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... messages to be delivered in a view identical to the one in which they are sent. These shortcomings are remedied by the Optimistic Virtual Synchrony (OVS) model, recently introduced by Sussman et al. =-=[SKM99]. In OVS, -=-each view installation is preceded by an optimistic view event, which provides the application with a "guess" what the next view will be. After this event, applications may optimistically se...

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... Sync layer of Ensemble, Section 2.2). To prevent this undesirable blocking problem, which reduces responsiveness, alternate and more complex solutions to handle membership changes have been proposed =-=[19, 24]-=-. These solutions implement a weaker property called same view delivery [13]. The implementation based on generic broadcast does not lead to blocking: the solution “naturally” implements the same view...

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...that processes (sites) moving together from view v to view v 0 should deliver the same set of messages in view v. Also that every message m be delivered in the same view v by all sites that deliver m =-=[14]-=-. The problem addressed in this paper is that since the multicast-based protocols have no 2 Phase Commit (or other atomic commit protocol), a transaction, t 1 , might be committed by operational sites...

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...updates which makes the implementation much more complex. In contrast, our proposal uses a simple mechanism that fits general purpose communication protocols. The work on Optimistic Virtual Synchrony =-=[35]-=- also uses semantic information to alleviate the cost of view changes by relaxing the ordering of views relatively to ordinary messages. A primary-backup protocol which discards messages and provides ...

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... might support or need. • jGCS provides support for recent research results that improve the performance of group communication systems, namely, semantic annotations [20, 321, 19] and early delivery =-=[18, 25, 24, 22]-=-. • open source implementations of jGCS for several major group communication systems have been already developed, namely, Appia [14], Spread [2] (including the FlushSpread variant), and JGroups [4]. ...

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...es on message semantics to relax causal order. Generic broadcast [19] is a relaxation of total order based on message semantics captured as a binary relation. The work on Optimistic Virtual Synchrony =-=[25]-=- also uses semantic information to alleviate the cost of view changes but, unlike our approach, does not address the issue of limiting the number of these changes. It would be interesting to combine t...

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... for further improvements and optimizations. 7.1 Future Developments Several future enhancements are planned. The VS client can be extended to support the Optimistic Virtual Synchrony semantics (OVS) =-=[12]-=-. OVS allows the user application to send messages while in a reconfiguration state, where the current implementation blocks. This would improve network utilization and performance. Based on the infor...

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...ce of membership from a group multicast will better enhance scalability of fault-tolerant distributed applications. This separation was later adopted by researchers who addressed the WAN environment (=-=[14, 15, 7]-=-). InterGroup [5] presents another WAN approach to address scalability. Several research projects in the past sought to relax the semantics of the middleware for distributed applications ([16, 17, 4, ...

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...ay is also avoided. 5 An extension of Virtual Synchrony, called Optimistic Virtual Synchrony, supports transmission of application messages in parallel with formation of new views (Sussman et al., in =-=[90, 89]-=-). With Optimistic Virtual Synchrony, the iads application can be modified to avoid the processing delay caused by blocking. 153 154 Chapter 11 Conclusions We have developed a formal design of a novel...