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Component Based Development (CBD)

Component-based development is a CBSE activity that occurs in parallel with domain engineering. Using analysis and architectural design methods discussed earlier, the software team refines an architectural style that is appropriate for the analysis model created for the application to be built. Once the architecture has been established, it must be populated by components that are available from reuse libraries and/or are engineered to meet custom needs.

For those requirements that are addressed with available components, the following software engineering activities must be done:
– Component qualification : It examines reusable components. These are identified by characteristics in their interfaces, i.e. the services provided, and the means by which consumers access these services. This does not always provide the whole picture of whether a component will fit the requirements and the architectural style. This is a process of discovery by the software Engineer. This ensures a candidate component will perform the function required, and whether it is compatible or adaptable to the architectural style of the system. The three important characteristics looked at are performance, reliability and usability.

– Component adaptation : It is required because very rarely will components integrate immediately with the system. Depending on the component type, different strategies are used for adaptation or wrapping. The most common approaches are:
* White box wrapping : The implementation of the component is directly modified in order to resolve any incompatibilities. This is, obviously, only possible if the source code is available for a component, which is extremely unlikely in the case of COTS.
* Grey box wrapping : This relies on the component library providing a component extension language or API that enables conflicts to be removed or masked.
* Black box wrapping : This is the most common case, where access to source code is not available, and the only way the component can be adapted is by pre/post processing at the interface level.
It is the job of the software engineer to determine whether the effort required to wrap a component adequately is justified, or whether it would be “cheaper” to engineer a custom component which removes these conflicts.

– Component composition : The component composition is a task assembles qualified, adapted, and engineered components to populate the architecture established for an application. To accomplish this, an infrastructure must be established to bind the components into an operational system. The infrastructure provides a model for the coordination with one another and performs common tasks. Among the many mechanisms for creating an effective infrastructure is a set of four “architectural ingredients” that should be present to achieve component composition.
* Data exchange model: Mechanism that enables users and applications to interact and transfer data. The data exchange mechanisms not only allow human-to-software and component-to-component data transfer but also transfer among system resources.
* Automation: A variety of tools, macros, and scripts should be implemented to facilitate interaction between reusable components.
* Structured storage: Heterogeneous data contained in a “compound document” should be organized and accessed as a single data structure, rather than a collection of separate files.
* Underlying object model: The object model ensures that components developed in different programming languages that reside on different platforms can be interoperable.

– Component update : When systems are implemented with COTS components, update is complicated by the imposition of a third party. The organization that developed the reusable component may be outside the immediate control of the software engineering organization.

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