17709_software engg(1).ppt

8/10/2019 17709_Software engg(1).ppt

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Software myths1. If we get behind schedule, we can just add morepeople

Fact: Adding people to a late project makes it evenlater

Someone has to teach the new people

2. A general statement of objectives is enough tostart programming

Fact: Incomplete requirements are a major cause forproject failures.3. Changes in requirements are easy to deal with

because software is flexible Fact: Changes are hard and expensive Especially during coding and after software

deployment


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Software myths4. Once we get the program running, we are done

Fact: Most effort comes after the software isdelivered for the first time.

Bug fixes, feature enhancements, etc.

5. The only product is the running program

Fact: Need the entire configuration Documentation of system requirements, design,

programming, and usage


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Software development life cyclemodel

A life cycle model prescribes the differentactivities that need to be carried out to

develop a software product andsequencing of these activities. A software life cycle is a series ofidentifiable stages that a software productundergoes during its lifetime.


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Why use a life cycle model?

It encourages development of a software insystematic and disciplined manner.

A single person can decide his own stepsto follow to develop a program.When a team works on a project then eachmember needs to abide by the modelspecified by the organisation.


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Phase entry and exit criteria

A good life cycle model should clearlydefine the entry and exit criteria for each

phase.

A phase can start only when thecorresponding phase-entry criteria issatisfied.


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Classical Waterfall Model

Feasibility Study

Requirement Analysis

Design

Coding and unit testing

Integration and system Testing

Maintenance


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Phases of classical waterfallmodel

Phases between feasibility study andtesting

known as development phases.

Among all life cycle phases maintenancephase consumes maximum effort.

Among development phases, testingphase consumes the maximum effort.


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Feasibility StudyMain aim of feasibility study : determine whetherdeveloping the product is

financially worthwhile technically feasible.First roughly understand what the customer wants:

different data which would be input to the system, processing needed on these data,

output data to be produced by the system, various constraints on the behaviour of the

system.


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Activities performed in FeasibilityStudy

Work out an overall understanding of theproblem.Formulate different solution strategies.

Examine alternate solution strategies interms of:

resources required,cost of development, anddevelopment time.


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Activities performed in FeasibilityStudy

Perform a cost/benefit analysis:to determine which solution is the best.you may determine that none of the

solutions is feasible due to:high cost,resource constraints,

technical reasons.


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Requirements Analysis andSpecification

Aim of this phase:

understand the exact requirements of the

customer,document them properly.

Consists of two distinct activities:

requirements gathering and analysis.requirements specification.


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Requirement Gathering and Analysis

Goal of Requirement gathering:

Collect all relevant information regardingthe product to be developed from thecustomer.

Clearly understand the customers

requirements.


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Requirement Gathering and Analysis (Contd..)

Goal of Requirement analysis:Weed out the inconsistencies andincompleteness in the requirements.

Inconsistent requirement is one wheresome part of requirement contradicts withsome other part.

Incomplete requirement is one where someparts of the requirement may have beenomitted advertently.


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Requirements Specification:

Requirements gathered and analyzed areorganized into a Software RequirementsSpecification (SRS) document.

Three most important contents of SRSdocument are:

The functional requirements.

The non-functional requirements.

The goals of implementation.


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SRS document:

Written according to end user terminology.

Serves as a contract between the

development team and customer.Future disputes between the two parties aresettled by examining the SRS document.

Also consists of formulation of usermanuals, system test plan, etc.


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Design

Design phase transforms requirementsspecification: into a form suitable for implementation in

some programming language.Software architecture is derived from SRSdocument.Two design approaches: traditional approach, object oriented approach.


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Traditional Design Approach

Consists of two activities: Structured analysis Structured design

Structure analysis of the requirementspecification is carried out where thedetailed structure of the problem isexamined.During structured design, the results ofstructured analysis are transformed intosoftware design.


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Structured Analysis Activity

Identify all the functions to be performed.Identify data flow among the functions.

Decompose each function recursively intosub-functions. Identify data flow among the sub-

functions as well.


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Structured Analysis Activity(Contd..)

It restricts itself to what needs to bedone aspects of the problem andneglect how to do it aspect.

functional requirements specified inSRS document are decomposed intosub functions and the data flow among

these sub-functions.


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Structured Design

Two main activities: Architectural Design (High level Design) Detailed Design (Low Level Design)

High-level design: decompose the system into modules, represent relationships among the

modules. Sometimes referred to as software

architecture.


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Structured Design (Contd..)

Detailed design: different modules designed in greater

detail: data structures and algorithms for each

module are designed.


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Object Oriented Design

First identify various objects (real worldentities) occurring in the problem: identify the relationships among the

objects. For example, the objects in a pay-roll

software may be:employees,

managers,pay-roll register,Departments, etc.


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Object Oriented Design(Contd..)

Object structure further refined to obtain the detailed

design.OOD has several advantages: lower development effort, lower development time, better maintainability.


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Coding and Unit testing

Also called as implementation phase.Each component of design isimplemented as a program module.End product is set of modules thathave been individually tested.


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Coding and Unit testing (Contd..)

During the implementation phase: each module of the design is coded, each module is unit tested

tested independently as a stand aloneunit, and debugged,

each module is documented.The purpose of unit testing: test if individual modules work correctly.The end product of implementation phase: a set of program modules that have been

tested individually.


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Integration and SystemTesting

Different modules are integrated in aplanned manner:

modules are almost never integrated inone shot. Normally integration is carried out through

a number of steps.During each integration step, the partially integrated system is tested.


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System Testing

After all the modules have beensuccessfully integrated and tested:

system testing is carried out.Goal of system testing: ensure that the developed system

functions according to its requirements asspecified in the SRS document.


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Maintenance

Maintenance of any software product: requires much more effort than the effort

to develop the product itself.development effort to maintenance effort istypically 40:60.


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Maintenance (Contd..)

Corrective maintenance: Correct errors which were not discovered

during the product development phases.

Perfective maintenance: Improve implementation of the system enhance functionalities of the system.

Adaptive maintenance: Port software to a new environment,

e.g. to a new computer or to a newoperating system


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Shortcomings of ClassicalWaterfall Model

Classical waterfall model considers transitionbetween two phases to be similar to a waterfall.However, developers do commit a large no. oferrors and thus they have to backtrack to some

previous stages to rectify that error, which isnot the case with this model.

All requirements are defined correctly at thebeginning of the project. The customershowever keep on changing the requirements.This model assumes that all phases aresequential. In practical environment phasesoverlap.


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Iterative Waterfall Model

Classical waterfall model is idealistic: assumes that no defect is introduced

during any development activity.

in practice: defects do get introduced in almost every

phase of the life cycle.Defects usually get detected much later inthe life cycle: For example, a design defect might go

unnoticed till the coding or testing phase.


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Iterative Waterfall Model(Contd..)

Once a defect is detected: we need to go back to the phase where it

was introduced

redo some of the work done during thatand all subsequent phases.

Therefore we need feedback paths in

the classical waterfall model.


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Feasibility Study

Req. Analysis

Design

Coding

Testing

Maintenance


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Errors should be detected in the same phase in which they are

introduced.

For example: if a design problem is detected in the

design phase itself,

the problem can be taken care of muchmore easily than, if it is identified at theend of the integration and system testingphase.


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Phase containment of errors

The principle of detecting errors as close toits point of introduction as possible:

is known as phase containment of errors.Iterative waterfall model is most widely usedmodel.

Almost every other model is derived fromthe waterfall model.


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PROTOTYPING MODEL

Before starting actual development, a working prototype of the system should

first be built.

A prototype is a toy implementation of asystem: limited functional capabilities, low reliability, inefficient performance.


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Reasons for developing a prototype

Fail early and inexpensively

Gather more accurate requirements

Technically understand the problemResolve conflicts

Rally financial support

File patents more easily


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Reasons for developing aprototype

Illustrate to the customer: input data formats, messages, reports, or

interactive dialogs.Examine technical issues associated withproduct development: Often major design decisions depend on

issues like:response time of a hardware controller,efficiency of a sorting algorithm, etc.


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Reasons for developing a prototype(Contd..)

The third reason for developing a prototypeis:

it is impossible to ``get it right'' the firsttime,

we must plan to throw away the firstproduct if we want to develop a goodproduct.


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Prototype Model (Contd..)

Start with approximate requirements.Carry out a quick design.Prototype model is built using several

short-cuts: Short-cuts might involve using inefficient,

inaccurate, or dummy functions. A function may use a table look-uprather than performing the actualcomputations.


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The developed prototype is submitted to thecustomer for his evaluation:

Based on the user feedback,requirements are refined. This cycle continues until the user

approves the prototype.The actual system is developed using theclassical waterfall approach.

Requirements

PR


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RequirementsGathering

Quick Design

Build Prototype

Customer Evaluationof Prototype

Refine RequirementsIncorporating

ustomer Suggestions

Maintain

Test

Implement

Design

ROTOTY

PE DEVELOPMENT

ID

Acceptance By Customer

ITERATIVE DEVELOPMENT


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Requirements analysis and specificationphase becomes redundant: final working prototype (with all user

feedbacks incorporated) serves as ananimated requirements specification.

Design and code for the prototype is usuallythrown away: However, the experience gathered from

developing the prototype helps a greatdeal while developing the actual product


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Even though construction of a workingprototype model involves additional cost ---overall development cost might be lowerfor: systems with unclear user requirements, systems with unresolved technical issues.Many user requirements get properly

defined and technical issues get resolved: these would have appeared later as

change requests and resulted in incurringmassive redesign costs.


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Evolutionary Model

Evolutionary model (aka successiveversions or incremental model): The system is broken down into several

modules which can be incrementallyimplemented and delivered.

First develop the core modules of thesystem.

The initial product skeleton is refined intoincreasing levels of capability: by adding new functionalities in

successive versions.


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Evolutionary Model (Contd..)

It is sometimes referred as design a little,test a little, deploy a little model.Successive version of the product: functioning systems capable of

performing some useful work. A new release may include new

functionality:also existing functionality in thecurrent release might have beenenhanced.


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Evolutionary Models Life Cycle Activities

AB

C

Evolutionary development of a software product


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In the evolutionary model, The s/w requirement is first broken down

into several modules, that can be

incrementally constructed and delivered. Development of core modules first. Non-core modules need the services from

core modules. This initial product skeleton is refined into

increasing levels of capability by addingnew functionalities in new versions.


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Each evolutionary version may bedeveloped using an iterative waterfallmodel of development.

Each successive version of theproduct is a fully functioning softwarecapable of performing more work than

previous one.

Rough Requirements Specification


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Rough Requirements Specification

Identify the core and other parts tobe developed incrementally

Develop the core part using theIterative waterfall model

Collect customer feedback andmodify requirements

Develop the next identified featuresUsing an iterative waterfall model

Maintenance

All Features Complete

Evolutionary Model of Software Development


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Advantages of EvolutionaryModel

Users get a chance to experiment with apartially developed system: much before the full working version is

released,Helps finding exact user requirements: much before fully working system is

developed.Core modules get tested thoroughly: reduces chances of errors in final

product.


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Disadvantages of EvolutionaryModel

Often, difficult to subdivide problems intofunctional units:

which can be incrementally implementedand delivered.evolutionary model is useful for very largeproblems,

where it is easier to find modules forincremental implementation.


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Evolutionary Model with Iteration

Many organizations use a combination ofiterative and incremental development:

a new release may include newfunctionality

existing functionality from the currentrelease may also have been modified.

l i d l i h


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Evolutionary Model withIteration

Several advantages:

Training can start on an earlier releasecustomer feedback taken into account

Markets can be created:for functionality that has never beenoffered.

Frequent releases allow developers to fixunanticipated problems quickly.


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Spiral Model

Proposed by Boehm in 1988.Each loop of the spiral represents a phaseof the software process:

the innermost loop might be concernedwith system feasibility, the next loop with system requirements

definition, the next one with system design, and so

on.There are no fixed phases in this model, the

phases shown in the figure are just


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Spiral model (Contd..)

The team must decide: how to structure the project into phases.Start work using some generic model:

add extra phasesfor specific projects or when problemsare identified during a project.

Each loop in the spiral is split into foursectors (quadrants).


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DetermineObjectives

and identifyalternativesolutions

Identify &Resolve Risks

Review andplan for thenext phase

Develop NextLevel of Product

Spiral model of software development


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Determineobjectives,alternative,constraints

Evaluatealternatives, identifyandresolverisks

Developverify nextlevel product

Plan nextphases

Quadrant 1: Determine


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Quadrant 1: Determineobjectives, alternatives, and

constraints:Objectives : performance,hardware/software interface ,functionality, etc.

Alternatives : design, reuse, buy, etc.

constraints : imposed on technology,cost, schedule, support, and risk.

Once the systems objectives,alternatives, and constraints areunderstood, Quadrant 2 (Evaluatealternatives, identify, and resolve risks)is performed

Quadrant 2: Evaluate


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For each identified project risk, a detailed analysis is carried out.

Steps are taken to reduce the risk.For example, if there is a risk that therequirements are inappropriate: a prototype system may be developed.

Quadrant 2: Evaluatealternatives, identify, resolve

risks:


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Quadrant 3 and Quadrant 4

Development and Validation (Thirdquadrant): develop and validate the next level of the

product.Review and Planning (Fourth quadrant): review the results achieved so far with

the customer and plan the next iterationaround the spiral.

With each iteration around the spiral: progressively more complete version of

the software gets built.


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Summary of Spiral steps:

Each successive phase in the project as a newspiral includes a four steps or phases.Software requirements in the design aregradually developed through a series ofprototypes.

The exact number of spirals necessary for theproject is flexible and depends on the number ofprototypes needed to reach a satisfactory design.Since each phase requires a certain level ofcommitment a cumulative cost of the projectrepresented by the width of the spiralOnce a satisfactory design is reached thesoftware is constructed according the final threeprocess of the waterfall model (Programming Integration-Delivery)


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Spiral Model as a meta model

Subsumes all discussed models: Uses waterfall model phases. uses an evolutionary approach --

iterations through the spiral are evolutionarylevels. enables understanding and reacting to

risks during each iteration along the

spiral. uses:

prototyping as a risk reduction mechanismretains the step-wise approach of the waterfallmodel.


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Strengths Introduces risk management

Prototyping controls costs Evolutionary development Release builds for beta testing Marketing advantage

Weaknesses Lack of risk management experience

Lack of milestones Management is dubious of spiral process Change in Management Prototype Vs Production

C i f Diff t Lif


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Comparison of Different LifeCycle Models

Iterative waterfall model most widely used model. But, suitable only for well-understood

problems .Prototype model is suitable forprojects not well understood:

user requirements technical aspects

Comparison of Different Life


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Comparison of Different LifeCycle Models (Contd..)

Evolutionary model is suitable forlarge problems: can be decomposed into a set of modules

that can be incrementally implemented, incremental delivery of the system is

acceptable to the customer.The spiral model:

suitable for development of technicallychallenging software products that aresubject to several kinds of risks.

17709_software engg(1).ppt

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