An Overview of Software Processes

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 introduce the software process model

concept To describe different generic process models

and their pros and cons


The Software Process

A structured set of activities required to develop a software system. These activities include:• Requirements (Specification)• Design• Implementation (Coding)• Testing (Validation)• Maintenance (Evolution)

A software process model is an abstract representation of a process.


Requirements

The process of establishing what services are required of the system the constraints on the system’s operation

and development


A Generic Requirements Process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument


Software Design

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


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


Generic Testing Planning


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.


System Evolution

Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems


Generic Software Process Models

The Waterfall model• Separate, non-overlapping 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


Waterfall Model

Requirementsdefinition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance


Waterfall Model Pros and Cons

Pros

Cons


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.

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

Process starts with the poorly understood requirements.


Evolutionary Development

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities


Exploratory Development Pros and Cons

Pros

Cons


Throw-away Prototyping Pros and Cons

Pros

Cons


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.


Reuse-oriented Development


Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation


Reuse-oriented Development Pros and Cons

Pros

Cons


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


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.


Incremental Development

Valida teincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida tesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem


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.


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

Documents

An Overview of Software Processes