02 soft eng intro

8/4/2019 02 Soft Eng Intro

1/15

Why software engineering?

Demand for software is growing dramatically Software costs are growing per system

Many projects have cost overrunsMany projects fail altogether

Software engineering seeks to find waysto build systems that are on time and within budget


2/15

Demand for larger software systems

What growth pattern do you see in the following? F4 fighter had no digital computer and software (Early 70s).

F16A had 50 digital processors and 135 KLOC (Late 70s).

F16D had 300 digital processors and 236 KLOC (Late 80s).

B-2 has over 200 digital processors and 5000 KLOC.

Software components are growing exponentially


3/15

Software development costs

Software costs are increasing ashardware costs continue to decline.

Hardware technology has made

great advancesSimple and well understood tasksare encoded in hardware

Least understood tasks are encodedin software

Demands of software are growingSize of the software applications isalso increasing

Hence the software crisis

Software

costs

Hardwarecosts

Time

Hardware costs vs. Software costsWhat can you infer from the graph?


4/15

State of the practice

Estimate Early On Time Delayed Canceled

13,000 6.06% 74.77% 11.83% 7.33%

130,000 1.24% 60.76% 17.67% 20.33%

1,300,000 0.14% 28.03% 23.83% 48.00%13,000,000 0.00% 13.67% 21.33% 65.00%

Source: Patterns of software failures and successes, Capers Jones, 1996

Delays common with mid- to large projects.Majority of the large projects are canceled.

What can you infer from this chart?


5/15

What can you infer from this chart?

0

10

20

30

40

50

60

Cost overrun Successful CancelledSource: The Standish Group, 1994

Successful projects (16.2%)- Delivered fully functional on time and on budget.

Challenged (52.7%)- Deliver less than full functionality, over-budget and late.

Impaired (31.1%)- Cancelled during development.


6/15

Software development costs and consequences

Failures resulting from software errors have varied consequencesranging from minor inconveniencesto catastrophic loss of life & property:

Air Traffic Control (FAA modernization):- $5.6 billion cost overrun.

- 8 year delay.- 2 systems are canceled- Requirements for the third have been decreased by 48%.

US Navy Finance System:- 4 times cost overrun.- Canceled after 9 years.

Flaw in Therac-25 control system caused radiation overdoses- Consequences were injury and deaths


7/15

Software development process has stages

Requirements analysis and definition: Establish the applications goals and constraints

in consultation with users

Design: Establish the systems architecture

Implementation and unit testing: Realize the design as a set of programs or program units

Unit testing verifies that each unit meets its specification

Integration and system testing: Integrate the program units and test as a complete system

Maintenance: Correct errors, improve implementation, and enhance

the systems services as new requirements are discovered


8/15

Relative costs to fix errors:What can you infer from this graph?

0

10

20

30

40

50

60

70

80

Require

ments

Design

Implementation

T

esting

Maintenance

Cost

Cost to fix an error increases as it is found later and laterin the software lifecycle


9/15

What is the primary driver of software costs?

What can you infer from the following graphic?

3%8%

7%

15%

67%

Requirements -- 3%Design -- 8%Implementation -- 7%Testing -- 15%Maintenance -- 67%

Most money and effort spentin testing and maintenance

But: 85% of errors are introduced duringrequirements analysis and design


10/15

Why are software projects late?

Estimating techniques are poorly developedEstimates are based on optimism:

- Programmers are optimistic.- Assume All will go well with the project.- Dont plan for slippage.- This is the last bug.

- Its going to work this time!

Optimism could be because of the nature of creativity:- Conception of an idea and its implementation.- Medium of creation constrains our ideas.- In case of software the medium is infinitely malleable.

- Expect a few problems in implementation.Our techniques of estimating are poorly developed.More seriously they reflect an unvoiced assumption which is quite untrue,that is, that all will go well.

-- Fred Brooks, The Mythical Man-Month


11/15

Why are software projects late? (contd..)

Does effort necessarily == progress? Is one man working six months equal to six men working one month?

Unit of man-month implies

that men and month are interchangeable.- True only when a task can be partitioned among many workerswith no communication between them.

- For sequential tasks, more effort has no effect on the schedule.- Many tasks in software engineering have sequential constraints.

Our estimating techniques fallaciously confuse effort with progress,hiding the assumption that men and months are interchangeable.- Fred Brooks, The Mythical Man-Month


12/15

Why software projects are late? (contd..)

Managers do not monitor progress effectively

Schedule slips day-by-day.Day-by-day slips are harder to recognize,

harder to prevent and harder to make up.

How does a software project get to be a year late?One day at a time!

Fred Brooks, The Mythical Man-Month


13/15

Why are software projects late ? (contd..)

When we recognize slippage, should we add more people?

Most tasks require communication among workers.Communication consists of:

- Training.

- Sharing information (intercommunication).Training affects effort at worst linearly, with the number of people.For n workers, intercommunication adds n(n-1)/2 to effort.

- If each worker must communicate with every other worker.Adding more people to an already late project is usually like

Adding gasoline to fire!

Brooks' Law:Adding manpower to a late software project makes it later.

Fred Brooks, The Mythical Man-Month


14/15

What software engineering is and is not..

Software engineering is concerned withengineering software systems, that is,building and modifying software systems: on time,

within budget,

meeting quality and performance standards, delivering the features desired/expected by the customer.

Software engineering is not Just building small or new systems.

Hacking or debugging until it works.

Easy, simple, boring or even pointless!


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Summary

Critical aspects of our day to day lives depend on software,yet software development lacks the rigor and discipline ofmature engineering disciplines: Too many projects get delayed, costs and schedules slip

Software engineering seeks to bring discipline and rigor

to the building and maintenance of software systems Study of software engineering focuses on three key elements:

process, methods and tools

Why is important to consider alternative models of thesoftware development process?

02 soft eng intro

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