unit3 software process

8/14/2019 Unit3 Software Process

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Ian Sommerville 2004 Software Engineering, 7th edition. Slide 2

Objectives

q To introduce software process modelsq To describe three generic process models and

when they may be used

q To describe outline process models forrequirements engineering, softwaredevelopment, testing and evolution

q To explain the Rational Unified Process model

q To introduce CASE technology to supportsoftware process activities


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Topics covered

q Software process models

q Process iteration

q Process activities

q The Rational Unified Process

q Computer-aided software engineering


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The software process

q A structured set of activities required to develop a

software system Specification Functionality of the software and constraints on its

operation must be defined;

Design and implementation- The software to meet the specification must

be produced;

Validation- Must be validated to ensure that it does what the customer

wants ;

Evolution The software must evolve to meet changing customers need.

q A software process model is an abstract representation of a

process. It presents a description of a process from someparticular perspective and thus provides only partial

information about that process.


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Generic software process models

q The waterfall model Separate and distinct phases of specification and

development.

q Evolutionary development

Specification, development and validation are interleaved.q Component-based software engineering

The system is assembled from existing components.

q There are many variants of these models .They are not

mutually exclusive and are often used together e.g. formaldevelopment where a waterfall-like process is used but thespecification is a formal specification that is refined throughseveral stages to an implementable design.


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Waterfall model

Requirements

definition

System and

software design

Implementation

and unit testing

Integration and

system testing

Operation and

maintenance


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Waterfall model phases

q Requirements analysis and definition-The systemsservices,constraints and goals are established by consultationwith system users.

q System and software design- The system design processpartitions the requirements to either hardware or softwaresystems.It establishes an overall system architecture.Softwaredesign involves identifying and describing the fundamentalsoftware system abstractions and their relationships.

q Implementation and unit testingq Integration and system testing

q Operation and maintenanceq The main drawback of the waterfall model is the difficulty of

accommodating change after the process is underway. Onephase has to be complete before moving onto the next phase.


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Evolutionary development

q Exploratory development

Objective is to work with customers and to evolve

a final system from an initial outline specification.

Should start with well-understood requirementsand add new features as proposed by the

customer.

q Throw-away prototyping

Objective is to understand the systemrequirements. Should start with poorly understood

requirements to clarify what is really needed.




Concurrentactivities

ValidationFinal

version

DevelopmentIntermediate

versions

Specification

Initial

version

Outlinedescription




q Problems

Lack of process visibility-need regular deliverables tomeasure progress,it is not cost effective to producedocuments that reflect every version of the system;

Systems are often poorly structured- Continual changetends to corrupt the software structure;

Special skills (e.g. in languages for rapid prototyping)may be required.

q Applicability

For small or medium-size interactive systems; For parts of large systems (e.g. the user interface);

For short-lifetime systems.



Component-based software engineering

(CBSE)

q Based on systematic reuse where systems are integratedfrom existing components or COTS (Commercial-off-the-shelf) systems.

q Process stages

Component analysis -Given the requirements specification, a search

is made for components to implement that specification;

Requirements modification The requirements are analysed usinginformation about the components that have been discovered andmodify to reflect the available components;

System design with reuse- The framework of the system is designedor an existing framework is reused;

Development and integration- System that cannot be externallyobtained is developed.

q This approach is becoming increasingly used as componentstandards have emerged.



Reuse-oriented development

Requirements

specification

Component

analysis

Development

and integration

System design

with reuse

Requirements

modification

System

validation



Process iteration

q 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.q Iteration can be applied to any of the generic

process models.

q Two (related) approaches

Incremental delivery;

Spiral development.



Incremental delivery

q 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.

q User requirements are prioritised and the highestpriority requirements are included in early increments.

q Once the development of an increment is started, the

requirements are frozen though requirements for later

increments can continue to evolve.


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Incremental development

Validate

increment

Develop system

increment

Design system

architecture

Integrate

increment

Validate

system

Define outlinerequirements

Assign requirementsto increments

System incomplete

Finalsystem


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Incremental development advantages

q Customer value can be delivered with each

increment so system functionality is available

earlier.

q Early increments act as a prototype to helpelicit requirements for later increments.

q Lower risk of overall project failure.

q

The highest priority system services tend toreceive the most testing.


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Spiral development

q The spiral model of the software was originally proposed

by Boehm.

q Process is represented as a spiral rather than as a

sequence of activities with backtracking.

q Each loop in the spiral represents a phase in the process.q No fixed phases such as specification or design - loops in

the spiral are chosen depending on what is required.

q Risks are explicitly assessed and resolved throughout the

process.

q The main difference between the spiral model and other

software process models is the explicit recognition of risk

in the spiral model


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Spiral model of the software process

Risk

analysis

Risk

analysis

Risk

analysis

Risk

analysis Proto-

type 1

Prototype 2

Prototype 3Opera-

tional

protoype

Concept of

Operation

Simulations, models, benchmar ks

S/W

requirements

Requirement

validation

Design

V&V

Product

designDetailed

design

Code

Unit test

Integration

testAcceptance

testService Develop , verify

next- level product

Evaluate alternatives,

identify, resolve risks

Determine objecti ves,

alternatives and

constraints

Plan ne xt phase

Integration

and test plan

Development

plan

Requirements plan

Life-cycle plan

REVIEW


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Spiral model sectors

q Objective setting

Specific objectives for the phase are identified.

q Risk assessment and reduction

Risks are assessed and activities put in place to reduce the

key risks.q Development and validation

After risk evaluation,a development model for the system is

chosen which can be any of the generic models.

q Planning

The project is reviewed and a decision made whether to

continue with a further loop of the spiral,If it is decided to

continue, the next phase of the project is planned.


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Process activities

q Software specification

q Software design and implementation

q Software validation

q Software evolution


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Software specification

q The process of establishing what services are

required and the constraints on the systems

operation and development.

q Requirements engineering process Feasibility study;

Requirements elicitation and analysis;

Requirements specification;

Requirements validation.


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The requirements engineering process

Feasibility

study

Requirements

elicitation and

analysis

Requirements

specification

Requirements

validationFeasibility

report

System

models

User and system

requirements

Requirements

document


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Software design and implementation

q The process of converting the systemspecification into an executable system.

q Software design

Design a software structure that realises thespecification;

q Implementation Translate this structure into an executable

program;

q The activities of design and implementationare closely related and may be inter-leaved.


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Design process activities

q Architectural design

q Abstract specification

q Interface design

q Component design

q Data structure design

q Algorithm design


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The software design process

Architectural

design

Abstract

specification

Interface

design

Component

design

Data

structure

design

Algorithm

design

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Data

structure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products


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Structured methods

q Systematic approaches to developing asoftware design.

q The design is usually documented as a set of

graphical models.q Possible models

Object model;

Sequence model;

State transition model; Structural model;

Data-flow model.


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Programming and debugging

q Translating a design into a program and

removing errors from that program.

q Programming is a personal activity - there is

no generic programming process.q Programmers carry out some program testing

to discover faults in the program and remove

these faults in the debugging process.


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The debugging process

Locate

error

Design

error repairRepair

error

Re-test

program


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Software validation

q Verification and validation (V & V) is intendedto show that a system conforms to itsspecification and meets the requirements ofthe system customer.

q Involves checking and review processes andsystem testing.

q System testing involves executing the systemwith test cases that are derived from thespecification of the real data to be processedby the system.


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Testing stages

q Component or unit testing Individual components are tested independently;

Components may be functions or objects orcoherent groupings of these entities.

q System testing Testing of the system as a whole. Testing of

emergent properties is particularly important.

q Acceptance testing

Testing with customer data to check that thesystem meets the customers needs.


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Testing phases

Requirements

specification

System

specification

System

design

Detailed

design

Module and

unit codeand test

Sub-system

integrationtest plan

System

integrationtest plan

Acceptance

test plan

ServiceAcceptance

test

System

integration test

Sub-system

integration test


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Software evolution

q Software is inherently flexible and can change.

q As requirements change through changing

business circumstances, the software that

supports the business must also evolve andchange.

q Although there has been a demarcation

between development and evolution

(maintenance) this is increasingly irrelevant as

fewer and fewer systems are completely new.


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System evolution

Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

New

system

Existing

systems


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The Rational Unified Process

q A modern process model derived from the

work on the UML and associated process.

q Normally described from 3 perspectives

A dynamic perspective that shows phases overtime;

A static perspective that shows process activities;

A practive perspective that suggests good

practice.


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RUP phase model

Phase iteration

Inception Elaboration Construction Transition


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RUP phases

q Inception

Establish the business case for the system.

q Elaboration

Develop an understanding of the problem domainand the system architecture.

q Construction

System design, programming and testing.

q Transition

Deploy the system in its operating environment.


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RUP good practice

q Develop software iteratively

q Manage requirements

q Use component-based architectures

q Visually model software

q Verify software quality

q Control changes to software


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Static workflows

Workflow Description

Business modelling The business processes are modelled using business use cases.

Requirements Actors who interact with the system are identified and use cases aredeveloped to model the system requirements.

Analysis and design A design model is created and documented using architectural

models, component models, object models and sequence models.

Implementation The components in the system are implemented and structured intoimplementation sub-systems. Automatic code generation from design

models helps accelerate this process.

Test Testing is an iterative process that is carried out in conjunction with

implementation. System testing follows the completion of the

implementation.

Deployment A product release is created, distributed to users and installed in their

workplace.

Configuration and

change management

This supporting workflow managed changes to the system (see

Chapter 29).

Project management This supporting workflow manages the system development (see

Chapter 5).

Environment This workflow is concerned with making appropriate software tools

available to the software development team.


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Computer-aided software engineering

q Computer-aided software engineering (CASE) is

software to support software development and

evolution processes.

q 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.


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Case technology

q 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 teaminteractions. CASE technology does not really

support these.


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CASE classification

q Classification helps us understand the different types

of CASE tools and their support for process activities.

q Functional perspective

Tools are classified according to their specific function.

q Process perspective

Tools are classified according to process activities that

are supported.

q Integration perspective

Tools are classified according to their organisation into

integrated units.


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Functional tool classification

Tool type Examples

Planning tools PERT tools, estimation tools, spreadsheets

Editing tools Text editors, diagram editors, word processors

Change management tools Requirements traceability tools, change control systems

Configuration management tools Version management systems, system building tools

Prototyping tools Very high-level languages, user interface generators

Method-support tools Design editors, data dictionaries, code generators

Language-processing tools Compilers, interpreters

Program analysis tools Cross reference generators, static analysers, dynamic analysers

Testing tools Test data generators, file comparators

Debugging tools Interactive debugging systems

Documentation tools Page layout programs, image editors

Re-engineering tools Cross-reference systems, program re-structuring systems


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Activity-based tool classification

Specification Design Implementation Verification

and

Validation

Re-engineering tools

Testing tools

Debugging tools

Program analysis tools

Language-processing

tools

Method support tools

Prototyping tools

Configuration

management tools

Change management tools

Documentation tools

Editing tools

Planning tools


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CASE integration

q Tools Support individual process tasks such as design

consistency checking, text editing, etc.

q Workbenches Support a process phase such as specification or

design, Normally include a number of integratedtools.

q Environments

Support all or a substantial part of an entiresoftware process. Normally include severalintegrated workbenches.


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Tools, workbenches, environments

Single-method

workbenches

General-purpose

workbenches

Multi-method

workbenches

Language-specific

workbenches

Programming TestingAnalysis and

design

Integrated

environments

Process-centred

environments

File

comparatorsCompilersEditors

EnvironmentsWorkbenchesTools

CASE

technology


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Key points

q Software processes are the activities involved inproducing and evolving a software system.

q Software process models are abstract representationsof these processes.

q General activities are specification, design andimplementation, validation and evolution.

q Generic process models describe the organisation ofsoftware processes. Examples include the waterfallmodel, evolutionary development and component-

based software engineering.q Iterative process models describe the software process

as a cycle of activities.


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Key points

q Requirements engineering is the process ofdeveloping a software specification.

q Design and implementation processes transform thespecification to an executable program.

q Validation involves checking that the system meets toits specification and user needs.

q Evolution is concerned with modifying the system afterit is in use.

q The Rational Unified Process is a generic processmodel that separates activities from phases.

q CASE technology supports software process activities.

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