This paper describes ``Object Group'', an object behavioral pattern for group communication and fault-tolerance in distributed systems. The Object Group pattern allows the implementation of replicated objects, of load sharing, and of efficient multicast communication over protocols like IP-multicast and UDP-broadcast. Application areas of the pattern are fault-tolerant client/server systems, groupware, and parallel text retrieval engines. Events within an Object Group honor the Virtual Synchrony model. Owing to Virtual Synchrony, the size of an object group can be varied at run-time, while client applications are interacting with the object. A replicated state remains consistent in spite of objects entering and leaving the group dynamically and in spite of failures. The Object Group pattern has been implemented in the Electra and in the Orbix+Isis CORBA Object Request Broker.
computer science; technical report
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