In a distributed collaborative application, a key requirement is that all users see the same copy of a shared window object at any given point in time (WYSIWIS) to maintain cohesive context of the problem. We recognize that user actions are generated based on their understanding of the context of a problem, where context is defined in terms global view of shared window object. We note however that, actions generated from different sites in response to the same globally agreed upon context may not be compatible to each other since execution of one action may change the context in a way such that other actions become irrelevant. This notion of compatibility of action is the crucial distinguishing factor between a distributed collaborative system and other distributed systems where independent actions are serializable. We formalize distributed collaboration as a finite state machine model (FSM) and propose an epoch-based implementation of this model. We formalize the notion of compatibility in terms of FSM, and also propose a pragmatic, rule based implementation of compatibility enforcement. We then provide distributed protocols for a system-level realization of our model. Sample applications are also described to bring out the salient features of our model.
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