AQuA: An

Abstract ⎯ Dependable distributed systems are difficult to build. This is particularly true if they have dependability requirements that change during the execution of an application, and are built with commercial off-the-shelf hardware. In that case, fault tolerance must be achieved using middleware software, and mechanisms must be provided to communicate the dependability requirements of a distributed application to the system and to adapt the system’s configuration to try to achieve the desired dependability. The AQuA architecture allows distributed applications to request a desired level of availability using the Quality Objects (QuO) framework and includes a dependability manager that attempts to meet requested availability levels by configuring the system in response to outside requests and changes in system resources due to faults. The AQuA architecture uses the QuO runtime to process and invoke availability requests, the Proteus dependability manager to configure the system in response to faults and availability requests, and the Ensemble protocol stack to provide group communication services. Furthermore, a CORBA interface is provided to application objects using the AQuA gateway. The gateway provides a mechanism to translate between process-level communication, as supported by Ensemble, and IIOP messages, understood by Object Request Brokers. Both active and passive replication are supported, and the replication type to use is chosen based on the performance and dependability requirements of particular distributed applications. 1