Component-Based Software Engineering

Objectives

1. To explain that CBSE is concerned with developing standardized components and composing these into applications
2. To describe components and component models
3. To show the principal activities in the CBSE process
4. To discuss approaches to component composition and problems that may arise

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 stand-alone service providers.

CBSE essentials

• Independent components specified by their interfaces. There should be a clear separation between the component interface and its implementation.
• Component standards to facilitate component integration. how component interfaces should be specified and how components communicate.

CBSE essentials

• Middleware that provides support for component inter-operability. To make independent, distributed components work together, you need middleware support that handles component communications.
• A development process that is geared to reuse. need a development process that allows requirements to evolve, depending, on the functionality of available components.

CBSE design principles

1. Components are independent so they do not interfere with each other’s operation. Implementation details are hidden. The component’s implementation can be changed without affecting the rest of the system.
2. Components communicate through well-defined interfaces. If these interfaces are maintained, one component can be replaced by another, which provides additional or enhanced functionality.
3. Component infrastructures offer a range of standard services that can be used in application systems. This reduces the amount of new code that has to be developed.

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?

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;

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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.

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.

Component characteristics 1

Component characteristics 2

Component interfaces

• Provides interface
• The ‘provides’ interface defines the services provided by the component. This interface, essentially, is the component API.
• Requires interface
• The ‘requires’ interface specifies what services must be provided by other components in the system if a component is to operate correctly. If these are not available, then the component will not work.

Component interfaces

A data collector component

Components and objects

• Components are deployable entities.
• Components do not define types.
• Component implementations are opaque.
• Components are language-independent.
• Components are standardized.

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.

Elements of a component model

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.

Component model services

Types of CBSE process

1. Development for reuse This process is concerned with developing components or services that will be reused in other applications. It usually involves generalizing existing components.
2. Development with reuse This is the process of developing new applications using existing components and services.

Component development for reuse

• Components developed for a specific application usually have to be generalized 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.

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.

CBSE with reuse

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.

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.

Reusable components

1. 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.
2. Generic components may be less space-efficient and may have longer execution times than their specific equivalents.

Key points

1. CBSE is a reuse-based approach to defining and implementing loosely coupled components into systems.
2. A component is a software unit whose functionality and dependencies are completely defined by its interfaces.
3. A component model defines a set of standards that component providers and composers should follow.
4. During the CBSE process, the processes of requirements engineering and system design are interleaved.