cmsc 345, version 1/03 an overview of software processes reference: software engineering, by ian...

CMSC 345, Version 1/03

An Overview ofSoftware Processes

Reference: Software Engineering, by Ian Sommerville, 6th edition, Chapter 3


Objectives To introduce the general phases of the

software (development) life cycle To describe various generic software

process models and their pros and cons


Software Life Cycle

The phases necessary to develop and maintain a software system. These phases include:• Requirements (Specification)• Design• Implementation (Coding)• Testing (Validation)• Maintenance (Evolution)

A software process model is an abstract representation of how these phases can be addressed.


Requirements

The process of establishing

• what services are required of the system

• the constraints on the system’s operation and development

The “what” of the software life cycle


A Generic Requirements Process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument


Design

The process of converting the system specification (requirements) into a software structure that realizes that specification

The “how” of the software life cycle


A Generic Software Design Process

Architecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products


Implementation 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.

The activities of design and implementation are closely related and may be interleaved.


Testing 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.


A Generic Testing Process

Sub-systemtesting

Moduletesting

Unittesting

Systemtesting

Acceptancetesting

Componenttesting

Integration testing Usertesting


V-Model of Test Planning

Requirementsspecification

Systemspecification

Systemdesign

Detaileddesign

Module andunit codeand tess

Sub-systemintegrationtest plan

Systemintegrationtest plan

Acceptancetest plan

ServiceAcceptance

testSystem

integration testSub-system

integration test


System Maintenance 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.


Software Process Models Waterfall model (Royce, 1970) Prototyping

• Throwaway• Evolutionary

Incremental development Spiral model (Boehm, 1988)


Waterfall Model

Requirements

Design

Implementation

Maintenance

Testing


Observations

The following phase should not start until the previous phase has finished.

In practice, • Phases overlap• May return to a previous phase

Still widely used, especially on very large projects


Waterfall Model Pros and Cons

Pros

Cons


Prototyping

Requirements

Implementation

Design

Testing

Design

Implementation

Testing

Maintenance


Observations

Used for requirements elicitation and validation

A “working” model (prototype) of the final system is developed during requirements

Is an iterative process Prototype can be thrown away or evolved

into the final system (evolutionary prototyping)


Prototyping Pros and Cons

Pros

Cons


Throwaway vs. Evolutionary

Throwaway pros

Evolutionary pros


Incremental Development

Valida teincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida tesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem


Observations 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.

Is an iterative process


Incremental Pros and Cons

Pros

Cons


Spiral Model

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


Observations

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.

Uses prototyping


Things to Think About

What about modifying existing software? What about using existing software?

• In-house modules• COTS (Commercial Off-The-Shelf)

cmsc 345, version 1/03 an overview of software processes reference: software engineering, by ian...

Documents

software system

generic testing process

software life cycle

software engineering

system testing

software structure

system maintenance

evolutionary process