Component-based software engineering 1 Ian Sommerville 2004

Component-based software engineering 1 Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 1

Objectives 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 2

Component-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 standalone service providers. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 3

CBSE 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 4

CBSE 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 5

Components 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 6

Component 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 7

Component 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 8

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 ‘requires’ interface specification. Composable For a component to be composable, all external interactions must take place through publicly defined interfaces. In addition, it must provide external access to information about itself such as its methods and attributes. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 9

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. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 10

Component interfaces Provides interface Defines the services that are provided by the component to other components. Requires interface Defines the services that specifies what services must be made available for the component to execute as specified. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 11

Requ er face Pr er fa De Define th from t Com by enviro usesto Component interfaces Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 12

Req er face Pr e f a r e senso st tS Dat te sens in re t lis A data collector component Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 13

Components and objects Components are deployable entities. Components do not define types. Component implementations are opaque. Components are language-independent. Components are standardised. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 14

Component models A component model is a definition of standards for component implementation, documentation and deployment. Examples of component models EJB model (Enterprise Java Beans) COM model (.NET model) Corba Component Model The component model specifies how interfaces should be defined and the elements that should be included in an interface definition. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 15

Cus Nam con Com Do Me Inter face Spe E P a in c acc defin inte face s t u Usa De Inter face info an Com Elements of a component model Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 16

Middleware support Component models are the basis for middleware that provides support for executing components. Component model implementations provide: Platform services that allow components written according to the model to communicate; Horizontal services that are application-independent services used by different components. To use services provided by a model, components are deployed in a container. This is a set of interfaces used to access the service implementations. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 17

Hori Com T r an Re man ma m Con Se P e rs Platf Inte fac Ex C Addr defi ma co Component model services Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 18

Component development for reuse Components developed for a specific application usually have to be generalised to make them reusable. A component is most likely to be reusable if it associated with a stable domain abstraction (business object). For example, in a hospital stable domain abstractions are associated with the fundamental purpose - nurses, patients, treatments, etc. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 19

Component development for reuse Components for reuse may be specially constructed by generalising existing components. Component reusability Should reflect stable domain abstractions; Should hide state representation; Should be as independent as possible; Should publish exceptions through the component interface. There is a trade-off between reusability and usability The more general the interface, the greater the reusability but it is then more complex and hence less usable. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 20

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 component adaptation. Integrate required components to reduce dependencies. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 21

Legacy system components Existing legacy systems that fulfil a useful business function can be re-packaged as components for reuse. This involves writing a wrapper component that implements provides and requires interfaces then accesses the legacy system. Although costly, this can be much less expensive than rewriting the legacy system. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 22

Reusable components The development cost of reusable components may be higher than the cost of specific equivalents. This extra reusability enhancement cost should be an organization rather than a project cost. Generic components may be less space-efficient and may have longer execution times than their specific equivalents. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 23

Key points CBSE is a reuse-based approach to defining and implementing loosely coupled components into systems. A component is a software unit whose functionality and dependencies are completely defined by its interfaces. A component model defines a set of standards that component providers and composers should follow. During the CBSE process, the processes of requirements engineering and system design are interleaved. Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 19 Slide 24