Software Enggniring

8/8/2019 Software Enggniring

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SOFTWARE ENGGNIRING

Software Engineering is the technological and managerial disciplineconcerned with systematic production and maintenance of software

products that are developed and modified on time and within cost

estimates.

SOFTWARE = Programs + Documentation required to develop, operate andmaintain programs.

Documentation is an essential component of software because it is notpossible to understand different aspects of large programs without

documentation.

WHAT IS A SOFTWARE PROCESS? A setofactivitieswhose goalisthe developmentorevolutionofsoftware. Genericactivitiesinallsoftwareprocessesare:

Specification - whatthesystemshould doand its developmentconstraints

Development - productionofthesoftwaresystem Validation - checking thatthesoftwareiswhatthecustomerwants Evolution - changing thesoftwareinresponsetochanging demands.

The identification of the software development as an engineering disciplineled to the need of a Software Development Model or Software Life

Cycle. The detailed model is still the topic of research but it is now clear

that a number of general models or paradigms of software development

can be identified.

SOFTWARE PROCESS MODELS Waterfall model Evolutionary development Component-based software engineering Incremental model Spiral model


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SOFTWARE PROCESS ACTIVITIES Specification Design and implementation Validation Evolution

THE RATIONAL UNIFIED PROCESS COMPUTER-AIDED SOFTWARE ENGINEERING

SOFTWARE PROCESS MODELS

Softwareprocess = organized setofactivitiesaimed at building asoftwaresystem

Softwareprocessmodel = anabstractrepresentationofasoftwareprocess

Fundamentalsoftwareprocessactivities: Softwarespecification Software design Softwareimplementation Softwarevalidation Softwareevolution

SOFTWARE PROCESS MODELS

1. THE WATER-FALL APPROACH:This approach views the software lifecycle as made up of a number of stages such as: requirements specification,

software design, implementation and so on. After the completion of one

stage the development proceeds to the next stage.

2. EXPLORATORYPROGRAMMING:This approach involves developinga working system as quickly as possible and then modifying that system

until it works in an adequate way. This is used in the de velopment of AI

systems where the users can not formulate a detailed requirement


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specification and where adequacy rather than correctness in the aim of

designers.

3. PROTOTYPING:This approach is similar to exploratory programming inthat first phase involves the quick development of a program for users

experiment. But here the objective is to establish the system requirements.This is developed from an outline specification. The users experiment with

the prototype to refine and improve the specification. This is followed by

the re-implementation of the software to produce a production quantity

system.

4. FORMAL TRANSFORMATION:This approach involves developing aformal specification of the system and transforming this specification to a

program using correctness - preserving transformations. Each

transformation is sufficiently close to the previous description that effort of

verifying the transformation is not excessive. It can then be guaranteed

that the developed program is the true implementation of the sp ecification.

5. SYSTEM ASSEMBLY FROM RE -USABLE COMPONENTS:Thisapproach involves developing software system from the existing

components. Thus the system development process becomes one of the

assembly rather than creation.

The first three of these approaches are all being used for practical

system development. Some systems have been built using formaltransformation but this is still an untried approach. The re-use-

oriented model is still not commercially viable because of the lack

of reusable component libraries.

1. THEWATERFALLMODELThis model is illustrated in the figure below. Because of the cascade from one

stage to other this model is called waterfall model. This is also called Classic

Life-Cycle. This model is the oldestand widely used one.

REQUIREMENTS ANALYSIS AND DEFINITION:The systems services,

constraints and goals are established by consultation with the system users.

They are then defined in a manner which is understandable by the users and

the development staff.


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SYSTEM AND SOFTWARE DESIGN: The system design process

partitions the requirements either to hardware or to software systems and

establishes an overall systems structure. The software design involves

representing the software system functions in such a way that may be

transformed to one or more executable programs.

IMPLEMENTATION AND UNI T TESTING:In this stage the software

design is realised as a set of working programs or program units. The unit -

testing means verifying each unit that meets its specifications and it is ver ified

that the defined input produces the desired results.

INTEGRATION AND SYSTEM TESTING :The individual program units or

programs are integrated and tested as a whole system to ensure that software

requirements are met. After testing the system is delivered to the customer.

OPERATION &MAINTENANCE :Normally (not necessarily) this is the

longest phase of the life cycle. The system is installed and put into practical

use. Maintenance involves correcting errors, which were not detected in

earlier stages of the life cycle, improving the implementation of the system

units and enhancing the systems services as new requirements are discovered.

ADVANTAGES: Organized approach, provides robust separation of phases

Requirementsdefinition

System and

software design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance


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Reflects common engineering practice DISADVANTAGES:

Doesnt cope well with changes required by the client Development teams might wait for each other A working version of the product is available only late

APPLICABILITY: When requirements are well known and few changes are likely to be

needed

Can be used also for parts of larger software systems2. EVOLUTIONARY DEVELOPMENTu

This is based on the idea of developing an initial implementation, exposing it to

the user and refining this through many stages until an adequate system has

been developed

y This method requires rapid development of the system & rapidincorporation of the modifications. To accomplish thus:

y A very high-level programming language such as LISP or PROLOG isrequired.

y Powerful dedicated hardware is required.y Integrated set of tools for the development of software is required.y Its main difference from the other approaches is in the verification &

validation:

y Without a specification verification is impossible because for this purposethe program is composed of its specifications.

y The notion of correctness is replaced by the notion of adequacy.y It has been mostly used to develop AI systems & has been little used in the

development of large, long life systems because :

y It is not economical to produce a great deal of documentation as thesystem goes on changing regularly.

y This results in a system whose structure is not well defined and if there isany it gets deteriorated.


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MAIN CHARACTERISTICS

The phases of the software construction are interleaved Feedback from the user is used throughout the entire process The software product is refined through many versions Types of evolutionary development: Exploratory development Throw-away prototyping Advantages:

Deals constantly with changes Provides quickly an initial version of the system Involves all development teams

Disadvantages: Quick fixes may be involved Invisible process, not well-supported by documentation The systems structure can be corrupted by continuous change Special tools and techniques may be necessary The client may have the impression the first version is very close to the

final product and thus be less patient

Applicability:

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities


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When requirements are not well understood When the client and the developer agree on a rapid prototype that will be

thrown away

Good for small and medium-sized software systems3. COMPONENT-BASED

SOFTWARE ENGINEERINGu

MAIN CHARACTERISTICS:

Makes intensive use of existing reusable components The focus is on integrating the components rather than on creating them

from the scratch

ADVANTAGES:y Reduces considerably the software to be developed in-housey Allows faster deliveryy In principle, more reliable systems, due to using previously tested

components

DISADVANTAGES:y Compromises in requirements are neededy Less control over the systems evolution APPLICABILITY:y When there is a pool of existing components that could satisfy the

requirements of the new product

y Emerging trend: integration of web services from a range of suppliers

Requirementsspecification

Componentanalysis

Development

and integration

System design

with reuse

Requirementsmodification

Systemvalidation


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4. INCREMENTALDEVELOPMENTu

MAIN CHARACTERISTICS :

y Hybrid model that combines elements of the waterfall and evolutionaryparadigms

y The specification, design, and implementation phases are broken in smallerincrements

Advantages:y Provides better support for process iterationy Reduces rework in the software construction processy Some decisions on requirements may be delayedy Allows early delivery of parts of the systemy Supports easier integration of sub-systemsy Lower risk of project failurey Delivery priorities can be more easily setDISADVANTAGES:

y Increments need be relatively smally Mapping requirements to increments may not be easyy Common software facilities may be difficult to identify

Applicability:

Validateincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Validatesystem

Define outlinerequirements

Assign requirementsto increments

System incomplete

Finalsystem


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y When it is possible to deliver the system part-by-partBOEHMS SPIRAL MODEL

Main characteristics:y Also a hybrid model that support process iterationy The process is represented as a spiral, each loop in the spiral representing a

process phase

y Four sectors per loop: objective setting, risk assessment and reduction,development and validation, planning

y Risk is explicitly taken into consideration

Advantages:y Risk reduction mechanisms are in placey Supports iteration and reflects real-world practicesy Systematic approach Disadvantages:y Requires expertise in risk evaluation and reduction

Risk

analysis

Risk

analysis

Risk

analysis

Riskanalysis Proto-

type 1

Prototype 2

Prototype 3Opera-

tionalprotoype

Concept of

Operation

Simulations, models, benchmarks

S/Wrequirements

Requirement

validation

DesignV&V

Productdesign Detailed

design

Code

Unit test

Integrationtest

AcceptancetestService Develop, verify

next-level product

Evaluate alternatives

identify, resolve risks

Determine objectivesalternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW


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y Complex, relatively difficult to follow strictlyy Applicable only to large systems Applicability:y Internal development of large systems

PROCESS ACTIVITIES1.SPECIFICATION

2.DESIGN& IMPLEMENTATION

3. TESTING..

The debugging process

The testing process

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and system

requirements

Requirementsdocument

hitecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

ystemhitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Designactivities

Designproducts

Locate

error

Design

error repair

Repair

error

Re-test

program


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Testing phases

System evolution

THE RATIONAL UNIFIED PROCESSRUP PHASES

Assess existing

systems

Define system

requirements

Propose system

changes

Modify

systems

New

system

Existing

systems


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CLASSIFICATION OF CASE TECHNOLOGY


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Single-methodworkbenches

General-purposeworkbenches

Multi-methodworkbenches

Language-specificworkbenches

Programming TestingAnalysis and

design

Integratedenvironments

Process-centredenvironments

Filecomparators

CompilersEditors

EnvironmentsWorkbenchesTools

CASEtechnology

Software Enggniring

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