Using active clients to minimize replication in primary-backup protocols

We consider a primary-backup approach to provide fault-tolerance service under a model in which the clients play an active role when their service requests are not fulfilled. Each client maintains an ordered list of servers and sends its service requests to the first server in its list. If the server does not respond within it specified timeout period, the client retransmits the request to the next server in its list. Under this model, we construct protocols that tolerate crash failures, send-omission failures, and receive-omission failures. For each type of failure, our protocol is optimal with respect 20 the degree of replication. More precisely, our protocols tolerate up to f server failures using only f+1 servers. In addition, these protocols tolerate an arbitrary number of client failures. Further, the protocols ensure that the service provided by the system is functionally equivalent to that provided by a single failure-free server.