Architectural pattern: Interceptor Source: POSA II pp 109 – 140POSA II 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

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

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

(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

Dynamics 1.Application instantiates a concrete interceptor and registers it with the appropriate dispatcher 2.Framework receives an event 3.Framework passes a context object to the dispatcher for that type of event 4.Dispatcher calls registered interceptors in the appropriate order, passing the context object 5.Interceptors do their thing, using the context object for event data and methods to control certain aspects of the framework’s behavior

Implementation 1.Model framework’s internal behavior 2.Identify and model interception points 3.Specify context objects 4.Specify the interceptors 5.Specify the dispatchers Registration interface Callback interface 6.Implement callback mechanisms 7.Implement concrete interceptors

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

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