©Sommerville 2000, Medvidovic 2006, Mejia 2009,Introduction to Software Engineering Slide 1...

©Sommerville 2000, Medvidovic 2006, Mejia 2009, Introduction to Software Engineering Slide 1

Ingeniería de Software Diseño, construcción y mantenimiento de

sistemas de software grandes.

Dr. Pedro Mejía Alvarez.CINVESTAV-IPN, México


Introduction

Getting started with software engineering

Objectives• To introduce software engineering and to explain its

importance

• To set out the answers to key questions about software engineering


Introduction

why is software so hard?

and what can we do about it?


How’s our personal software?

Software warranties, 2007Apple

“Except for the limited warranty on media ... software is

provided “as is”, with all faults and without warranty of

any kind...”

Google

“as is, with no warranties whatsoever”

Microsoft

“substantially in accordance with the accompanying

materials, for a period of 90 days...”


Is your PC secure?

typical patch size

‣ 100MB

typical time to download

‣ 10 minutes

average time to infection*

‣ 4 minutes[Windows XP, default firewall settings] Unprotected PCs Fall To Hacker Bots In

Just Four Minutes

Gregg Keizer; Nov 30, 2004; http://www.techweb.com/wire/security/54201306

From: Security Absurdity: The Complete, Unquestionable, And Total Failure of

Information Security, Noam Eppel; http://securityabsurdity.com


What about our operating systems ?


What about our web browsers ?

dependences between DLLs ‣ disciplined layering

why IE killed Netscape? ‣ spaghetti code in both ‣ but IE3 rebuilt from scratch

dependences in internet explorer

graph from http://www.spinellis.gr/blog/20031003for Netscape story see:Competing on Internet Time: Lessons From Netscape & Its Battle with Microsoftby Michael A. Cusumano and David B. Yoffie


Sample failures in Systems build for the government?

Navy enterprise resource planning

‣ $1B wasted on systems that don’t interoperate

NASA financial systems

‣ after 12 years and $120M spent, on third attempt

expected to cost $1B

‣ still cannot produce auditable financial statements

Department of Veterans’ Affairs

‣ supplies not available for patients due to bad

inventory control

‣ implementation halted after spending $250M


Sample failures in Systems build for the government (FBI)?

reacting to 9/11

‣ had to send photos of suspected hijackers by fax

‣ no PCs for most employees, no secure email for images

Trilogy

‣ new network, thousands of PCs, software system (“VCF”)

‣ contract awarded to SAIC

National Research Council report, 2004

‣ agents can’t take copies of cases into the field

‣ no bookmarking or history to help navigation, no sorting

outcome

‣ $600M later, no system; Sentinel ($425M) planned for 2009


Critical systems (why they fail ?)

South Africa, October 2007

‣ antiaircraft cannon kills 9 soldiers and injures 14 others

‣ cause not known, but software suspected

http://blog.wired.com/defense/2007/10/robot-cannon-ki.html


Critical systems (why they fail ?)A radar system that was supposed to warn low-flying planes of nearby obstacles was plagued with problems

and fixed nationwide only after a 1997 fatal airplane crash on Guam, according to a published report. In

some cases, programming errors caused the Minimum Safe-Altitude Warning system not to operate over

wide areas, including near busy airports such as those in Chicago and Dallas-Ft. Worth. In other cases,

false alarms were so numerous that air traffic controllers placed cardboard over warning speakers to

silence the noise. The Federal Aviation Administration was warned about the trouble after a business jet

Crashed outside Washington in 1994, but it did not take decisive action to resolve it until after a Korean Air

jumbo jet slammed into a hill on approach to Guam in August 1997, killing 228. AP, Oct 1999;

http://ns.gov.gu/guam/indexmain.html

most aviation deaths from “controlled flight into terrain”


Critical systems (why they fail ?)

ARIANE Flight 501 Disintegration after 39 sec Caused by large correction for attitude

deviation Caused by wrong data being sent to On

Board Computer Caused by software exception in

Inertial Reference System after 36 sec.

IEEE Computer, jan. 1997, p. 129-130

http://www.cs.vu.nl/~hans/ariane5report.html


How do we get here ?

Magnetic disks, US$/gigabyte

From Frans Kaashoek and Jerome Saltzer, Topics in the Engineering of

Computer Systems, to appear.


How do we get here ?

operating system growth size in millions of lines of code

From Frans Kaashoek and Jerome Saltzer, Topics in the Engineering of Computer Systems.


Fundamental challenges:context, state space, coupling

a software system is a component ‣ interacts with physical environment ‣ and organizational context of operators & users

sources of defects ‣ < 3% of software failures due to bugs in code ‣ >90% from poor understanding of requirements

consequences ‣ requirements analysis is critical ‣ not just function, also assumptions


Fundamental challenges:context, state space, coupling

state space complexity

software systems have huge state space ‣ in lifetime, small proportion covered ‣ in testing, hardly any covered

implications “‣ Program testing can be used to show the

presence of bugs, but never to show their absence!” ‣ often running in uncharted territory

*E.W. Dijkstra, Structured programming (EWD268)http://www.cs.utexas.edu/users/EWD/

©Sommerville 2000, Medvidovic 2006, Mejia 2009, Introduction to Software Engineering Slide 17 17

Central themes

SE is concerned with BIG programs

complexity is an issue software evolves development must be

efficient

you’re doing it together software must

effectively support users involves different

disciplines SE is a balancing act


Relative distribution of software/hardware costs

HardwareDevelopment

Software

Maintenance

1955 1970 1985Year

100

60

20

Per

cen

t of

tot

al c

ost

Why does software maintenance cost so much?


Global distribution of effort

testing 45%

coding 20%design 15%

requirementsengineering 10%

specification 10%


Engineering Engineering is …

• The application of scientific principles and methods• To the construction of useful structures & machines

Examples• Mechanical engineering• Civil engineering• Chemical engineering• Electrical engineering• Nuclear engineering• Aeronautical engineering

Why other areas of science and engineering are doing things better than Software Engineering ?


Electrical Engineering


Architecture & Civil Engineering


Control Systems


Process & Chemical Engineering


Software Engineering The term is 40 years old: NATO

Conferences• Garmisch, Germany, October 7-11, 1968• Rome, Italy, October 27-31, 1969

The reality is finally beginning to arrive• Computer science as the scientific basis

• Other scientific bases?


Software Engineering in a Nutshell

Development of software systems whose size/complexity warrants team(s) of engineers• multi-person construction of multi-version software

[Parnas 1987]

Scope• study of software process, development principles, techniques,

and notations

Goal• production of quality software, delivered on time, within budget,

satisfying customers’ requirements and users’ needs


The economies of ALL developed nations are dependent on software

More and more systems are software controlled Software engineering is concerned with theories,

methods and tools for professional software development

Software engineering expenditure represents a significant fraction of GNP in all developed countries

Software failures are ever more visible and costly

Why software engineering


Ever-Present Difficulties Few guiding scientific principles Few universally applicable methods As much

managerial / psychological / sociologicalas technological


Why These Difficulties? SE is a unique brand of engineering

• Software is malleable

• Software construction is human-intensive

• Software is intangible

• Software problems are unprecedentedly complex

• Software directly depends upon the hardware• It is at the top of the system engineering “food chain”

• Software solutions require unusual rigor

• Software has discontinuous operational nature


Software Engineering ≠ Software Programming

Software programming• Single developer

• “Toy” applications

• Short lifespan

• Single or few stakeholders• Architect = Developer = Manager = Tester = Customer = User

• One-of-a-kind systems

• Built from scratch

• Minimal maintenance


Software Engineering ≠ Software Programming

Software engineering• Teams of developers with multiple roles• Complex systems• Indefinite lifespan• Numerous stakeholders

• Architect ≠ Developer ≠ Manager ≠ Tester ≠ Customer ≠ User

• System families• Reuse to amortize costs• Maintenance accounts for over 60% of overall

development costs


Software costs often dominate system costs. The costs of software on a PC are often greater than the hardware costs

Software costs more to maintain than it does to develop. For systems with a long life, maintenance costs may be several times development costs

Software engineering is concerned with cost-effective software development

Software costs


Economic and Management Aspects of SE

Software production =development + maintenance (evolution)

Maintenance costs > 60% of all development costs• 20% corrective• 30% adaptive• 50% perfective

Quicker development is not always preferable• higher up-front costs may defray downstream costs• poorly designed/implemented software is a critical cost factor


Relative Costs of Fixing Software Faults

Requirements Specification Planning Design Implementation Integration Maintenance

1 2 3 410

30

200


Mythical Man-Monthby Fred Brooks

Published in 1975, republished in 1995• Experience managing development of OS/360 in 1964-65

Central argument• Large projects suffer management problems different in kind than small

ones, due to division in labor• Critical need is the preservation of the conceptual integrity of the

product itself Central conclusions

• Conceptual integrity achieved through chief architect• Implementation achieved through well-managed effort

Brooks’s Law• Adding personnel to a late project makes it later


FAQs about software engineering1. What is software?

2. What is software engineering?

3. What is the difference between software engineering and computer science?

4. What is the difference between software engineering and system engineering?

5. What is a software process?

6. What are the costs of software engineering?

7. What are software engineering methods?

8. What are the attributes of good software?

9. What are the key challenges facing software engineering?


What is software?

Computer programs and associated documentation• Often referred to as “artifacts”

Software products may be developed for a particular customer or may be developed for a general market

Software products may be• Generic - developed to be sold to a range of different customers• Custom - developed for a single customer according to the

customer’s specification


What is software engineering?

Software engineering is an engineering discipline which is concerned with all aspects of software production

Software engineers should adopt a systematic and organised approach to their work and use appropriate tools and techniques depending on

• the problem to be solved,• the development constraints, and• the resources available

A key software engineering “axiom”• Better• Cheaper pick any two• Faster


What is the difference between software engineering and computer science?

Computer science is concerned with theory and fundamentals

Software engineering is concerned with the practicalities of developing and delivering useful software

Computer science theories are currently insufficient to act as a complete underpinning for software engineering


What is the difference between software engineering and system engineering?

System engineering is concerned with all aspects of computer-based systems development including hardware, software and process engineering.• Software engineering is a “component” in this process

System engineers are involved in overall system specification, architectural design, integration and deployment


What is a software process?

A set of activities whose goal is the development or evolution of software

Generic activities in all software processes are:• Specification - what the system should do and its development

constraints

• Development - production of the software system

• Validation - checking that the software is what the customer wants

• Evolution - changing the software in response to changing demands


What are the costs of software engineering?

Roughly 60% of costs are development costs, 40% are testing costs

Evolution costs often far exceed development costs Costs vary depending on

• The type of system being developed• E.g., custom built vs. mass market software

• The requirements of system attributes such as performance and system reliability

• The experience of the development team(s)

Distribution of costs depends on the development model that is used


What are software engineering methods? Structured approaches to software development

which include system• Models

• Why are models needed?

• Notations• Such as?

• Rules - Constraints applied to system models

• Design advice - recommendations on good design practice

• Process guidance - what activities to follow

What are some example methods?


What are the attributes of good software? Software should deliver the required functionality and performance, and

should be maintainable, dependable and usable Maintainability

• Software must evolve to meet changing needs Dependability

• Software must be trustworthy Efficiency

• Software should not waste system resources Usability

• Software must be usable by the users for which it was designed

There are many others!


What are the key challenges facing software engineering?

Coping with• Legacy systems

• Increasing diversity

• Demands for reduced delivery times

Legacy systems• Old, valuable systems must be maintained and updated

Heterogeneity• Systems are distributed and include a mix of hardware and software

Delivery• There is increasing pressure for faster delivery of software


Essential software engineering difficulties

Complexity• no two software parts are alike

• complexity grows non-linearly with size

Conformity• software is always required to conform

• often the “last kid on the block”

Changeability• software is viewed as infinitely malleable

• change originates with new applications, users, machines, standards, laws

Invisibility• the reality of software is not embedded in space

• software is not representable as a familiar geometric entity


Key points Software engineering is an engineering discipline which is concerned

with all aspects of software production.

Software products consist of developed programs and associated documentation. Essential product attributes are maintainability, dependability, efficiency and usability.

The software process consists of activities which are involved in developing software products. Basic activities are software specification, development, validation and evolution.

Methods are organised ways of producing software. They include suggestions for the process to be followed, the notations to be used, rules governing the system descriptions which are produced and design guidelines.

©Sommerville 2000, Medvidovic 2006, Mejia 2009,Introduction to Software Engineering Slide 1...

Documents

Transcript of ©Sommerville 2000, Medvidovic 2006, Mejia 2009,Introduction to Software Engineering Slide 1...