Ch17 CBSE

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Chapter 17 Component-based software engineering

Lecture 1

1Chapter 17 Software reuse

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Topics covered

Components and component models

CBSE processes

Component composition


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Component-based development

Component-based software engineering (CBSE) is anapproach to software development that relies on thereuse of entities called ‘software components’.

t emerged from the failure of ob!ect-oriented

development to support effective reuse. Single ob!ectclasses are too detailed and specific.

Components are more abstract than ob!ect classes andcan be considered to be stand-alone service providers."he# can e$ist as stand-alone entities.


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CBSE essentials

ndependent components specified b# their interfaces.

Component standards to facilitate component

integration.

%iddleware that provides support for component inter-operabilit#.

& development process that is geared to reuse.


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CBSE and design principles

&part from the benefits of reuse' CBSE is based onsound software engineering design principles Components are independent so do not interfere with each

other

Component implementations are hidden Communication is through well-defined interfaces Component platforms are shared and reduce development

costs.


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Component standards

Standards need to be established so that components

can communicate with each other and inter-operate.

*nfortunatel#' several competing component standards

were established

Sun’s Enterprise +ava Beans

%icrosoft’s C,% and .E"

C,B&’s CC%

n practice' these multiple standards have hindered the

upta/e of CBSE. t is impossible for componentsdeveloped using different approaches to wor/ together.


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CBSE problems

Component trustworthiness - how can a component with

no available source code be trusted0

Component certification - who will certif# the 1ualit# of

components0

Emergent propert# prediction - how can the emergent

properties of component compositions be predicted0

e1uirements trade-offs - how do we do trade-off

anal#sis between the features of one component and

another0


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Components

Components provide a service without regard to wherethe component is e$ecuting or its programminglanguage & component is an independent e$ecutable entit# that can be

made up of one or more e$ecutable ob!ects "he component interface is published and all interactions are

through the published interface


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Component definitions

Councill and 2einmann

A software component is a software element that conforms to a

component model and can be independently deployed and

composed without modification according to a composition

standard. S3#pers/i

A software component is a unit of composition with contractually

specified interfaces and explicit context dependencies only. A

software component can be deployed independently and is

subject to composition by third-parties.


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Component characteristics

Componentcharacteristic

Description

Standardi3ed Component standardi3ation means that a component used in a CBSEprocess has to conform to a standard component model. "his modelma# define component interfaces' component metadata'documentation' composition' and deplo#ment.

ndependent & component should be independent4it should be possible tocompose and deplo# it without having to use other specificcomponents. n situations where the component needs e$ternall#provided services' these should be e$plicitl# set out in a ‘re1uires’

interface specification.

Composable 5or a component to be composable' all e$ternal interactions mustta/e place through publicl# defined interfaces. n addition' it mustprovide e$ternal access to information about itself' such as itsmethods and attributes.


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Component characteristics

Componentcharacteristic

Description

6eplo#able "o be deplo#able' a component has to be self-contained. t must be

able to operate as a stand-alone entit# on a component platform thatprovides an implementation of the component model. "his usuall#means that the component is binar# and does not have to becompiled before it is deplo#ed. f a component is implemented as aservice' it does not have to be deplo#ed b# a user of a component.ather' it is deplo#ed b# the service provider.

6ocumented Components have to be full# documented so that potential users candecide whether or not the components meet their needs. "he s#nta$and' ideall#' the semantics of all component interfaces should bespecified.


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Component as a service provider

"he component is an independent' e$ecutable entit#. t

does not have to be compiled before it is used with other

components.

"he services offered b# a component are made available

through an interface and all component interactions ta/e

place through that interface.

"he component interface is e$pressed in terms of

parameteri3ed operations and its internal state is never

e$posed.


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Component interfaces

7rovides interface

6efines the services that are provided b# the component to other

components.

"his interface' essentiall#' is the component &7. t defines the

methods that can be called b# a user of the component. e1uires interface

6efines the services that specifies what services must be made

available for the component to e$ecute as specified.

"his does not compromise the independence or deplo#abilit# of

a component because the ‘re1uires’ interface does not define

how these services should be provided.


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Component interfaces

Note UML notation. a!! an" so#$ets #an %t to&ether.


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model of a data collector component


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Component models

& component model is a definition of standards for

component implementation' documentation and

deplo#ment.

E$amples of component models

E+B model (Enterprise +ava Beans)

C,%8 model (.E" model)

Corba Component %odel

"he component model specifies how interfaces should

be defined and the elements that should be included inan interface definition.


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Basic elements of a component model


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Elements of a component model

nterfaces

Components are defined b# specif#ing their interfaces. "he

component model specifies how the interfaces should be defined

and the elements' such as operation names' parameters and

e$ceptions' which should be included in the interface definition. *sage

n order for components to be distributed and accessed

remotel#' the# need to have a uni1ue name or handle associated

with them. "his has to be globall# uni1ue.

6eplo#ment

"he component model includes a specification of how

components should be pac/aged for deplo#ment as

independent' e$ecutable entities.


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!iddleware support

Component models are the basis for middleware thatprovides support for e$ecuting components.

Component model implementations provide 7latform services that allow components written according to the

model to communicate Support services that are application-independent services used

b# different components.

"o use services provided b# a model' components aredeplo#ed in a container. "his is a set of interfaces used

to access the service implementations.


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!iddleware services defined in a component

model


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CBSE processes

CBSE processes are software processes that support

component-based software engineering.

"he# ta/e into account the possibilities of reuse and the different

process activities involved in developing and using reusable

components. 6evelopment for reuse

"his process is concerned with developing components or

services that will be reused in other applications. t usuall#

involves generali3ing e$isting components.

6evelopment with reuse

"his process is the process of developing new applications using

e$isting components and services.


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CBSE processes


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Supporting processes

Component ac1uisition is the process of ac1uiring

components for reuse or development into a reusable

component.

t ma# involve accessing locall#- developed components or

services or finding these components from an e$ternal source.

Component management is concerned with managing a

compan#’s reusable components' ensuring that the# are

properl# catalogued' stored and made available for

reuse. Component certification is the process of chec/ing a

component and certif#ing that it meets its specification.

Chapter 17 Software reuse 23

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"e# points

CBSE is a reuse-based approach to defining andimplementing loosel# coupled components into s#stems.

& component is a software unit whose functionalit# anddependencies are completel# defined b# its interfaces.

& component model defines a set of standards thatcomponent providers and composers should follow.

"he /e# CBSE processes are CBSE for reuse and CBSEwith reuse.


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Chapter 17 Component-based software engineering

Lecture $


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CBSE for reuse

CBSE for reuse focuses on component development.

Components developed for a specific application usuall#

have to be generalised to ma/e them reusable.

& component is most li/el# to be reusable if it associatedwith a stable domain abstraction (business ob!ect).

5or e$ample' in a hospital stable domain abstractions

are associated with the fundamental purpose - nurses'

patients' treatments' etc.


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Component development for reuse

Components for reuse ma# be speciall# constructed b#generalising e$isting components.

Component reusabilit#

Should reflect stable domain abstractions

Should hide state representation Should be as independent as possible Should publish e$ceptions through the component interface.

"here is a trade-off between reusabilit# and usabilit#

"he more general the interface' the greater the reusabilit# but

it is then more comple$ and hence less usable.


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Changes for reusabilit#

emove application-specific methods.

Change names to ma/e them general.

&dd methods to broaden coverage.

%a/e e$ception handling consistent.

&dd a configuration interface for component adaptation.

ntegrate re1uired components to reduce dependencies.


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E%ception handling

Components should not handle e$ceptions themselves'

because each application will have its own re1uirements

for e$ception handling.

ather' the component should define what e$ceptions can arise

and should publish these as part of the interface.

n practice' however' there are two problems with this

7ublishing all e$ceptions leads to bloated interfaces that are

harder to understand. "his ma# put off potential users of the

component.

"he operation of the component ma# depend on local e$ception

handling' and changing this ma# have serious implications for

the functionalit# of the component.


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Legac# s#stem components

E$isting legac# s#stems that fulfil a useful business

function can be re-pac/aged as components for reuse.

"his involves writing a wrapper component that

implements provides and re1uires interfaces then

accesses the legac# s#stem.

&lthough costl#' this can be much less e$pensive than

rewriting the legac# s#stem.


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&eusable components

"he development cost of reusable components ma# be

higher than the cost of specific e1uivalents. "his e$tra

reusabilit# enhancement cost should be an organi3ation

rather than a pro!ect cost.

9eneric components ma# be less space-efficient and

ma# have longer e$ecution times than their specific

e1uivalents.


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Component management

Component management involves deciding how to

classif# the component so that it can be discovered'

ma/ing the component available either in a repositor# or

as a service' maintaining information about the use of

the component and /eeping trac/ of different componentversions.

& compan# with a reuse program ma# carr# out some

form of component certification before the component is

made available for reuse. Certification means that someone apart from the developer

chec/s the 1ualit# of the component.


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CBSE with reuse

CBSE with reuse process has to find and integratereusable components.

:hen reusing components' it is essential to ma/e trade-offs between ideal re1uirements and the services

actuall# provided b# available components. "his involves

6eveloping outline re1uirements Searching for components then modif#ing re1uirements

according to available functionalit#.

Searching again to find if there are better components that meetthe revised re1uirements.

Composing components to create the s#stem.


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CBSE with reuse


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The component identification process


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Component identification issues

"rust. ;ou need to be able to trust the supplier of acomponent. &t best' an untrusted component ma# notoperate as advertised at worst' it can breach #oursecurit#.

e1uirements. 6ifferent groups of components willsatisf# different re1uirements.

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Component validation

Component validation involves developing a set of test

cases for a component (or' possibl#' e$tending test

cases supplied with that component) and developing a

test harness to run component tests.

"he ma!or problem with component validation is that thecomponent specification ma# not be sufficientl# detailed to allow

#ou to develop a complete set of component tests.

&s well as testing that a component for reuse does what

#ou re1uire' #ou ma# also have to chec/ that thecomponent does not include an# malicious code or

functionalit# that #ou don’t need.


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riane launcher failure ' validation failure(

n =>>?' the =st test flight of the &riane @ roc/et ended in

disaster when the launcher went out of control A

seconds after ta/e off.

"he problem was due to a reused component from a

previous version of the launcher (the nertial avigationS#stem) that failed because assumptions made when

that component was developed did not hold for &riane @.

"he functionalit# that failed in this component was not

re1uired in &riane @.


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Component composition

"he process of assembling components to create a

s#stem.

Composition involves integrating components with each

other and with the component infrastructure.

ormall# #ou have to write ‘glue code’ to integrate

components.


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T#pes of composition

Se1uential composition where the composedcomponents are e$ecuted in se1uence. "his involvescomposing the provides interfaces of each component.

2ierarchical composition where one component calls on

the services of another. "he provides interface of onecomponent is composed with the re1uires interface ofanother.

&dditive composition where the interfaces of twocomponents are put together to create a new

component. 7rovides and re1uires interfaces ofintegrated component is a combination of interfaces ofconstituent components.


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T#pes of component composition


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)nterface incompatibilit#

7arameter incompatibilit# where operations have the

same name but are of different t#pes.

,peration incompatibilit# where the names of operations

in the composed interfaces are different.

,peration incompleteness where the provides interface

of one component is a subset of the re1uires interface of

another.


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Components with incompatible interfaces


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daptor components

&ddress the problem of component incompatibilit# b#

reconciling the interfaces of the components that are

composed.

6ifferent t#pes of adaptor are re1uired depending on the

t#pe of composition.

&n address5inder and a mapper component ma# be

composed through an adaptor that strips the postal code

from an address and passes this to the mapper

component.


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Composition through an adaptor

"he component postCodeStripper is the adaptor that

facilitates the se1uential composition of address5inder

and mapper components.


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n adaptor lin*ing a data collector and a sensor


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+hoto librar# composition


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)nterface semantics

;ou have to rel# on component documentation to decide

if interfaces that are s#ntacticall# compatible are actuall#

compatible.

Consider an interface for a 7hotoibrar# component


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+hoto Librar# documentation

'(his )etho" a""s a photo&raph to the !i*rar+ an"asso#iates the photo&raph i"enti%er an" #ata!o&ue

"es#riptor with the photo&raph.,

'what happens if the photo&raph i"enti%er is a!rea"+asso#iate" with a photo&raph in the !i*rar+-,

'is the photo&raph "es#riptor asso#iate" with the #ata!o&ue

entr+ as we!! as the photo&raph i.e. if "e!ete thephoto&raph/ "o a!so "e!ete the #ata!o&ue infor)ation-,


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The ,bect Constraint Language

"he ,b!ect Constraint anguage (,C) has been

designed to define constraints that are associated with

*% models.

t is based around the notion of pre and post condition

specification D common to man# formal methods.


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The ,CL description of the +hoto Librar#

interface

-- "he conte$t /e#word names the component to which the conditions appl#

conte%t addtem

-- "he preconditions specif# what must be true before e$ecution of addtem

pre 7hotoibrar#.libSi3e() F

7hotoibrar#.retrieve(pid) G null

-- "he postconditions specif# what is true after e$ecution

postlibSi3e () G libSi3e()Hpre 8 =

7hotoibrar#.retrieve(pid) G p

7hotoibrar#.catEntr#(pid) G photodesc

conte%t delete

pre 7hotoibrar#.retrieve(pid) I null

post 7hotoibrar#.retrieve(pid) G null

7hotoibrar#.catEntr#(pid) G 7hotoibrar#.catEntr#(pid)Hpre

7hotoibrar#.libSi3e() G libSi3e()Hpre4=


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+hoto librar# conditions

&s specified' the ,C associated with the 7hoto ibrar#

component states that

"here must not be a photograph in the librar# with the same

identifier as the photograph to be entered

"he librar# must e$ist - assume that creating a librar# adds asingle item to it

Each new entr# increases the si3e of the librar# b# =

f #ou retrieve using the same identifier then #ou get bac/ the

photo that #ou added

f #ou loo/ up the catalogue using that identifier' then #ou getbac/ the catalogue entr# that #ou made.


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Composition trade-offs

:hen composing components' #ou ma# find conflicts

between functional and non-functional re1uirements' and

conflicts between the need for rapid deliver# and s#stem

evolution.

;ou need to ma/e decisions such as :hat composition of components is effective for delivering the

functional re1uirements0

:hat composition of components allows for future change0

:hat will be the emergent properties of the composed s#stem0


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Data collection and report generation

components


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"e# points

6uring the CBSE process' the processes ofre1uirements engineering and s#stem design areinterleaved.

Component composition is the process of ‘wiring’

components together to create a s#stem.:hen composing reusable components' #ou normall#

have to write adaptors to reconcile different componentinterfaces.

:hen choosing compositions' #ou have to considerre1uired functionalit#' non-functional re1uirements ands#stem evolution.

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Ch17 CBSE