The Common Object Request Broker Architecture (CORBA) is the most successful representative for an object based distributed computing architecture. Although CORBA simplifies the implementation of complex, distributed systems significantly, support of techniques for reliable fault-tolerant software, such as online-replacement or replication is not within the scope of today's CORBA or Real-time CORBA. The Simplex architecture developed at the Software Engineering Institute at Carnegie Mellon University supports the online replacement of software components within a fault-tolerant, real time control application. Simplex middleware consists of both a hard real time publisher/subscriber service and a soft real time service to support human-in-the-loop supervisor control activities. We have replaced Simplex' soft real time services with CORBA services while retaining its hard real time publication and subscription service. This composite approach allows us to take advantage of both the strength of CORBA in distributed computing and Simplex' strength in hard real time control applications. The authors discuss the trade-off issues in this composite system approach and present the tele-laboratory experiment, which is based on the extended Simplex system.
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