Inroduction to Software Engineering

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SOFTWAREENGINEERING

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Introduction


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This Course

SE is unlike other CS topicsOS , DBMS , Compilers etc talk about

specific types of software productSW Engg. focuses on general softwareSoftware Engineering is the systematic

approach to development, operation,maintenance, and retirement of sw.Basic Q. of SW Engg.: How to developindustrial-strength software?


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What this course will give ?

Main objective: Give an idea of how industrial-strength software gets developed

At the end you should have the ability to plan,

execute, and manage small software projectsLectures will discuss how to perform differenttasks in a project

In the project , the techniques will be applied


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Project

A group project with 3-5 peopleWill develop software for some

customer to solve some real problemsApproximately 10 hours per week perstudent

Will start from 3rd week and last tillthe semester endExtra lectures in start to kick-start theproject


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Evaluation and Grading

Project will have a weight of 50-60%a poor project cannot get a good grade

One mid sem exam and an end sem exam.Project group grade; marks equally dividedunless the team specifies a diff distribution(based on contribution)

Review by a group of some other groups workproducts


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Software

Q : If you have to write a 10,000 line programin C to solve a problem, how long will it take?

Answers: generally range from 2-4 months

Let us analyze the productivityProductivity = output/input resources

In SW output is considered as LOC

Input resources is effort - person months; overheadcost modeled in rate for person month

Though not perfect, some productivity measure isneeded, as project has to keep it high


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Software

The productivity is 2.5-5 KLOC/PM

Q: What is the productivity in a typical

commercial SW organization ?A: Between 100 to 1000 LOC/PM

Q: Why is it low, when your productivity is so

high? (people like you work in the industry)A: What the student is building and what theindustry builds are two different things


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Software

In a univ a student system is built while thecommercial org builds industrial strength sw

What is the difference between a studentprogram and industrial strength sw for thesame problem?

Software (IEEE): collection of programs,procedures, rules, and associateddocumentation and data


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Software

Student

Developer is the

userbugs are tolerable

UI not important

No documentation

IndustrialStrength

Others are the usersbugs not tolerated

UI v. imp. issue

Documents needed forthe user as well as forthe organization and theproject


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Software

Student

SW not in critical use

Reliability, robustnessnot important

No investment

Dont care aboutportability

Industrial Strength

Supports importantfunctions / business

Reliability , robustnessare very important

Heavy investment

Portability is a key issuehere


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Industrial strength software

Student programs for a problem & industrialstrength software are two different things

Key difference is in quality (includingusability, reliability, portability, etc.)High quality requires heavy testing, which consumes30-50% of total development effort

Requires development be broken in stages such thatbugs can be detected in each

Good UI, backup, fault-tolerance, following of stds etcincrease the size for the same functionality


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Industrial strengthsoftware

If 1/5th productivity, and increase in size bya factor of 2, industrial strength software willtake 10 times effort

Brooks thumb-rule: Industrial strength swcosts 10 time more than student sw

Domain of SW Engg: Industrial strength sw

In SW Engg. and in this course, softwaremeans industrial strength software


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Software is Expensive

Let us look at costs involvedProductivity = 500 LOC/PM

Cost to the company = $10K/PM

Cost per LOC = $20I.e, each line of delivered code costs about $20.

A simple application for a business may have20KLOC to 50KLOCCost = $100K to $1Million

Can easily run on $10K-$20K hardware

So HW costs in an IT solution are small as compared to SWcosts.


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Software is Expensive

The HW/SW ratio for a computer systemhas shown a reversal from the early years.In 50s , HW:SW :: 80:20

In 80s , HW:SW :: 20:80

So , SW is very expensive

Importance of optimizing HW is not muchMore important to optimize SW


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Late & Unreliable

20-25% of SW projects never completeBecause after some time they realize that the finalcost will be much higher

Many companies report runawaysbudget & cost out of control

consulting companies to help control them

One defence survey found that 70% of theequipment problems are due to SW

Many examples of software failures


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Unreliable

SW failures are different from failures of mechanical or electrical systemsIn software, failures are not due to aging relatedproblemsFailures occur due to bugs or errors that getintroduced during developmentI.e. the bug that causes a failure exists fromstart, only manifests later


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Maintenance

Once sw delivered, it enters maintenancephase

Why is maintenance needed for sw when itdoes not wear with age?Residual errors requiring corrective maint

Upgrades and environment changes adaptivemaint

Over sw life, maint can cost more than thedevelopment cost of sw


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SE Challenges

Problem domain discussed before,now we discuss the area of SE

SE (IEEE): systematic approach todevelopment,., of software

Systematic approach: methodologiesand practices that can be used tosolve a problem from problem domain


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SE Challenges

The problem of producing software tosatisfy user needs drives the approachesused in SESoftware is Industrial strength sw

But there are other factors that drive theselection of approaches

These factors include considerations of scale, quality, productivity, consistency,change,


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Scale

SE must deal with problem of scalemethods for solving small problems do not scale up forlarge problems

industrial strength SW problems tend to be large

SE methods must be scalable

Two clear dimensions in this

engineering methodsproject management

For small, both can be informal or ad-hoc, for largeboth have to be formalized


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Scale

Click to edit Master text stylesSecond level

Third level

Fourth level Fifth level


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Scale

An illustration of issue of scale is counting thenumber of people in a room vs taking a census

Both are counting problems

Methods used in first not useful for census

For large scale counting problem, must usedifferent techniques and models

Management will become critical


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Productivity

An engg project driven by cost and scheduleIn sw cost is mainly manpower cost, hence

measured in person-monthsSchedule is in months/weeks very importantin business contextProductivity capture both of theseIf P is higher, cost is lowerIf P is higher, time taken can be lesser

Approaches used by SE must deliver high P


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Quality ISO standard


Third level Fourth level Fifth level


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Quality ISO std

ISO std has six attributes

Functionality

Reliability

Usability

Efficiency

Maintainability

Portability


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Quality

Multiple dimensions mean that not easyto reduce Q to a single number

Concept of Q is project specificFor some reliability is most important

For others usability may be more

importantReliability is generally considered themain Q criterion


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Consistency andrepeatability

Sometimes a group can deliver one goodsoftware system

Key SE challenge: how to ensure thatsuccess can be repeatedSE wants methods that can consistentlyproduce high Q sw with high P

A sw org, wants to deliver high Q&Pconsistently across projectsFrameworks like ISO and CMM focus on thisaspect a lot


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Change

Only constant in business is change!

Software must change to supportthe changing business needs

SE practices must accommodatechange

Methods that disallow change, evenif high Q and P, are of little use


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SE Approach

We understand the problem domain, thefactors that drive SE

Consistently develop sw with high Q&P forlarge scale problems and under changesQ&P are the basic objectives to be achievedunder large scale and changes

Q&P governed by people, processes, andtechnology


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Iron Triangle


Third level

Fourth level Fifth level


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SE Approach

SE focuses mostly on processes forachieving the goals

Systematic approach is really aboutprocesses being usedSE separates process for developing swfrom the developed product (i.e sw)

Premise: Process largely determines Q&P,hence suitable processes will lead to highQ&P


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SE Approach

Design of proper processes and theircontrol is a key challenge SE faces

This focus on process makes SEdifferent from many CS courses

Sw process is the equivalent of manufacturing process


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SE Approach

The development process used in SE istypically phased

Phases separate concerns with each phasefocusing on some aspectRequirements, architecture, design, coding, testingare key phases

This phased process has to be properlymanaged to achieve the objectivesMetrics and measurement important for this


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Summary

The problem domain for SE isindustrial strength software

Software comprises programs,documentation, and dataSE aims to provide methods for

systematically developing SWMain goal achieve high quality andproductivity (Q&P)


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Summary

Must have high Q&P withconsistency, under large scale and

changesBasic approach of SE is to separateprocess from products and focus on

process and managing the process