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Architectural pattern: Interceptor

Architectural pattern: Interceptor. Source: POSA II pp 109 – 140 Environment: developing frameworks that can be extended transparently Recurring problem: Frameworks cannot anticipate all of the services they will need

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Architectural pattern: Interceptor

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  1. Architectural pattern: Interceptor • Source: POSA II pp 109 – 140 • Environment: developing frameworks that can be extended transparently • Recurring problem: • Frameworks cannot anticipate all of the services they will need • The benefit of a framework is lost if new services have to be implemented by applications

  2. Requirements for extensibility • Add a service without requiring modifications to the framework’s core architecture • Adding application-specific services should not break existing components or applications • Applications using a framework may need to monitor and control its behavior

  3. Examples of framework services that might use such an extension mechanism • Security • Event logging • Load balancing • Transaction management

  4. (Core of a) solution - interceptors • Interceptor interfaces provide hooks that can call service code when certain events occur • Event example: arrival of a client request • Dispatchers manage the concrete interceptors registered for framework events • Context objects provide access to event information and the framework’s internal state and behavior

  5. Dynamics • Application instantiates a concrete interceptor and registers it with the appropriate dispatcher • Framework receives an event • Framework passes a context object to the dispatcher for that type of event • Dispatcher calls registered interceptors in the appropriate order, passing the context object • Interceptors do their thing, using the context object for event data and methods to control certain aspects of the framework’s behavior

  6. Implementation • Model framework’s internal behavior • Identify and model interception points • Specify context objects • Specify the interceptors • Specify the dispatchers • Registration interface • Callback interface • Implement callback mechanisms • Implement concrete interceptors

  7. Known uses • Component-based application servers • EJB, CORBA components, COM+ • CORBA ORBs • Web browsers • Plug-ins are concrete interceptors • Non-software: paper mail forwarding

  8. Consequences • Benefits • Extensibility and flexibility • Separation of concerns • Support for monitoring and control • Layer symmetry • Reuse of interceptors across applications • Liabilities • Complex design issues • Malicious or erroneous interceptors • Risk of interception cascades

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