

l Automated process support

The software process

l A structured set of activities required to develop asoftware system• Specification

• Design

• Validation

• Evolution

l A software process model is an abstractrepresentation of a process. It presents adescription of a process from some particularperspective

Software process models

Generic software process models

l The waterfall model• Separate and distinct phases of specification and development

l Evolutionary development• Specification and development are interleaved

l Formal systems development• A mathematical system model is formally transformed to an

implementation

l Reuse-based development• The system is assembled from existing components

Waterfall modelRequirements

definition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance

Waterfall model phases

l Requirements analysis and definition

l System and software design

l Implementation and unit testing

l Integration and system testing

l Operation and maintenance

l The drawback of the waterfall model is thedifficulty of accommodating change after theprocess is underway

Waterfall model problems

l Inflexible partitioning of the project into distinctstages

l This makes it difficult to respond to changingcustomer requirements

l Therefore, this model is only appropriate whenthe requirements are well-understood

Evolutionary development

l Exploratory development• Objective is to work with customers and to evolve a final

system from an initial outline specification. Should start withwell-understood requirements

l Throw-away prototyping• Objective is to understand the system requirements. Should start

with poorly understood requirements

Validation Finalversion

Development Intermediateversions

Specification Initialversion

Outlinedescription

Concurrentactivities

l Problems• Lack of process visibility

• Systems are often poorly structured

• Special skills (e.g. in languages for rapid prototyping) may berequired

l Applicability• For small or medium-size interactive systems

• For parts of large systems (e.g. the user interface)

• For short-lifetime systems

Formal systems development

l Based on the transformation of a mathematicalspecification through different representations toan executable program

l Transformations are ‘correctness-preserving’ so itis straightforward to show that the programconforms to its specification

l Embodied in the ‘Cleanroom’ approach tosoftware development

Requirementsdefinition

Formalspecification

Formaltransformation

Integration andsystem testing

Formal transformations

R2Formalspecification R3 Executable

program

P2 P3 P4

T1 T2 T3 T4

Proofs of transformation correctness

Formal transformations

l Problems• Need for specialised skills and training to apply the technique

• Difficult to formally specify some aspects of the system such asthe user interface

l Applicability• Critical systems especially those where a safety or security case

must be made before the system is put into operation

Reuse-oriented development

l Based on systematic reuse where systems areintegrated 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 more important butstill limited experience with it

Reuse-oriented development

Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation

Process iteration

l System requirements ALWAYS evolve in thecourse of a project so process iteration whereearlier stages are reworked is always part of theprocess for large systems

l Iteration can be applied to any of the genericprocess models

l Two (related) approaches• Incremental development

• Spiral development

Incremental development

l Rather than deliver the system as a singledelivery, the development and delivery is brokendown into increments with each incrementdelivering part of the required functionality

l User requirements are prioritised and the highestpriority requirements are included in earlyincrements

l Once the development of an increment is started,the requirements are frozen though requirementsfor later increments can continue to evolve

Incremental development

Validateincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Validatesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

Incremental development advantages

l Customer value can be delivered with eachincrement so system functionality is availableearlier

l Early increments act as a prototype to help elicitrequirements for later increments

l Lower risk of overall project failure

l The highest priority system services tend toreceive the most testing

Extreme programming

l New approach to development based on thedevelopment and delivery of very smallincrements of functionality

l Relies on constant code improvement, userinvolvement in the development team andpairwise programming

Spiral development

l Process is represented as a spiral rather than as asequence of activities with backtracking

l Each loop in the spiral represents a phase in theprocess.

l No fixed phases such as specification or design -loops in the spiral are chosen depending on whatis required

l Risks are explicitly assessed and resolvedthroughout the process

Spiral model of the software process

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2

Prototype 3Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

Unit 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

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

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

Software specification

l The process of establishing what services arerequired and the constraints on the system’soperation and development

l Requirements engineering process• Feasibility study

• Requirements elicitation and analysis

• Requirements specification

• Requirements validation

The requirements engineering process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument

Software design and implementation

l The process of converting the systemspecification 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 areclosely related and may be inter-leaved

Design process activities

l Architectural design

l Abstract specification

l Interface design

l Component design

l Data structure design

l Algorithm design

The software design process

Architecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Design activities

Design products

Design methods

l Systematic approaches to developing a softwaredesign

l The design is usually documented as a set ofgraphical models

l Possible models• Data-flow model

• Entity-relation-attribute model

• Structural model

• Object models

Programming and debugging

l Translating a design into a program and removingerrors from that program

l Programming is a personal activity - there is nogeneric programming process

l Programmers carry out some program testing todiscover faults in the program and remove thesefaults in the debugging process

The debugging process

Locateerror

Designerror repair

Repairerror

Re-testprogram

Software validation

l Verification and validation is intended to showthat a system conforms to its specification andmeets the requirements of the system customer

l Involves checking and review processes andsystem testing

l System testing involves executing the systemwith test cases that are derived from thespecification of the real data to be processed bythe system

The testing process

Sub-systemtesting

Moduletesting

Unittesting

Systemtesting

Acceptancetesting

Componenttesting

Integration testing Usertesting

Testing stagesl Unit testing

• Individual components are tested

l Module testing• Related collections of dependent components are tested

l Sub-system testing• Modules are integrated into sub-systems and tested. The focus

here should be on interface testing

l System testing• Testing of the system as a whole. Testing of emergent properties

l Acceptance testing• Testing with customer data to check that it is acceptable

Testing phases

Systemspecification

Systemdesign

Detaileddesign

Module andunit codeand tess

Sub-systemintegrationtest plan

Systemintegrationtest plan

Acceptancetest plan

Service Acceptancetest

Systemintegration test

Sub-systemintegration test

Software evolution

l Software is inherently flexible and can change.

l As requirements change through changingbusiness circumstances, the software thatsupports the business must also evolve andchange

l Although there has been a demarcation betweendevelopment and evolution (maintenance) this isincreasingly irrelevant as fewer and fewersystems are completely new

System evolution

Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems

Automated process support

l Computer-aided software engineering (CASE) issoftware to support software development andevolution 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

Case technology

l Case technology has led to significantimprovements in the software process though notthe order of magnitude improvements that wereonce 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 doesnot really support these

CASE classification

l Classification helps us understand the differenttypes of CASE tools and their support for processactivities

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

Functional tool classificationTool type ExamplesPlanning tools PERT tools, estimation tools,

spreadsheetsEditing tools Text editors, diagram editors, word

processorsChange management tools Requirements traceability tools, change

control systemsConfiguration management tools Version management systems, system

building toolsPrototyping tools Very high-level languages,

user interface generatorsMethod-support tools Design editors, data dictionaries, code

generatorsLanguage-processing tools Compilers, interpretersProgram analysis tools Cross reference generators, static

analysers, dynamic analysersTesting tools Test data generators, file comparatorsDebugging tools Interactive debugging systemsDocumentation tools Page layout programs, image editorsRe-engineering tools Cross-reference systems, program re-

structuring systems

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

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

Tools, workbenches, environments

Single-methodworkbenches

General-purposeworkbenches

Multi-methodworkbenches

Language-specificworkbenches

Programming TestingAnalysis anddesign

Integratedenvironments

Process-centredenvironments

FilecomparatorsCompilersEditors

EnvironmentsWorkbenchesTools

CASEtechnology

Key points

l Software processes are the activities involved inproducing and evolving a software system. Theyare represented in a software process model

l General activities are specification, design andimplementation, validation and evolution

l Generic process models describe the organisationof software processes

l Iterative process models describe the softwareprocess as a cycle of activities

Key points

l Requirements engineering is the process ofdeveloping a software specification

l Design and implementation processes transformthe specification to an executable program

l Validation involves checking that the systemmeets to its specification and user needs

l Evolution is concerned with modifying thesystem after it is in use

l CASE technology supports software processactivities

l coherent sets of activities for specifying, designing

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