CompBasedSoftwEng Ian Sommervile Super Za Uvod

8/3/2019 CompBasedSoftwEng Ian Sommervile Super Za Uvod

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Ian Sommerville 2006 MSc module: Advanced Software Engineering Slide 1

Component-based software

engineering


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Objectives

To explain that CBSE is concerned withdeveloping standardised components andcomposing these into applications

To describe components and componentmodels

To show the principal activities in the CBSEprocess

To discuss approaches to componentcomposition and problems that may arise


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Topics covered

Components and component models

The CBSE process

Component composition


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Component-based development

Component-based software engineering(CBSE) is an approach to softwaredevelopment that relies on software reuse.

It emerged from the failure of object-orienteddevelopment to support effective reuse. Singleobject classes are too detailed and specific.

Components are more abstract than objectclasses and can be considered to be stand-alone service providers.


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CBSE essentials

Independent components specified by theirinterfaces.

Component standards to facilitate component

integration. Middleware that provides support for

component inter-operability.

A development process that is geared toreuse.


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CBSE and design principles

Apart from the benefits of reuse, CBSE isbased on sound software engineering designprinciples:

Components are independent so do not interferewith each other;

Component implementations are hidden;

Communication is through well-defined interfaces;

Component platforms are shared and reducedevelopment costs.


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CBSE problems

Component trustworthiness - how can a componentwith no available source code be trusted?

Component certification - who will certify the quality of

components?

Emergent property prediction - how can the emergentproperties of component compositions be predicted?

Requirements trade-offs - how do we do trade-off

analysis between the features of one component and

another?



Components

Components provide a service without regardto where the component is executing or itsprogramming language A component is an independent executable entity

that can be made up of one or more executableobjects;

The component interface is published and allinteractions are through the published interface;



Component definitions

Councill and Heinmann: A software component is a software element that

conforms to a component model and can beindependently deployed and composed without

modification according to a composition standard.

Szyperski: A software component is a unit of composition with

contractually specified interfaces and explicit

context dependencies only. A software componentcan be deployed independently and is subject tocomposition by third-parties.



Component as a service provider

The component is an independent, executableentity. It does not have to be compiled before itis used with other components.

The services offered by a component aremade available through an interface and allcomponent interactions take place through thatinterface.



Component characteristics 1

Standardised Component standardisation means that a component that is

used in a CBSE process has to conform to some standardised

component model. This model may define component

interfaces, component meta-data, documentation, composition

and deployment.

Independent A component should be independent it should be possible to

compose and deploy it without having to use other specific

components. In situations where the component needs

externally provided services, these should be explicitly set out

in a requiresinterface specification.

Composable For a component to be composable, all external interactionsmust take place through publicly defined interfaces. In

addition, it must provide external access to information about

itself such as its methods and attributes.



Component characteristics 2

Deployable To be deployable, a component has to be self-contained and

must be able to operate as a stand-alone entity on some

component platform that implements the component model.This usually means that the component is a binary component

that does not have to be compiled before it is deployed.

Documented Components have to be fully documented so that potential

users of the component can decide whether or not they meet

their needs. The syntax and, ideally, the semantics of all

component interfaces have to be specified.



Component interfaces

Provides interface

Defines the services that are provided by thecomponent to other components.

Requires interface Defines the services that must be made available

for the component to execute as specified.



Component interfaces

Provides int erf aceRequires int erf ace

Component

Defines th e services

fromthecomponents

environment t hat i t

uses

Defines th e services

that are prov ided

by the componen t

to other componen ts



Provides/requires interfaces

These interfaces are NOT the same as input/outputinterfaces.

Both of these interfaces define both the inputs neededand the outputs produced by the provided and

required services Requires

You can think of this as defining the methods that arecalled by a component or before a component executes

Provides

You can think of this as defining how the component iscalled.


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A data collector component

Provides int erf aceRequires int erf ace

Data collector

addSensor

removeSensorstartSensor

stopSensor

testSensor

listAllreport

initialise

sensorManagement

sensorData


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Components and objects

Components are deployable entities.

Components do not define types.

Component implementations are opaque.

Components are language-independent.

Components are standardised.


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Component models

A component model is a definition of standardsfor component implementation, documentationand deployment.

Examples of component models EJB model (Enterprise Java Beans) COM+ model (.NET model)

Corba Component Model

The component model specifies how interfacesshould be defined and the elements thatshould be included in an interface definition.


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Elements of a component model

Component model

Interfaces

Usage

information

Deployment

and use

Interface

definition

Speci fic

interfaces

Composi tion

Naming

convention

Meta-data

access

Customisation

Packaging

Documentation

Evolution

support


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Middleware support

Component models are the basis for middleware thatprovides support for executing components.

Component model implementations provide:

Platform services that allow components writtenaccording to the model to communicate;

Horizontal services that are application-independentservices used by different components.

To use services provided by a model, components are

deployed in a container. This is a set of interfaces usedto access the service implementations.


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Component model services

Platform services

AddressingInter face

definitionComponent

communications

Exception

management

Horizontal services

Security

Transaction

management

Concurrency

Component

management

Persistence

Resource

management


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Component development for reuse

Components are rarely created by simplyusing part of the code of another applicationsystem

Rather, components for reuse have to bespecifically developed so that they arereusable across a range of applications


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Components developed for a specificapplication usually have to be generalised tomake them reusable.

A component is most likely to be reusable if itassociated with a stable domain abstraction(business object).

For example, in a hospital stable domainabstractions are associated with thefundamental purpose - nurses, patients,treatments, etc.


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Components for reuse may be specially constructed bygeneralising existing components.

Component reusability

Should reflect stable domain abstractions;

Should hide state representation; Should be as independent as possible;

Should publish exceptions through the componentinterface.

There is a trade-off between reusability and usability

The more general the interface, the greater thereusability but it is then more complex and hence lessusable.


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Changes for reusability

Remove application-specific methods.

Change names to make them general.

Add methods to broaden coverage.

Make exception handling consistent.

Add a configuration interface for componentadaptation.

Integrate required components to reducedependencies.


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Legacy system components

Existing legacy systems that fulfil a usefulbusiness function can be re-packaged ascomponents for reuse.

This involves writing a wrapper component thatimplements provides and requires interfacesthen accesses the legacy system.

Although costly, this can be much lessexpensive than rewriting the legacy system.

We will return to issues of legacy system reusein the discussion of service-oriented systems.


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Reusable components

The development cost of reusable componentsmay be higher than the cost of specificequivalents. This extra reusability

enhancement cost should be an organizationrather than a project cost.

Generic components may be lessspace-efficient and may have longer execution

times than their specific equivalents.


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CBSE Processes

As discussed, conventional softwareengineering processes have to be adapted forreuse.


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The CBSE process

When reusing components, it is essential tomake trade-offs between ideal requirementsand the services actually provided by availablecomponents.

This involves: Developing outline requirements;

Searching for components then modifyingrequirements according to available functionality.

Searching again to find if there are bettercomponents that meet the revised requirements.


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The CBSE process


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The component identification process


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Component identification issues

Trust. You need to be able to trust the supplier of acomponent. At best, an untrusted component may notoperate as advertised; at worst, it can breach your

security.

Requirements. Different groups of components willsatisfy different requirements.

Validation.

The component specification may not be detailed

enough to allow comprehensive tests to be developed. Components may have unwanted functionality. How

can you test this will not interfere with your application?


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Designing a system by integrating a number ofcomponents.


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The process of assembling components tocreate a system.

Composition involves integrating components

with each other and with the componentinfrastructure.

Normally you have to write glue code to

integrate components.


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Types of composition

Sequential composition where the composedcomponents are executed in sequence. This involvescomposing the provides interfaces of each component.

Hierarchical composition where one component calls

on the services of another. The provides interface ofone component is composed with the requires interfaceof another.

Additive composition where the interfaces of two

components are put together to create a newcomponent.


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Types of composition

(a)

A A

B B

A B

(b) (c)


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Sequential composition

In this case, the components are executed insequence to provide some required effect

The outputs from the provides interface from

the first component executed become theinputs for the provides interface for the 2ndunit called (perhaps with some modificationthrough an adapter component)

Each component is executed independentlyand does not have to be aware of the othercomponents in the sequence.


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Hierarchical composition

In this case, one component (defined in therequires interface) is called directly from withinthe body of the other component.

The calling component must know the nameand the interface signature of the calledcomponent.


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Additive composition

In this case, we put two components togetherso that the provides interface includesoperations that come from both of the

composed components. Essentially, this is normally implemented by

defining a new small component that offers thecombined interface and which than calls one of

the composed components, depending on thecall to the composed component.


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Interface incompatibility

Parameter incompatibility where operationshave the same name but are of different types.

Operation incompatibility where the names of

operations in the composed interfaces aredifferent.

Operation incompleteness where the providesinterface of one component is a subset of the

requires interface of another.


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Incompatible components

addressFinder

phoneDatabase (string command )s tring l ocati on(s tring p n)

s tring owner (string pn)

s tring propertyType (string pn)

mapp er

mapDB (string command)dis play Map (s tring postCode, scale)

printMap (s tring p ost Code, scale)


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Incompatibility

The component addressFinder through itslocation method produces a string which is theaddress of the property, including street

number and name and town The component mapper through its

displayMap method, expects a string which isa postcode only (not a complete address)


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Adaptor components

Address the problem of componentincompatibility by reconciling the interfaces ofthe components that are composed.

Different types of adaptor are requireddepending on the type of composition.

An addressFinder and a mapper componentmay be composed through an adaptor (calledpostCodeStripper) that strips the postal codefrom an address and passes this to themapper component.


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address = addressFinder.location (phonenumber) ;

postCode = postCodeStripper.getPostCode (address) ;

mapper.displayMap(postCode, 10000)

Composition through an adaptor

The component postCodeStripper is theadaptor that facilitates the sequentialcomposition of addressFinder and mapper

components.


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Adaptor for data collector

Data collector

addSensor

removeSensorstartSensor

stopSensor

testSensor

listAll

report

initialise

sensorManagement

sensorData

Adaptersensor

start

getdata

stop


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Adaptor functionality

The data collector component requires asensorManagement component that providesfacilities to stop and start sensors and to query

sensors for data The adapter component in this case turns a

string of sensor management commands e.g.sensor.stop into the commands required for a

specific sensor device.


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Interface semantics

You have to rely on component documentationto decide if interfaces that are syntacticallycompatible are actually compatible.

Consider an interface for a PhotoLibrarycomponent:

public void addItem (Identifier pid ; Photograph p; CatalogEntry photodesc) ;public Photograph retrieve (Identi fier pid) ;public CatalogEntry catEntry (Identifier pid) ;


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Photo library composition

PhotoLibrary

adaptorImage

Manager

getImage

User

Inter face

getCatalogEntry

addItem

retrieve

catEntry


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Photo Library documentation

This method adds a photograph to the library and

associates the photograph identifier and catalogue

descriptor with the photograph.

what happens if the photograph identifier is already

associated with a photograph in the library?

is the photograph descriptor associated with the

catalogue entry as well as the photograph i.e. if I deletethe photograph, do I also delete the catalogue

information?


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The Object Constraint Language

The Object Constraint Language (OCL) hasbeen designed to define constraints that areassociated with UML models.

It is based around the notion of pre and postcondition specification - similar to the approachused in Z as described in Chapter 10.


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Formal description of photo library

-- The context keyword names the component to which the condi tions apply

context addItem

-- The preconditions specify what must be true before execution of addItem

pre: PhotoLibrary.libSize() > 0

PhotoLibrary.retrieve(pid) = null

-- The postconditions specify what is true after execution

post: libSize () = libSize()@pre + 1

PhotoLibrary.retrieve(pid) = p

PhotoLibrary.catEntry(pid) = photodesc

context delete

pre: PhotoLibrary.retrieve(pid) null ;

post: PhotoLibrary.retrieve(pid) = null

PhotoLibrary.catEntry(pid) = PhotoLibrary.catEntry(pid)@pre

PhotoLibrary.libSize() = libSize()@pre - 1


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Photo library conditions

As specified, the OCL associated with the PhotoLibrary component states that:

There must not be a photograph in the library with thesame identifier as the photograph to be entered;

The library must exist - assume that creating a libraryadds a single item to it;

Each new entry increases the size of the library by 1;

If you retrieve using the same identifier then you getback the photo that you added;

If you look up the catalogue using that identifier, thenyou get back the catalogue entry that you made.


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Composition trade-offs

When composing components, you may findconflicts between functional and non-functionalrequirements, and conflicts between the needfor rapid delivery and system evolution.

You need to make decisions such as: What composition of components is effective for

delivering the functional requirements?

What composition of components allows for future

change? What will be the emergent properties of the

composed system?


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Data collection and report generation

(a) Data

collection

(b)

Data

management

Report

generator

Data

collectionData base

Report

Report


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Composition trade-offs

For composition (a), reporting and datamanagement are separate so there is moreflexibility if changes in one of these functions

but not the other have to be made. For composition (b), there are fewer discrete

components so faster communications withinthe component offering both data management

and report generation.


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Key points

CBSE is a reuse-based approach to defining andimplementing loosely coupled components intosystems.

A component is a software unit whose functionality and

dependencies are completely defined by its interfaces.

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

During the CBSE process, the processes of

requirements engineering and system design areinterleaved.


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Key points

Component composition is the process ofwiring components together to create a

system.

When composing reusable components, younormally have to write adaptors to reconciledifferent component interfaces.

When choosing compositions, you have to

consider required functionality, non-functionalrequirements and system evolution.

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