Component-based software engineeringObjectivesTopics coveredComponent-based developmentCBSE essentialsCBSE and design principlesCBSE problemsComponentsComponent definitionsComponent as a service providerComponent characteristics 1Component characteristics 2Component interfacesSlide 14A data collector componentComponents and objectsComponent modelsElements of a component modelMiddleware supportComponent model servicesComponent development for reuseSlide 22Changes for reusabilityLegacy system componentsReusable componentsThe CBSE processSlide 27The component identification processComponent identification issuesAriane launcher failureComponent compositionTypes of compositionSlide 33Interface incompatibilityIncompatible componentsAdaptor componentsComposition through an adaptorAdaptor for data collectorInterface semanticsPhoto library compositionPhoto Library documentationThe Object Constraint LanguageFormal description of photo libraryPhoto library conditionsComposition trade-offsData collection and report generationKey pointsSlide 48©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 1Component-based software engineering©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 2ObjectivesTo explain that CBSE is concerned with developing standardised components and composing these into applicationsTo describe components and component modelsTo show the principal activities in the CBSE processTo discuss approaches to component composition and problems that may arise©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 3Topics coveredComponents and component modelsThe CBSE processComponent composition©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 4Component-based developmentComponent-based software engineering (CBSE) is an approach to software development that relies on software reuse.It emerged from the failure of object-oriented development to support effective reuse. Single object classes are too detailed and specific.Components are more abstract than object classes and can be considered to be stand-alone service providers.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 5CBSE essentialsIndependent components specified by their interfaces.Component standards to facilitate component integration.Middleware that provides support for component inter-operability.A development process that is geared to reuse.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 6CBSE and design principlesApart from the benefits of reuse, CBSE is based on sound 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 7CBSE problemsComponent trustworthiness - how can a component with no available source code be trusted?Component certification - who will certify the quality of components?Emergent property prediction - how can the emergent properties of component compositions be predicted?Requirements trade-offs - how do we do trade-off analysis between the features of one component and another?©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 8ComponentsComponents provide a service without regard to where the component is executing or its programming language•A component is an independent executable entity that can be made up of one or more executable objects;•The component interface is published and all interactions are through the published interface;©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 9Component definitionsCouncill and Heinmann:•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.Szyperski:•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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 10Component as a service providerThe component is an independent, executable entity. It does not have to be compiled before it is used with other components.The services offered by a component are made available through an interface and all component interactions take place through that interface.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 11Component characteristics 1Standardised Component standardisation means that a component that isused in a CBSE process has to conform to some standardisedcomponent model. This model may define componentinterfaces, component meta-data, documentation, compositionand deployment.Independent A component should be independent – it should be possible tocompose and deploy it without having to use other specificcomponents. In situations where the component needsexternally provided services, these should be explicitly set outin a ‘requires’ interface specification.Composable For a component to be composable, all external interactionsmust take place through publicly defined interfaces. Inaddition, it must provide external access to information aboutitself such as its methods and attributes.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 12Component characteristics 2Deployable To be deployable, a component has to be self-contained andmust be able to operate as a stand-alone entity on somecomponent platform that implements the component model.This usually means that the component is a binary componentthat does not have to be compiled before it is deployed.Documented Components have to be fully documented so that potentialusers of the component can decide whether or not they meettheir needs. The syntax and, ideally, the semantics of allcomponent interfaces have to be specified.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 13Component interfacesProvides interface•Defines the services that