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2005 IAS, Universitt Stuttgart 1

SER

Objectives of chapter 2

To define the exact meaning of software engineering

To introduce the most important terms and acronyms related to it

To outline general software engineering principles and methods

To understand what are the major development stages in a softwareproject

Software Engineering for Real-Time Systems

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SERSoftware Engineering for Real-Time Systems

2 Fundamentals

2.1 Terminology

2.2 Quality Characteristics

2.3 Software Development Phases

2.4 Summary

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SER

What can I do, if ...

2.1 Terminology

Quality

characteristics

Procedure

Concept

Method

Reliability

Terms

are important

for a

common

understanding

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SER

General Terms

2.1 Terminology

Definition Software:Computer programs, procudures, rules and possible associated

documentation and data pertaining to operation of a computer system.

[IEEE 610.12]

Definition Software Product:

Self-contained result of a successfully completed software project, projectpart or software process.

Definition Software System:

Part of the physical or intellectual world consisting of software elements

and a software structure in which the software elements are arranged.

Exterior view on software

(customers point of view)

Interior view on software

(developers point of view)

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SER2.1 Terminology

Definition Software Engineering:

Targeted provision and systematical use of principles, methods, concepts,

notations and tools for the partitioned, engineering-like development and

utilization of large software systems. Targeted means the consideration of

e.g. cost, time and quality.

[Balzert, Lehrbuch der Softwaretechnik]

Quality Cost

Time

Magic triangle:

Software

engineering

aims to

achieve theoptimum

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SER

Characteristics

universal, abstract, general

constitute the theoretical basis

no statement about how the principle can be accomplished

independent of application domain or development phases

2.1 Terminology

Definition:

Principles are fundamental assumptions that can be taken as a basis for

actions.


Example

principle of structuring (basis of structured programming, e.g.)

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SER

General principles of software engineering

Abstraction

Decomposition

Forming perspectives

2.1 Terminology

Abstraction

a generalization which is achieved by omitting properties

separation of essential and unimportant aspects

the decision on what unimportant is is not obvious and strongly depends

on the problem under consideration

especially in the early development stages abstractions are useful

in software development they concentrate on data and information flows

Advantages:

by omitting unimportant properties complexity can be reduced

the solution is getting more general and may be used in a broader context

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SER

Decomposition

Delimiting elements from a whole

Separate consideration of each element

May be performed based on different aspects: hierarchy or modules

Always leads to some kind of structure, therefore the principle of

decomposition is also known as principle of structuring

2.1 Terminology

Advantages:

Complexity of elements is less than of the whole, i.e. decomposition results

in a better understanding and less errors

Elements may be realized in parallel

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SER

Forming perspectives

Separate consideration or analysis of a circumstance from different pointsof view

Seeing the same thing from different angles is the key ability of a good

system analyst an important to understand what the customer is expecting

2.1 Terminology

Advantages: Consideration of all relevant aspects of a given problem

Less errors in early development stages

Example: Office automation

Economic perspective

Perspective of working place arrangement

Perspective of integration into existing EDP infrastructure

Perspective of user friendliness (one of the most important!)

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SER

Definition:

Methods are systematically used, well-founded procedures for reaching

predefined goals (in general within the scope of predefined principles)Balzert, Lehrbuch der Softwaretechnik]

2.1 Terminology

Characteristics

make principles ready to use concrete in terms of separation into single steps

utilization and success has to be verifiable and measurable

state when and how which concept has to be used

methods are put in concrete form by procedures

Example

principle of structuring

associated method is structured programming, i.e.

designing programs by using sequences, selections and loops only

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SER

General methods of software engineering

2.1 Terminology

Top-down method

Bottom-up method

Yo-yo approach

The direction of the development process being described allows todistinguish between different method classes:

User sphere or view

Basis sphere or machine

Top-down Bottom-up

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SER

Top-down method

Starting point: abstract and complete description of the system to bedeveloped

Decomposition into subsystems and concretion at each development step

Consistency between abstract an concrete levels must be guaranteed

Also known as outside-in method

2.1 Terminology

Advantages: User sphere is the central point of the system development

Complete and precise specifications lead exactly to the expected system

The overall view facilitates finding appropriate abstraction levels and

getting a good system structure

Disadvantages: Highly developed ability of abstract thinking is essential

Hard to achieve transitions towards more concrete levels

Realization-related problems (constraints due to hardware, operating

system, or programming language) can be detected at the very end only

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SER

Bottom-up method

Starting point: concrete basis machine with known functionality Step by step abstraction and combination of known elements to larger

system units

Last step consists of realizing the user machine

Also known as inside-out method

2.1 Terminology

Advantages:

The concrete starting point guarantees a solid basis for the system

development

Better chances that existing partial solutions are searched for and reused

Disadvantages:

Hard to match the user machine, which constitutes the actual objective of

the development, with what has been expected

Potential risk that low-level system components are created which later

turn out not to be relevant any more

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SER

Yo-yo method

Pragmatic connection of top-down and bottom-up methods

Basically the same as top-down but switch to bottom-up allowed whenever

it is needed

Reason for switch: abstract description not possible or useful without

having considered some more concrete aspects and properties of the

system

2.1 Terminology

Advantages:

Allows flexible adaptation of the method to current situation

lower demand for abstract thinking ability

Disadvantages:

Decision of when to switch the development direction is arbitrary

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SER2.1 Terminology

Definition:A concept is an abstraction allowing to model a defined fact from one or

several points of view.


Definition:

A notation represents one or several concepts by means of symbols.


Examples

Concepts of structured programming: sequence, selection, loop

Suitable notation: structure chart

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SER2.1 Terminology

Definition:Tools provide an automated support for methods, procedures, concepts

and notations.


Characteristics

force to follow methods, procedures, standards, concepts and notations

increase the productivity

in software development tools are necessary but not sufficient

Examples

compilers, debuggers

test systems like micro-controller emulators or network packet generators

CASE tools (Computer Aided Software Engineering)

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SER

2 Fundamentals

2.1 Terminology



2.4 Summary

Software Engineering for Real-Time Systems

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SER2.2 Quality Characteristics

Quality characteristics = development goals

How reliable is

the software?

Is the software

easy to use?

How efficient is

the software?

How easy can the

software be

modified?

How easy can the

software transfered

to another environment?

Are the required functionsavailable in the software?

Functionality

Reliability

Usability

Efficiency

Transferability

Maintainability

ISO/IEC

9126-1

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Definition:

Functionality is the set of attributes that bear on the existence of a set of

functions and their specified properties. The functions are those that

satisfy stated or implied needs.

[ISO/IEC 9126:1991]

Subdivision in:

Suitabilityappropriateness of the functions for specified tasks,

e.g. the search function of a text editor may be based on regular

expressions but wouldnt be suited for non-advanced users then

Accurateness

provision of right or agreed results or effects,

e.g. the correctness and the required accuracy of computed values

Interoperability

ability to interact with specified systems

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Compliance

adherence to application related standards or conventions or regulations

in laws and similar prescriptions

Security

ability to prevent unauthorized access, whether accidental or deliberate,

to programs or data

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Definition:

Usability is the set of attributes that bear on the effort needed for use,

and on the individual assessment of such use, by a stated or implied set

of users.

[ISO/IEC 9126:1991]

Subdivision in:

Understandabilitythe users effort for recognizing the logical concept and its applicability

Learnability

the users effort for learning an application,

i.e. the introduction

Operability

the users effort for operation and operation control

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Definition:

Efficiency is the set of attributes that bear on the relationship between

the level of performance of the software and the amount of resources

used, under stated conditions.

[ISO/IEC 9126:1991]

Subdivision in:

Time behaviorresponse and processing times and on throughput rates in performing

the function

Resource behavior

amount of resource used and the duration of such use in performing the

function

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Definition:

Maintainability is the set of attributes that bear on the effort needed to

make specified modifications.

[ISO/IEC 9126:1991]

Analyzability

effort needed for diagnosis of deficiencies or causes of failures, or foridentification of parts to be modified

Changeability

effort needed for modification, fault removal or for environmental change

Stability

risk of unexpected effect of modifications

Testability

effort needed for validating the modified software

Subdivision in:

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Example of a maintainable development: defining the

architecture of an embedded web server

Running on a micro-controller platform with Ethernet connectivity

Allows to serve HTML or XML documents from within a device to a

standard web browser

Incorporates an optimized implementation of needed Internet protocols

HTTP

TCP/IP

ARP

Ethernet

Option 1:

Whole protocol stack in a singlemodule

TCP

Option 2:

Each protocol layer in its own module

IP

ARP

Ethernet

HTTP

Hard to analyze because all layers

have to be regarded at once

Modifications are very likely to

cause side effects

Protocol layers can be inspected

changed and tested independently

of each other

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SER

Definition:

Portability is the set of attributes that bear on the ability of software to be

transferred from one environment.

[ISO/IEC 9126:1991]

Adaptability

opportunity for its adaptation to different specified environments withoutapplying other actions or means than those provided for this purpose for

the software considered

Installability

effort needed to install the software in a specified environment

Conformanceadherence to standards or conventions relating to portability

Replacability

opportunity and effort using a software in the place of specified other

software in the environment of that software

Subdivision in:


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SER

Example of a portable development: migrating the

embedded web server to another micro-controller platform

HTTP

TCP/IP

ARP

Ethernet

Different micro-controller platform means there is different hardware

environment (CPU, memory resources, Ethernet chip, other peripherals)

But: most of the protocol layers dont access any hardware resources; the

Ethernet device driver constitutes the only exception

Option 1:

Whole protocol stack in a singlemodule

Option 2:

Each protocol layer in its own module

The whole software package is

getting involved

High probability of errors cropping

up while porting to new platform

Only the Ethernet device driver

needs to be touched


TCP

IP

ARPEthernet

HTTP

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Weighting of quality characteristics

quality characteristic compete

in general: accurateness is more important than efficiency

order of precedence depends on the system

Examples of competing quality characteristics

maintainabiliy, portability efficiency

resource consumption fault tolerance

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SER

How to increase the system quality

attach great importance to a complete and careful project documentation

decompose the system into modules, hide information, keep interfaces

smart

structure the system, adapt it to environment and resources

use problem-oriented modeling and programming languages, take care fortestability and readability, minimize fault probability

consider the qualification of your staffing and plan training programs


general measures

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SER

Overall view


Requirements

engineering

System

analysis

System and

software design

Implementation

Test

Requirements

specification

ProblemPhases can be

iterated and

may overlap

Xxx

Legend:

Development phase

Development resultYyy

System

model

Design

specification

Realized

system

Test

documentation

Maintenance,

enhancements

System

ready for use

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SER

Requirements engineering

Finding out and describing the services the system is expected to provideand the constraints under which it must operate

Consultation and negotiation with the system users


Examples of typical requirements:

required functions

description of correct and invalid input data as well as

the required output data in either cases

start-up and shutdown phase

optional system enhancements

documentation-related requirements

performance parameters

quality characteristics

major test scenarios

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SER2.3 Software Development Phases

Examples of constraints to be considered:

user profile

number of users

expertise of users

system environment

hardware performance

existing system software (operating system, common services)

existing software packages (other applications or software libraries)

Requirements engineering result:

Requirements specification

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SER

System and software design

planning of the method applied for solving the problem

description of the parallel processes, of how they have to be synchronized,

their relationships with each other (interfaces and data) and their

relationships with the technical process (external interfaces and events)

description of involved devices and computers

accomplishment of the requirements as defined by the requirements

specification


Design specification

Design result:

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Implementation

Software realization of a program on the basis of the design specification using a

programming language

allocation of tasks and threads for the parallel processes

selection of appropriate control flow operators

selection of appropriate data types for the data, the interfaces and the

events

addition of comments to the program code

compiling, linking, loading and executing the realized programs


Hardware

realization of the hardware required by the design specification

creation or provision of needed devices

Program code and executables, assembled devices

Implementation results:

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Test

Unit test regarding single systems parts

verification against design specification

Integration test

regarding the whole system

verification against design specification

System test

regarding the whole system

validation against requirements

performance benchmarks, optimizations


Test specification and reports

Test results:

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Maintenance and enhancements

Life span of huge systems may last longer than


the underlying hardware is available

the staff having been involved in the development is available

Maintenance tasks:

elimination of errors enhancements due to new requirements

update of underlying hardware system or parts of it

General rule:

10 % of the development costs

= maintenance costs per year

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SERSoftware Engineering for Real-Time Systems

2.1 Terminology



2.4 Summary

2 Fundamentals

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SER

Summary

Software Engineering means to achieve a high software quality whilekeeping software costs at the very minimum

Software Engineering guides to a systematic development by adopting

principles, methods, concepts, notations and tools

Software quality has a great variety of aspects; different quality

characteristics often influence and compete each other

Software quality is not an absolute term but has to be defined a new in

every upcoming software project

There are characteristic assignments and activities to be performed during

a software development which can be summarized in development phases

Each development phase consists of a set of activities to be carried out

and a set of expected results at the end of the phase

2.4 Summary

Documents

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