Aqua: a Framework for Providing Adaptive Fault Tolerance to Distributed Applications By

Dependable distributed systems are diÆcult to build. This is particularly true if they have dependability requirements that change during the execution of an application and are built with commercial o -the-shelf hardware. In that case, fault tolerance can be achieved using middleware, and mechanisms must be provided to communicate the dependability requirements of a distributed application to the system and to adapt the system's con guration to achieve the desired dependability. In this thesis, we developed and implemented the AQuA architecture, which allows a distributed CORBA application to request a desired level of dependability during the application's runtime. AQuA provides fault-tolerance mechanisms to ensure that a CORBA client can obtain reliable services, even if the CORBA server object that provides the desired services may su er from crash failures and value faults. AQuA includes a replicated dependability manager that provides dependability management by con guring the system in response to applications' requests and changes in system resources due to faults. It uses Maestro/Ensemble to provide group communication services, and it provides applications with di erent types of groups so that they can achieve better scalability as well as dependability. The AQuA gateway is designed to receive standard CORBA IIOP messages to allow any standard CORBA application to use AQuA. The gateway also includes a set of handlers that implement replication schemes to forward messages reliably to the remote replicated objects.