software engineering cs2301

8/12/2019 software engineering cs2301

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Unit-III

Design Concepts and PrinciplesDesign Concepts:

1. Abstraction:

It concentrate on the essential features and ignore details that are not relevant

Procedural abstraction = a named sequence of instructions that has a specific & limited

function e.g. Open (for the door)

Data abstraction= a named collection of data that describes a data object e.g. Door

Control abstraction= implies a program control mechanisms without specif internal

details

2. Software architecture

It defines overall structure of the software components and the was in which that structure

provides conceptual integrit for a sstem. Patterns

! design pattern defines a design structure that solves a particular design problem within

a specific conte"t and aimed #forces$ that ma have an impact on the manner in which thepattern is applied and used.

%ach design pattern must determine the following

'hether the pattern applicable to the current wor

'hether the pattern can be reused.

'hether the pattern can serve as a guide for developing a similar but

functionalit or structurall different pattern.

!. Infor"ation #iding:

Information (data and procedure) contained within a module is inaccessible to modulesthat have no need for such information

$. %efine"ent:

*rocess of elaboration where the designer provides successivel more detail for each

design component.

&.'odularit(+he software is divided into separatel named and addressable components sometimes

called modularit.

+here are , criteria to evaluate design method with effective modular design

-odular decomposabilit provides sstematic means for breaing problem into

sub problems

-odular composabilit supports reuse of e"isting modules in new sstems

-odular understandabilit module can be understood as a standalone unit

-odular continuit sideeffects due to module changes minimi/ed

-odular protection sideeffects due to processing errors minimi/ed

). *unctional Independence:

+he functional independence is achieved b developing the functional modules with

single minded approach.

!njalai !mmal -ahalingam %ngineering 0ollege

Department of 0omputer 1cience and %ngineering


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Cohesion:

2unctional strength of the module.

! cohesive module perform a single tas. 3ow tightl bound or related 4a

module5s6 internal elements are to one another.

Coincidental: module performs unrelated functions #bad$ +ogical: module performs functions that are related onl logicall

,e"poral: module performs more than one function but the all happen within awelldefined time span.

Procedural: module performs more than one function but the all do functions

that are related

Co""unicational: module performs more than one function but the are all onthe same bod of data.

*unctional :

Coupling:

+he degree of interdependence between modules.

Content coupling: modules can directl refer to the contents of each other.

Co""on coupling: modules communicate via global data.

Control coupling: modules communicate b data that allows one module to

directl affect the behavior of the other.

Sta"p coupling: modules communicate b a heterogeneous set of items not allof which are used.

Data coupling: modules communicate b parameters where each parameter is a

single item or a homogeneous set that incorporate no control element.

o coupling: there is no relationship.

. %efactoring:

7efactoring is a reorgani/ation technique that simplifies the design(or code) of a componentwithout changing its function or behavior.

/. Data structure:

7epresentation of the logical relationship among individual data elements (requires at

least as much attention as algorithm design)

10. Software procedure:

*recise specification of processing (event sequences decision points repetitive

operations data organi/ation8structure)

11. Design Classes:

User interface classes9 define abstractions necessar for 30I.

usiness do"ain classes9 refinements of analsis classes.

Process classes9 lowerlevel business abstractions that manage business domain classes. Persistent classes9 data stores (databases) that persist beond e"ecution of the software

S(ste" classes9 management and control functions that enable the sstem to operateand communicate within its computing environment and with the outside world.

Design Process:

Design *rocess is a sequence of activities to develop the software design product.

Design uidelines:


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! design should e"hibit good architectural structure

! design should be modular

! design should contain distinct representations of data architecture interfaces and

components (modules)

! design should lead to data structures that are appropriate for the objects to be

implemented and be drawn from recogni/able design patterns ! design should lead to components that e"hibit independent functional characteristics

! design should lead to interfaces that reduce the comple"it of connections between

modules and with the e"ternal environment.

! design should be derived using a reputable method that is driven b information

obtained during software requirements analsis.

Design Principles:

+he design process should not suffer from tunnel vision

+he design should be traceable to the analsis model

+he design should not reinvent the wheel

+he design should minimi/e intellectual distance between the software and the problem

as it e"ists in the real world

+he design should e"hibit uniformit and integration

+he design should be structured to accommodate change

+he design should be structured to degrade gentl even with bad data events or

operating conditions are encountered

+he design should be assessed for qualit as it is being created

+he design should be reviewed to minimi/e conceptual (semantic) errors.

Design #euristics for 3ffecti4e 'odularit(:

%valuate the first iteration of the program structure to reduce coupling and improve

cohesion.

!ttempt to minimi/e structures with high fanout: strive for fanin as structure depthincreases.

;eep the scope of effect of a module within the scope of control for that module.

%valuate module interfaces to reduce comple"it reduce redundanc and improve

consistenc.

Define modules whose function is predictable and not overl restrictive (e.g. a module

that onl implements a single sub function).

1trive for controlled entr modules avoid pathological connection (e.g. branches into themiddle of another module)

Design 'odel:


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Data Design 3le"ents:

Data design creates a model of data that is represented at a high level of

abstraction

7efined progressivel to more implementationspecific representation for

processing b the computer base sstem +ranslation of data model into a data base is pivotal to achieving business

objective of a sstem

!t Progra" co"ponent le4el-< data structures and associated algorithms are

designed.

!t application le4el< +ranslation of data model into a data base is pivotal to

achieving business objective of a sstem

!t business le4el-< transformation of database into data warehouse.

Architectural Design 3le"ents:

!rchitectural design is equivalent to the floor plan of the house.

Derived from three sources

() Information about the application domain of the software

(>) !nalsis model such as dataflow diagrams or analsis classes.

(?) !rchitectural pattern and stles

Interface Design ele"ents:

1et of detailed drawings constituting

() @ser interface

(>) %"ternal interfaces to other sstems devices etc

(?) Internal interfaces between various components

Co"ponent le4el design ele"ents:

2ull describe the internal details of each software component

%.g. wiring plumbing within each room

@-A diagram can be used


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'odels for Architectural Design:

#ori7ontal Partitioning:

It defines separate branches of the modular hierarch for each major program function Defines ? partition = Input *rocess Output

enefits:

eas to test software

eas to maintain software

Drawbac8s:

causes more data to be passed

across module interfaces complicate overall

control of program flow

9ertical Partitioning:

control (decision maing) & wor should be distributed +opDown in program structure

top level modules 9 do control function

low level modules 9 do the wors( input computation output tass ) ! change in control module will affect the subordinates.

! change in worer module will less liel effect others.

7epresentation of the logical relationship among individual elements of data.


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enefits:

%as to maintain the changes.

7educe the change impact and error propagation.

Data Design:

+ransforming the analsis information model (data dictionar and %7D) into data

structures required to implement the software.

Data Design at architectural +e4el:

Data Design at Co"ponent +e4el:

. +he sstematic analsis principles applied to function and behavior should also beapplied to data.

>. !ll data structures and the operations to be performed on each should be identified.

?. ! data dictionar should be established and used to define both data and program design.


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B. Aow level data design decisions should be deferred until late in the design process.

,. +he representation of data structure should be nown onl to those modules that must

mae direct use of the data contained within the structure.C. ! librar of useful data structures and the operations that ma be applied to them should

be developed.

. ! software design and programming language should support the specification andreali/ation of abstract data tpes.

Architectural St(les:

%ach stle describes a sstem categor that encompasses() ! set of components (database, clients, servers and filters)

(>) ! set of connectorsthat enables #communication and coordination(call event

broadcasting pipes)(?) Constraintsthat define how components can be integrated to form the sstem

(B Semantic modelsto understand the overall properties of a sstem

Data Centered Architecture:

! data store (e.g. a file or database) resides at the center of this architecture and is

accessed frequentl b other components that update add delete or otherwise modifdata within the store.

It promotes integrabilit.

Data *low Architecture:

1hows the flow of input data its computational components and output data

1tructure is also called pipe and 2ilter

*ipe provides path for flow of data

2ilters manipulate data and wor independent of its neighboring filter


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Call and %eturn Architecture:

!chieves a structure that is eas to modif and scale

+wo sub stles

() -ain program8sub program architecture

0lassic program structure

-ain program invoes a number of components which in turn invoe still othercomponents

(>) 7emote procedure call architecture

0omponents of main program8subprogram are distributed across computers over

networ

;b


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+a(ered Architecture:

! number of different laers are defined

Inner Aaer( interface with O1)

Intermediate Aaer @tilit services and application function)

Outer Aaer (@ser interface)

Architectural Design:

+he architectural design begins with representing the sstem in the conte"t i.e. definingthe e"ternal entities.

Once all e"ternal software has been described the designer specifies the structure of the

sstem b defining and refining software components.

%epresenting the s(ste" in the conte=t:


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*ig: Architectural Conte=t Diagra" for Safe #o"e Securit( S(ste

Defining Archet(pes:


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*ig: Arche,(pe for Safe #o"e Securit( S(ste"

%efining the Architecture into Co"ponents:


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,ransfor" 'apping:

,ransfor" *low:

Data coming from e"ternal forms must be converted into an internal form for processing.

Design Steps for ,ransfor" 'apping:

7eview the fundamental sstem model to identif the information flow.

7eview and refine the data flow diagrams for the software.

Determine if the D2D has the transform or transaction flow characteristics.

Isolate the transform centre b specifing incoming and outgoing flow

boundaries.

*erform first level factoring.

*erform second level factoring.

7efine the firstiteration architecture using design heuristics for improved

software qualit.

,ransaction 'apping:

,ransaction *low:

Information flow is characteri/ed b a single data item called a transaction that triggers

other data flow along one of man paths.

Design Steps for ,ransaction 'apping:

7eview the fundamental sstem model to identif the information flow.

7eview and refine the data flow diagrams for the software.

Determine if the D2D has the transform or transaction flow characteristics.

Identif the transaction centre and flow characteristics along each of the action

paths.

-ap D2D into transaction processing structure


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2actor and refine the transaction structure and structure of each action path.

7efine the firstiteration architecture using design heuristics for improved

software qualit.

User Interface Design Principles:

Define interaction modes in a wa that does not force a user into unnecessar or

undesired actions.

*rovide for fle"ible interaction.

!llow user interaction to be interruptible and undoable.

1treamline interaction as sill levels advance and allow the interaction to be customi/ed.

3ide technical internals from the casual user.

Design for direct interaction with objects that appear on the screen.

'hiledesigning the user interface the capabilit need and e"perience of sstem

user should be considered.

User *a"iliarit(: Instead of using computer terminolog mae use of user oriented

terminologies.

Consistenc(: +he appropriate level of consistenc should be maintained in the user

interface.

'ini"al Surprise:+he commands should operate in a nown wa.

%eco4erabilit(:+he sstem should provide a recovering facilit to user from his errors

so that user can correct those errors.

User uidance: +he user interface can be effectivel used b a novice user if some user

guidance such as help sstems online manuals etc.

%eal ,i"e S(ste"s:

! realtime sstem is a software sstem where the correct functioning of the sstem

depends on the results produced b the sstem and the time at which these results are produced.

Soft%eal ,i"e S(ste":-! Esoft5 realtime sstem is a sstem whose operationis degraded if results are not produced according to the specified timing requirements

#ard %eal ,i"e S(ste" ! Ehard5 realtime sstem is a sstem whose operationis incorrect if results are not produced according to the timing specification.

%eal ,i"e Software Design:

1timulus8response sstemFiven a stimulus the sstem must produce a response within aspecified time. +here are two tpes

Periodic sti"uli 1timuli which occur at predictable time intervals

G 2or e"ample a temperature sensor ma be polled H times per second

Aperiodic sti"uli:1timuli which occur at unpredictable timesG 2or e"ample a sstem power failure ma trigger an interrupt which must be

processed b the sstem


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%eal ,i"e S(ste" Design:

Design both the hardware and the software associated with sstem. *artition functions toeither hardware or software

Design decisions should be made on the basis on nonfunctional sstem requirements

3ardware delivers better performance but potentiall longer development and less scope

for 0hange %eal ,i"e S(ste" Design Process:

. Identif the stimuli to be processed and the required responses to these stimuli

>. 2or each stimulus and response identif the timing constraints?. !ggregate the stimulus and response processing into concurrent processes. !

process ma be associated with each class of stimulus and response

B. Design algorithms to process each class of stimulus and response. +hese mustmeet the given timing requirements

,. Design a scheduling sstem which will ensure that processes are started in time to

meet their deadlines

C. Integrate using a realtime e"ecutive or operating 1stem

,i"ing constraints: -a require e"tensive simulation and e"periment to ensure that these are met b the

sstem

-a mean that certain design strategies such as objectoriented design cannot be used

because of the additional overhead involved

-a mean that lowlevel programming language features have to be used forperformance reasons

%eal ,i"e S(ste" 'odeling:

7eal +ime sstems have to respond to events occurring at irregular intervals.

+he effect of a stimulus in a realtime sstem ma trigger a transition from one state

to another.

2inite state machines can be used for modeling realtime sstems.

+he @-A includes notations for defining state machine models


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Full power

Enabled

do: operateoven

Fullpower

Halfpower

Halfpower

Fullpower

Number

TimerDooropen

Doorclosed

Doorclosed

Systemfault

Start

do: set power = 600

Half power

do: set power = 300

Set time

do: get numberexit: set time

Disabled

Operation

Timer

Cancel

Waiting

do: display time

Waiting

do: display time

do: display 'Ready'

do: display 'Waiting'

%eal ti"e Progra""ing:

3ardreal time sstems ma have to programmed in assembl language to ensure that

deadlines are met

Aanguages such as 0 allow efficient programs to be written but do not have constructs tosupport concurrenc or shared resource management

!da as a language designed to support realtime sstems design so includes a general

purpose concurrenc mechanism

>a4a as a %eal ti"e s(ste":

ava supports lightweight concurrenc (threads and snchroni/ed methods) and can beused for some soft realtime sstems

ava >.H is not suitable for hard 7+ programming or programming where precise control

of timing is required. +he reasons are

G Jot possible to specif thread e"ecution timeG @ncontrollable garbage collection

G Jot possible to discover queue si/es for shared resources

G Kariable virtual machine implementationG Jot possible to do space or timing analsis

%eal ,i"e 3=ecuti4es:

7ealtime e"ecutives are speciali/ed operating sstems which manage the processes in

the 7+1 7esponsible for process management and resource (processor and memor) allocation

-a be based on a standard 7+% ernel which is used unchanged or modified for a

particular application

Does not include facilities such as file -anagement

,he e=ecuti4e co"ponents are:

%eal-ti"e cloc8 *rovides information for process scheduling.


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Interrupt handler -anages aperiodic requests for service.

Scheduler 0hooses the ne"t process to be run.

%esource "anager !llocates memor and processor resources.

Despatcher 1tarts process e"ecution.

Configuration "anager-7esponsible for the dnamic reconfiguration of thesstem software and hardware. 3ardware modules ma be replaced and software

upgraded without stopping the sstems

*ault "anager 7esponsible for detecting software and hardware faults and

taing appropriate actions (e.g. switching to bacup diss) to ensure that the

sstem continues in operation

+he processing of some tpes of stimuli must sometimes tae priorit

Interrupt le4el priorit(-3ighest priorit which is allocated to processes

requiring a ver fast response

Cloc8 le4el priorit(-!llocated to periodic processes

Process resourcerequirements

Scheduler

Schedulinginformation

Resourcemanager

Despatcher

Real-timeclock

Processesawaitingresources

Readylist

Interrupthandler

Availableresource

list

Processorlist

Executingprocess

Readyprocesses

Releasedresources

Process 'anage"ent:

Resource manager

Allocatememoryand processor

Scheduler

Choose processfor execution

Despatcher

Start executionon anavailableprocessor

0oncerned with managing the set of concurrent processes


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*eriodic processes are e"ecuted at prespecified time intervals

+he e"ecutive uses the realtime cloc to determine when to e"ecute a process

*rocess period time between e"ecutions

*rocess deadline the time b which processing must be complete

+he scheduler chooses the ne"t process to be e"ecuted b the processor. +his depends on

a scheduling strateg which ma tae the process priorit into account +he resource manager allocates memor and a processor for the process to be e"ecuted

+he despatcher taes the process from read list loads it onto a processor and startse"ecution

on pre-e"pti4e scheduling

G Once a process has been scheduled for e"ecution it runs to completion or until it

is bloced for some reason (e.g. waiting for I8O)

Pre-e"pti4e scheduling

G +he e"ecution of e"ecuting processes ma be stopped if a higher priorit process

requires service

Scheduling algorith"s

G 7oundrobinG 7ate monotonicG 1hortest deadline first

Data Ac?uisition S(ste":

0ollect data from sensors for subsequent processing and analsis.

Data collection processes and processing processes ma have different periods and

deadlines.

Data collection ma be faster than processing e.g.collecting information about an

e"plosion.

0ircular or ring buffers are a mechanism for smoothing speed differences.

%eactor Data Collector:

DisplayProcessdata

Sensor databuffer

Sensorprocess

Sensoridentifier and

value

Processedflux level

Sensors (each data flowis a sensor value)

Consumerprocess

Producerprocess


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! sstem collects data from a set of sensors monitoring the neutron flu" from a nuclear

reactor.

2lu" data is placed in a ring buffer for later processing.

+he ring buffer is itself implemented as a concurrent process so that the collection and

processing processes ma be snchroni/ed.

'utual 3=clusion:

*roducer processes collect data and add it to the buffer. 0onsumer processes tae data

from the buffer and mae elements available

*roducer and consumer processes must be mutuall e"cluded from accessing the same

element.

+he buffer must stop producer processes adding information to a full buffer andconsumer processes tring to tae information from an empt buffer.

Software Configuration 'anage"ent @SC':

10- identifies organi/es and controls modifications to the software being built b asoftware development team throughout the software process.

eed for SC': +he software configuration management is concerned with managing the changes in the

evolving software. If the changes are not controlled at all then this stream of uncontrolledchange can cause the wellrunning software project into chaos. 3ence it is essential to

identif change control the changes ensure that the changes are properl implemented

and report these changes to others. !nd to carr out these set of activities the softwareconfiguration is essential.

3le"ents of Configuration 'anage"ent S(ste":

Co"ponent ele"ents :-! set of tools coupled with a file management (e.g. database)sstem that enables access to and management of each software configuration item

Process ele"ents:-! collection of procedures and tass that define an effective

approach to change management for all participants

Construction ele"ents:-! set of tools that automate the construction of software b

ensuring that the proper set of valid components (i.e. the correct version) is assembled

#u"an ele"ents:- ! set of tools and process features used b a software team toimplement effective 10-

aselines:

I333 Definition:! specification or product that has been formall reviewed and agreedupon and that thereafter serves as the basis for further development and that can be

changed onl through formal change control procedures

It is a milestone in the development of software and is mared b the deliver of one or

more computer software configuration items (010Is) that have been approved as aconsequence of a formal technical review

aseline Processes:

! series of software engineering tass produces a 010I

+he 010I is reviewed and possibl approved

+he approved 010I is given a new version number and placed in a project database (i.e.

software repositor)


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+he 10- repositor is the set of mechanisms and data structures that allow a software

team to manage change in an effective manner

+he repositor performs or precipitates the following functions

Dataintegrit(=>Kalidates entries ensures consistenc cascades modifications

Information sharing=

Methodology enforcement=Defines objects in the repositor to guarantee a

standard approach for creation of software engineering documents

SC' *eatures:

9ersioning.

o saves all of these versions to enable effective management of product releases and

to permit developers to go bac to previous versions

Dependenc( trac8ing and change "anage"ent.

o +he repositor manages a wide variet of relationships among the data elements

stored in it.

%e?uire"ents tracing.

o *rovides the abilit to trac all the design and construction components and

deliverables that result from a specific requirement specification

Configuration "anage"ent.

o ;eeps trac of a series of configurations representing specific project milestones

or production releases. Kersion management provides the needed versions and

lin management eeps trac of interdependencies.

Audit trails.


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o %stablishes additional information about when wh and b whom changes are

made.

SC' Process:

+he 10- process defines a series of tass that have four primar objectives. +he are

Identif all items that collectivel define the software configuration

-anage changes to one or more of these items

2acilitate construction of different versions of an application

%nsure the software qualit is maintained as the configuration evolves over time

*rovide information on changes that have occurred

Identification of ;b


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! version control sstem implements or is directl integrated with four major

capabilities

aproject database (repository)that stores all relevant configuration objects

a version managementcapabilit that stores all versions of a configuration object (or

enables an version to be constructed using differences from past versions):

a make facility that enables the software engineer to collect all relevant configuration

objects and construct a specific version of the software.

an issues tracking(also called bug tracking) capabilit that enables the team to record

and trac the status of all outstanding issues associated with each configuration object.

+he 10- repositor maintains a change set

1erves as a collection of all changes made to a baseline configuration

@sed to create a specific version of the software

0aptures all changes to all files in the configuration along with the reason for changes

and details of who made the changes

Change Control:

0hange control is a procedural activit that ensures qualit and consistenc as changes

are made to a configuration object

! change request is submitted to a configuration control authorit which is usuall a

change control board (00M)

+he request is evaluated for technical merit potential side effects overall impact on other

configuration objects and sstem functions and projected cost in terms of mone time

and resources

!n engineering change order (%0O) is issued for each approved change request

Describes the change to be made the constraints to follow and the criteria for review and

audit

+he baselined 010I is obtained from the 10- repositor


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!ccess control governs which software engineers have the authorit to access and modif

a particular configuration object

1nchroni/ation control helps to ensure that parallel changes performed b two different

people donNt overwrite one another.


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Distribute the new 4ersion

Configuration Audit: 0onfiguration auditing is an 1! activit that helps to ensure that qualit is maintained

as changes are made

It complements the formal technical review and is conducted b the 1! group

It addresses the following questions

o 3as the change specified in the %0O been madeP 3ave an additional

modifications been incorporatedP

o 3as a formal technical review been conducted to assess technical correctnessP

o 3as the software process been followed and have software engineering standards

been properl appliedP

o 3as the change been QhighlightedQ and QdocumentedQ in the 010IP 3ave the

change data and change author been specifiedP Do the attributes of the

configuration object reflect the changeP

o 3ave 10- procedures for noting the change recording it and reporting it been

followedP

o 3ave all related 010Is been properl updatedP

! configuration audit ensures that the correct 010Is (b version) have been incorporated

into a specific build that all documentation is uptodate and consistent with the version that has

been built

Status %eporting:


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0onfiguration status reporting (017) is also called status accounting.

*rovides information about each change to those personnel in an organi/ation with a need

to now

!nswers what happened who did it when did it happen and what else will be affectedP

1ources of entries for configuration status reporting. %ach time a 010I is assigned new or

updated information

%ach time a change is approved b the 00M and an %0O is issued. %ach time a

configuration audit is conducted

+he configuration status report. *laced in an online database or on a website for software

developers and maintainers to read

Fiven to management and practitioners to eep them appraised of important changes to

the project 010Is

software engineering cs2301

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