Topic (1)Software Engineering (601321)1 Introduction Complex and large SW. SW crises. 1950 1960 1970...

Topic (1) Software Engineering (601321) 1

Introduction

• Complex and large SW.• SW crises.

1950 1960 1970 1980 1990 2000

•Expensive HW.•Custom SW.•Batch execution

•Structured programming• Product SW.

•Data orientation•Expert systems•AI•distributed systems•parallel programming


Introduction

• Software Engineering is concerned with theories, methods and tools needed to develop high quality software products in a cost effective way.

• Cost effective: – limited resources.– Time schedule.


Software Engineering

• 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

• Computer science is concerned with theory and fundamentals; software engineering is concerned with the practicalities of developing and delivering useful software


Software products

• A Software: computer programs and associated documentation and configuration files

• Software products may be: Generic products

developed to be sold to a range of different customers (on the open market to any customer).

Customised products (Bespoke)

developed for a specific customer according to their specification.


The software development process

• Structured set of activities required to develop or evolve a software product

• Generic activities in all software processes are:– Requirement specification - what the system should do

and its development constraints– Design & Development – design and production of the

software system– Validation - checking that the software is what the

customer wants– Evolution - changing the software in response to

changing demands


Software Process Model• A simplified representation of a software process,

presented from a specific perspective• Generic process models:

– The waterfall model• Separate and distinct phases of specification and development

– Evolutionary development• Specification and development are interleaved

– Formal system development• A mathematical system model is formally transformed to an

implementation

– Reuse-oriented development• The system is assembled from existing components


Waterfall modelRequirements

definition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance


Evolutionary development

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities


Evolutionary model

• Exploratory prototyping – Objective is to work with customers and to evolve a

final system from an initial outline specification. Should start with well-understood requirements

• Throw-away prototyping– Objective is to understand the system requirements.

Should start with poorly understood requirements


Formal System Development

• Requirement definition

• Formal specification

• Formal transformation

executable program

• Integration and system testing


Reuse-oriented development

• Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.

• Process stages– Requirement specification– Component analysis;– Requirements modification;– System design with reuse;– Development and integration.– System validation


Spiral model

Riskanalysis

Riskanalysis

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2Prototype 3

Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

CodeUnit test

IntegrationtestAcceptance

testService Develop, verifynext-level product

Evaluate alternativesidentify, resolve risks

Determine objectivesalternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW

(1) (2)

(3)(4)


Sectors of the spiral model• Objective setting

– Specific objectives and constraints of the phase.

• Risk assessment and reduction– Risks are analyzed and information is sought to reduce

these risks

• Development and validation– An appropriate model is chosen and used for the

development of this phase

• Planning: The project is reviewed and plans drawn up for the next round of the spiral


Template for a spiral round

• Objectives• Constraints• Alternatives• Risks• Risk resolution• Results• Plans• Commitment


Software process-characteristics

• Visibility– Is the process progress externally visible?

• Supportability– Can the process be supported by CASE tools?

• Reliability– Are process errors discovered before they result in

product errors?

• Robustness– Can the process continue in spite of unexpected

problems?


Process visibility

• Software systems are intangible so managers need documents to assess progress

• However, this may cause problems– The time taken to review and approve documents is

significant– The need to produce documents constraints process

iteration


Computer-aided software Engineering

• Software tools that support the software development process

• CASE technology has resulted in significant improvements in quality and productivity

• But, they should be carefully chosen and staff should be properly trained.


CASE classification• CASE systems can be classified according to their

– Functionality - what functions do they provide– Process support - what software process activities do they

support– The breadth of support which they provide

• Classification allows tools to be assessed and compared

• Tools unevenly support phases of the software cycle. (e.g. design and implementation are well supported, but not maintenance and specification)


CASE classification• Classification according to breadth:

– Toolssupport single tasks (e.g. documentation, compilation)

– Workbenchessupport activities in a process phase (e.g. design,

specification)

– Environmentsupport all or most of the software development process.


Software quality/ attributes

TestabilityMaintainability

PortabilityInteroperabilityReusability

Efficiency SecurityUsability Reliability

operational

Productrevision

Producttransition



• Reliability :The extent to which the program can be expected to perform its intended function with required precision under all conditions.

• Efficiency : The amount of computing resources required by the program.

• Security : The extent to which access to data and software by unauthorized persons can be controlled.



• Usability : The effort required to learn and operate the software. (user interface, documentation).

• Maintainability : The effort required to meet changes in user needs.

• Testability : The effort required to test a program to ensure that it performs its intended function.



• Portability : The effort required to translate the program from one HW and/or SW environment to another.

• Reusability : The extent to which a program or part of it can be reused in other applications.

• Interoperability : The effort required to couple one system to another.


System Engineering

• Designing, implementing and installing systems which include hardware, software and people

• System: a collection of interrelated components working together towards some common objective

• A system may include software, mechanical, electrical and electronic hardware and be operated by people

• Large systems are used to solve ‘wicked problems’


System Engineering

• The behaviour of system components depend on other components

• System properties can be ‘emergent’ (e.g. reliability)

• Emergent properties are a consequence of the relationships between system components

• System and its environment


System failuresSystem failures often occur because of unforeseen inter-

relationships between components• Hardware reliability

• What is the probability of a hardware component failing and how long does it take to repair that component?

• Software reliability • How likely is it that a software component will produce an incorrect output.

Software failure is usually distinct from hardware failure in that software does not wear out.

• Operator reliability • How likely is it that the operator of a system will make an error?

Topic (1)Software Engineering (601321)1 Introduction Complex and large SW. SW crises. 1950 1960 1970...

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