Distributed applications are realized by cooperation of multiple processes which manipulate data objects like databases. Objects in the systems are replicated to make the systems fault-tolerant. We discuss a system where read and write request messages are issued to replicas in a quorum-based scheme. In this paper, a quorum-based (QB) ordered (QBO) relation among request messages is defined to make the replicas consistent. We discuss a group protocol which supports a group of replicas with the

more »

... BO delivery of request messages. read write ( :QBO) System Model Computers p 1 , . . ., p n are interconnected in an asynchronous network where messages may be lost and the delay time is not bounded in the network. Applications are realized in a 3-tier client server model. Replicas of data objects are stored in data servers and transactions in application servers is-