©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 4 Slide 1 Chapter 4 Software Processes Modified by Randy K. Smith Additional Material.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 4 Slide 1

Chapter 4Software Processes

Modified by Randy K. Smith

Additional Material taken from:•Object-Oriented and Classical Software Engineering, Fifth Edition, WCB/McGraw-Hill, 2002, Stephen R. Schach•Software Engineering A Practitioner’s Approach, Fifth Edition, WCB/McGraw-Hill, 2001, Roger S. Pressman


Software Processes Coherent sets of activities for specifying,

designing, implementing and testing software systems

Objectives• To introduce software process models• To describe a number of different process models and when

they may be used• To describe outline process models for requirements

engineering, software development, testing and evolution• To introduce CASE technology to support software process

activities


The software process A structured set of activities required to develop a

software system• Specification• Design• Validation• Evolution

A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective


System/informationengineering

Generic software process model

analysis design code test

Software Engineering A Practitioner’s Approach, Fifth Edition, WCB/McGraw-Hill, 2001, Roger S. Pressman


Generic software process models

The waterfall model• Separate and distinct phases of specification and

development Evolutionary development

• Specification and development are interleaved Formal systems development

• A mathematical system model is formally transformed to an implementation

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 Phases

• Requirements analysis and definition• System and software design• Implementation and unit testing• Integration and system testing• Operation and maintenance• The drawback of the waterfall model is the difficulty of

accommodating change after the process is underway Problems

• Inflexible partitioning of the project into distinct stages• Makes it difficult to respond to changing customer

requirements• Model is appropriate when requirements are well-understood


Waterfall model (Another View)

Object-Oriented and Classical Software Engineering Fifth Edition, WCB/McGraw-Hill, 2002Stephen R. Schach


Evolutionary Models

Build/Revise mock-upListen to

Customer

Customer test drives

mock-up

Prototyping

•Evolutionary•Build to keep

•Throwaway•Built not to keep

Sommerville


Evolutionary development 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

Throw-away prototyping• Objective is to understand the

system requirements. Should start with poorly understood requirements

Build/Revise

mock-upListen to Customer

Customer test drives

mock-up


Evolutionary development

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities


Evolutionary developmentBuild-and-Fix

Object-Oriented and Classical Software Engineering Fifth Edition, WCB/McGraw-Hill, 2002, Stephen R. Schach


Evolutionary development

Problems• 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


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 for large systems

Iteration can be applied to any of the generic process models

Two (related) approaches• Incremental development• Spiral development


Incremental development Do not deliver the system as a single delivery

• development and delivery broken down into increments

• each increment delivering part of the required functionality User requirements are prioritized (highest first) Once an increment is started, requirements are frozen

• requirements for later increments can continue to evolve

Valida teincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida tesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem


Incremental development

Object-Oriented and Classical Software Engineering, Fifth Edition, WCB/McGraw-Hill, 2002Stephen R. Schach


The Incremental Model






increment 2

increment 3

increment 4

increment 1

delivery of1st increment

delivery of2nd increment

delivery of3rd increment

delivery of4th increment

calendar time


Incremental development advantages 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


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


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


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


The Rational Unified Process A modern process model derived from the

work on the UML and associated process. Normally described from 3 perspectives

• A dynamic perspective that shows phases over time;

• A static perspective that shows process activities;

• A perspective that suggests good practice.


The (Rational) Unified Process


RUP good practice

Develop software iteratively Manage requirements Use component-based architectures Visually model software Verify software quality Control changes to software


Formal systems development Based on the transformation of a mathematical

specification through different representations to an executable program

Transformations are ‘correctness-preserving’ so it is straightforward to show that the program conforms to its specification

Embodied in the ‘Cleanroom’ approach to software development

Requirementsdefinition

Formalspecification

Formaltransformation

Integration andsystem testing


Formal systems development Problems

• Need for specialised skills and training to apply the technique

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

Applicability• Critical systems especially those where a safety

or security case must be made before the system is put into operation


Synchronize-and Stabilize Model Microsoft’s life-cycle model Requirements analysis—interview potential customers Draw up specifications Divide project into 3 or 4 builds Each build carried out by small teams in parallel At the end of the day—synchronize (test and debug) At the end of the build—stabilize (freeze build) Components always work together

• Get early insights into operation of product

Object-Oriented and Classical Software Engineering, Fifth Edition, WCB/McGraw-Hill, 2002Stephen R. Schach


Agile Methods Effective (rapid and adaptive) response to change Effective communication among all stakeholders Drawing the customer onto the team Organizing a team so that it is in control of the work

performed

Yielding … Rapid, incremental delivery of software Extreme Programming (XP)

• Test First, Pair Programming







Software specification

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


The requirements engineering process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument







Software design and implementation

The process of converting the system specification into an executable system

Software design• Design a software structure that realises the

specification Implementation

• Translate this structure into an executable program The activities of design and implementation are

closely related and may be inter-leaved


Design process activities

Architectural design Abstract specification Interface design Component design Data structure design Algorithm design


The software design process

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 (UML)

Architecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products







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

Locateerror

Designerror repair

Repairerror

Re-testprogram







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


The testing process

Sub-systemtesting

Moduletesting

Unittesting

Systemtesting

Acceptancetesting

Componenttesting

Integration testing Usertesting


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


Testing phases

Requirementsspecification

Systemspecification

Systemdesign

Detaileddesign

Module andunit codeand tess

Sub-systemintegrationtest plan

Systemintegrationtest plan

Acceptancetest plan

ServiceAcceptance

testSystem

integration testSub-system

integration test


Software evolution Software is inherently flexible and can change. As requirements change (changing business

circumstances), software must evolve and change The demarcation between development and

evolution (maintenance) is increasingly irrelevant as fewer and fewer systems are completely new

Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems


Automated process support (CASE)

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

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


Key points

Software processes are the activities involved in producing and evolving a software system. They are represented in a software process model

General activities are specification, design and implementation, validation and evolution

Generic process models describe the organisation of software processes

Iterative process models describe the software process as a cycle of activities


Key points

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

CASE technology supports software process activities

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 4 Slide 1 Chapter 4 Software Processes Modified by Randy K. Smith Additional Material.

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