Short note: component-based software engineering

Component-based software engineering (CBSE) is an approach to software development that relies on reuse of independent, executable software components. Systems are built by composing components that are defined by their interfaces, rather than writing all code from scratch. It grew from the failure of object-oriented development to give large-scale reuse, since single classes are too detailed and specific.

Component characteristics: standardised (conform to a component model), independent, composable (all interaction through interfaces), deployable (run without being compiled in) and documented.

Each component has two interfaces:

• Provides interface – services the component offers to others.
• Requires interface – services it needs from other components.

            +-----------------+
  provides  |                 |  requires
   O--------|    Component    |--------C
            |                 |
            +-----------------+

Essentials of CBSE

1. Independent components fully specified by their interfaces.
2. Component standards / models (e.g. EJB, .NET, CORBA Component Model) that define interfaces, packaging and deployment.
3. Middleware that handles component communication and services like transactions and security.
4. A development process geared to reuse: outline requirements → identify candidate components → modify requirements to suit available components → architectural design → identify more components → compose components.

Benefits: faster development, lower cost, reliable tested parts, easier maintenance. Problems: trusting components without source code, certifying quality, predicting emergent properties of the composed system, and trade-offs between requirements and available components.