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2005 IAS, Universitt Stuttgart 1

SER

1 Introduction and Motivation

1.1 Why is Software Engineering Important?

1.2 Why is Software Engineering Error-Prone?

1.3 FAQs on Software Engineering

1.4 Professional and Ethical Responsibility

1.5 Summary

Software Engineering for Real-Time Systems

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SER

Why is Software Engineering Important?

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


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SER1.1 Why is Software Engineering Important?

Intensification of problems

The complexity of programs is increasing

System lifetime increases

New applications for the use of computers emerge.

In 2001 ca. 8Bio Microprocessors were delivered. Only 2% ended up in

PCs. The rest is embedded in systems -> This is what this lecture is all

about.

Software development is an integral component of system development

More and more developers are occupied with the maintenance of old

systems

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Life cycle of a software product

flood of information

high complexity

costly and faulty maintenance

software development

software

maintenance

requirement specification(2000 requirements)

product(1m DIN A4,1 M BYTE Code)

system analysis(10 000 detailed

requirements)

software programs(100 000 lines of code)

rough design(50 module packages)

detailed design( 1500 modules )

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Software costs

Software costs often dominate system costs. The costs of software on a

PC are often greater than the hardware cost. Modern telecommunication

systems attribute 80..90% of total system cost to software cost.

Software costs more to maintain than it does to develop. For systems with

a long lifetime such as real-time or embedded systems - , maintenancecosts are several times the development costs.

Software engineering is concerned with cost-effective software

development.

Design to Cost!

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SERSoftware Engineering for Real-Time Systems






1.5 Summary

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SER1.2 Why is Software Engineering Error-Prone?

Characteristics of Software

software is an immaterial product

software is not subject to wear out

software ages but continues to live

software normally can be changed easier and quicker

than a technical product

there are no spare parts for software

Deeply embedded into todays business and society

processes makes it vulnerable to all type of attacks.

duplication is trivialsoftware theft

security

Standing on quicksand

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Denver-Airport (1993 - 1995)

October 1993: Problems in the high tech baggage distribution plant.

April 1994: Postpone of the opening after debacle:

Pieces of baggage were thrown on cars not there at all; cars fell out of the

lanes; etc.

The computer-controlled baggage distribution plant consisted of over 150

computers, 4000 cars, 5000 sensors, 56 bar code scanners; value of the plant:

$200 million, additional $100 million for building modifications.

Main cause:Too many messages via Ethernet; sorting instructions didn't arrive in time because

the network (LAN) is overloaded.

The whole problem is too complex.

August 1994: Installation of a (second) conventional system with usual

conveyor belts: additional $50 million.

First official opening: March 1, 1995 - but out of service soon after;

reopening: October 1995

Total costs: $5200 million, instead of $2000 million projected initially.

Software faults are expensive

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Case Study - Ariane 5 (1)

An European rocket designed to send commercial payloads (e.g.communication satellites, etc.) into orbit

Successor of successful Ariane 4 launcher

Ariane 5 can carry heavier payloads than Ariane 4

June 4, 1996

Total failure of the Ariane 5 launcher on its

maiden flight

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Launcher failure

Approximately 37 seconds after the successful lift-off, the Ariane 5

launcher lost control.

Incorrect control signals were sent to the engines and these swivelled so

that unsustainable stresses were imposed on the rocket.

It started to break up and self-destructed.

The system failure was a direct result of a software failure. However, it was

symptomatic of a more general system validation failure.

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The problem

The attitude and trajectory of the rocket are measured by a computer-

based inertial reference system. This transmits commands to the engines

to maintain attitude and direction.

The software failed and this system and the backup system shut down.

Diagnostic commands were transmitted to the engines, which interpretedthem as real data and which swivelled to an extreme position.

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SER

Software failure


Software failure occurred when attempting to convert a 64-bit floating point

number to a signed 16-bit integer which caused the number to overflow.

There was no application-defined exception handler associated with the

conversion so the default exception management facilities provided by the

system were invoked. They shut down the whole software. The backup software was just a copy and behaved in exactly the same

way.

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Avoidable failure?

The software that failed was reused from the Ariane 4 launch vehicle. Thespecific part leading towards the overflow was not used in Ariane 5.

Decisions were made:

to reuse working algorithms and their implementation in a modular

(black box) way.

not to remove/change the reused component as this could introduce

new faults.

not to provide any overflow exception handling because the processor

was heavily loaded. For dependability reasons, it was thought

desirable to have some spare processor capacity.


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Why not Ariane 4?

The physical characteristics of Ariane 4 (a smaller vehicle) are such that it

has a lower initial acceleration than Ariane 5.

The value of the variable on Ariane 4 could never reach a level that caused

overflow during the launch period.

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SER

Validation failure

As the facility that failed was not required for Ariane 5, there was no

requirement associated with it.

As there was no associated requirement, there were no tests of that part of

the software and hence no possibility of discovering the problem.

During system testing, simulators of the inertial reference systemcomputers were used. These did not generate the error as there was no

requirement!


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Review failure

The design and code of all software should be reviewed for problems

during the development process.

However:

The inertial reference system software was not reviewed because it

had been used in a previous version.

The review failed to expose the problem or that the test coverage

would not reveal the problem.

The review failed to appreciate the consequences of system shutdown

during a launch.

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SER

Lessons learned

As well as testing for what the system should do, you may also have to testfor what the system should not do

Do not have a default exception handling response which is system shut-

down in systems that have no fail-safe state

In critical computations, always return best effort values even if the

absolutely correct values cannot be computed

Wherever possible, use real equipment and not simulations

Improve the review process to include external participants and review all

assumptions made in the code


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The failure was avoidable

The developers of the Ariane 5 made a critical and elementary

mistake:

They designed a system which took the whole system to the fall by a

single component fault.


Extensive listing of software problems

http://www.csl.sri.com/users/neumann/illustrative.html

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1.5 Summary

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SER

FAQs about Software Engineering

What is software? What is software engineering?

What is the difference between software engineering and computer

science?

What are real-time systems?

Why is software engineering for real-time systems difficult? What is a software process?

What is a software process model?

What are the costs of software engineering?

What are software engineering methods?

What is CASE (Computer-Aided Software Engineering) What are the attributes of good software?

What are the reasons for poor software quality?

What are the key challenges facing software engineering?


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SER

What is software?

Computer programs andassociated documentation

Software products may be

generic

-> developed to be sold to

a range of differentcustomers

Custom-made

-> developed for a single

customer according to

their specification


software

product

designdocumentation

systemdesignprototypes

configuration

managementtest environment

users

guide

support

userrequirements

specification

Software is much more than code.

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SER

What is software engineering?

Software engineering is the use of qualified methods, tools and process

models for building and using software aiming


on the one hand to reduce the software costs at the development,

maintenance and enhancement of program systems

on the other hand to realize a higher level of system quality.

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What is the difference between software engineering and

computer science?

Software engineering constructive approach

guidelines from experiences and empirical

methods

development and delivery of useful

software products


Computer Science analytical, inductive approach

theoretical valid, comprehensive solutions

research and definition of principles,

methods and software tools that may beapplied to software product development

practical

theoretical

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SER

What are real-time systems?

Many of today Information systems seat reservation

bank account management

logistic systems

Telecommunication

Industrial automation systems

Production and manufacturing automation

Air-conditioning and building automation systems

Energy generation (automation, monitoring, safety)

Traffic management, train systems, air flight systems

Further micro-controller / embedded systems


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SER

Why is software engineering for real-time systems difficult?

Being in time is the critical requirement

programming languages with real time features

parallel processing

synchronization

interrupt processing

process input/output

integrated hardware / software systems

embedded into technical processes that are difficult to simulate

real-time operating systems, database systems, etc

test of dynamic aspects

simulation of complex parallel processes

safety and reliability aspects


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SER

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


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SER

Problem: quality assurance

Ensuring, that quality requirements are met

in every development phase.


accompanying development

Ensuring, that the software process is worked on basis of proven products,

specifications and documents only.

standardized procedure

Ensuring, that unexamined products, without specifications or documents

do not leave the company.

release through SQA

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SER

What is a software process model?

A simplified representation of a software process, presented from aspecific perspective

Examples of process perspectives are

Workflow perspective - sequence of activities

Data-flow perspective - information flow

Role/action perspective - who does what

Generic process models

Waterfall

Evolutionary development

Formal transformation Integration from reusable components


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SER

What are the costs of software engineering?

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

custom software, evolution costs often exceed development costs. During

maintenance activities, cost of test increases.

Costs vary depending on the type of system being developed and the

requirements of system attributes such as performance and system

reliability

Distribution of costs depends on the development model that is used


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SER

What are software engineering methods?

Structured approaches to software development which include systemmodels, notations, rules, design advice and process guidance

Model descriptions

Descriptions of graphical models which should be produced

Rules

Constraints applied to system models

Recommendations

Advice on good design practice

Process guidance

What activities to follow


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SER

Methods of programming


20202010200019901980197019601950

build systematical new development configure

todayprogramming as an art

structured

object oriented

adaptive

SW crisis

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SER

What is CASE (Computer-Aided Software Engineering)?


Software systems which are intended to provide automated support forsoftware process activities. CASE systems are often used for method

support

Upper-CASE

Tools to support the early process activities of requirements and

design

Lower-CASE Tools

Tools to support later activities such as programming, debugging and

testing

requirements, design

programming, debugging,

test

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SER

What are the attributes of good software?


The software should deliver the required functionality and performance tothe user and should be maintainable, dependable and usable

Maintainability

Software must evolve to meet changing needs

Dependability

Software must be trustworthy

Efficiency

Software should not make wasteful use of system resources

Usability

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

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SER

What are reasons for poor software quality?


organization and management unclear object in mind

unclear responsibility

inadequate project planning and control

insufficient estimation of the project costs

quality control not independent

methodology

missing/ insufficient registration of needs

no use of development methods

incomplete documentation

no standards or no use of standards

no quality assurance

technology

no development models

no strategies for process improvement

not enough use of CASE-Tools

no analysis of metrics

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SER

Distribution of the error costs


requirementsreview

designreview

codereview

componenttest

systemtest

deployment

error costs100%

80%

60%

40%

20%

0%

The later an error is discovered, the more expensive is the elimination!

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What are the key challenges facing software engineering?


Legacy systems Old, valuable systems must be maintained and updated

Heterogeneity

Systems are distributed and include a mix of hardware and software

Better, faster, cheaper There is increasing pressure for faster delivery of software at a lower

cost with improving quality

Security, reliability, availability, safety

Modern society and businesses depend heavily on software. They are

vulnerable to poor quality and attacks.

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SER

Problem: communication (1)


Team 1

component

A

component

B

Team 2

component

C

component

D

If Team 1 changes the interface of component A ...

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Problem: communication (2)


Team 1

component

A

component

B

Team 2

component

C

component

D

... can you ensure, that Team 2 will consider this when constructingcomponent C?

Problem: different teams, products, companies

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1.5 Summary

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Professional and ethical responsibility


Software engineering involves wider responsibilities than simply the

application of technical skills

Software engineers must behave in an honest and ethically responsible

way if they are to be respected as professionals

Ethical behaviour is more than simply upholding the law.

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Issues of professional responsibility


Confidentiality

Engineers should normally respect the confidentiality of their

employers or clients irrespective of whether or not a formal

confidentiality agreement has been signed.

Competence Engineers should not misrepresent their level of competence.

They should not knowingly accept work which is out with their

competence.

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Issues of professional responsibility


Intellectual property rights Engineers should be aware of local laws governing the use of

intellectual property such as patents, copyright, etc. They should

be careful to ensure that the intellectual property of employers

and clients is protected.

Computer misuse Software engineers should not use their technical skills to misuse

other peoples computers. Computer misuse ranges from relatively

trivial (game playing on an employers machine, say) to extremely

serious (dissemination of viruses).

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ACM/IEEE Code of Ethics


The professional societies have cooperated to produce a code ofethical practice.

Members of these organisations sign up to the code of practice when

they join.

The Code contains eight Principles related to the behaviour of and

decisions made by professional software engineers, including practitioners,educators, managers, supervisors and policy makers,

as well as trainees and students of the profession.

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Code of ethics - principles (1)


1. PUBLIC Software engineers shall act consistently with the public interest.

2. CLIENT AND EMPLOYER

Software engineers shall act in a manner that is in the best interests

of their client and employer consistent with the public interest.

3. PRODUCT

Software engineers shall ensure that their products and related

modifications meet the highest professional standards possible.

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4. JUDGMENT Software engineers shall maintain integrity and independence in their

professional judgment.

5. MANAGEMENT

Software engineering managers and leaders shall subscribe to and

promote an ethical approach to the management of softwaredevelopment and maintenance.

6. PROFESSION

Software engineers shall advance the integrity and reputation of the

profession consistent with the public interest.

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7. COLLEAGUES

Software engineers shall be fair to and supportive of their colleagues.

8. SELF

Software engineers shall participate in lifelong learning regarding the

practice of their profession and shall promote an ethical approach tothe practice of the profession.

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Ethical dilemmas


Disagreement in principle with the policies of senior management

Your employer acts in an unethical way and releases a safety-critical

system without finishing the testing of the system

Participation in the development of military weapons systems or

nuclear systems

These dilemmas depend on ones own values which might differ

from the clients or employers values

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SER


Software Engineering for Real-Time Systems




1.4 Professional and Ethical Responsibility1.5 Summary

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SER1.5 Summary

Summary

Software engineering is an engineering discipline which is concernedwith 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 indeveloping 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 anddesign guidelines.

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SER

Further Information

1.5 Summary

Ian Sommerville: Software

Engineering

Addison Wesley, 20016. Edition

ISBN: 3827370019

Software Engineering Body ofKnowledge

http://www.swebok.org/

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