Interceptors Add Object Functionality

In COM today, the COM library is not directly involved when a client calls a method in a COM object. Instead, the call goes directly from the client to the object. In COM+, this is no longer true. All COM+ method calls pass through the COM+ library, although Microsoft promises that the overhead this incurs will be negligible (see the figure "Two COM+ Method Calls" ). Since COM+ is i nvolved in every method call, it's now possible to insert other objects in that path. A COM+ object that is automatically invoked during access of another COM+ object is called an interceptor .

One or more interceptors can lie in wait along the path (as shown in the figure ). For example, an interceptor might perform a security check on the client, then cause the method call to fail if the necessary permissions aren't in p lace. Doing this allows keeping the authorization logic separate from the business logic in the component the client is using.

Interceptors could also be used to manage the state of the destination object. An interceptor might stand in for the object itself, allowing a client to believe the object still exists when it has actually deactivated. Doing this allows building much more scalable applications, and it is very similar to what the Microsoft Transaction Server (MTS) does today. In fact, Microsoft plans to restruct ure MTS using interceptors when COM+ is available. An interceptor could even perform load balancing, transparently distributing method calls to various equivalent objects in turn.

By placing specific attributes in the client code, the client can determine which interceptors load when using a particular COM+ object. But, despite their convenience, interceptors suffer from one potential drawback: performance. Having to traverse a sequence of interceptors on every method call to an object is bound to slow things down.

Microsoft's expectation is that interceptors will primarily be for system-level functions: for the most part, a few highly optimized COM+ objects will be the most commonly used choices. Still, there are likely to be many situations where the convenience and flexibility of interceptors outweigh the performance hit they bring.

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Two COM+ Method Calls

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Interceptors in the path can check client security, stand in for the object, or perform other tasks, but they increase the call's overhead.