Using Interceptors to Enhance CORBA

T he integration of distributed computing and the object model leads to distributed object computing, in which objects rather than processes are distributed across multiple computers. A well-established standard for distributed object computing is the Common Object Request Broker Architecture (CORBA).1 Distributed object frameworks like CORBA have many attractive features but provide little support for alternative protocols, profiling and monitoring, security, or reliability. Previously, you would have had to create—and enable the application to use—the components that provide such additional capabilities. Even if the components that provide these capabilities already exist, using them requires substantial effort, as well as specialized knowledge and understanding of problems outside the application domain. With the advent of interceptors—nonapplication components that can alter application behavior—you can enhance CORBA applications at runtime with components whose operation is transparent to both the application and the CORBA framework; this means that you can modify application behavior without having to modify the application or the CORBA framework. The Object Management Group—the CORBA standards body—recognizes the value of interceptors.2 Other distributed object computing standards, such as Microsoft’s Distributed Component Object Model (DCOM), provide for interceptor-like components through custom marshaling mechanisms,3 which enable an application to bypass the standard communication mechanisms and to use custom ones. Interceptors can also allow you to chain together multiple components (each with its own functionality) to achieve new functionality transparently. Over the past few years, we have developed a system—called Eternal—that exploits interceptors transparently.4-6 The Eternal system enhances unmodified CORBA applications with reliability by using an interceptor to chain together protocol, monitoring, scheduling, and replication management components at runtime.