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I was wondering what is the actual relationship between Paxos based replication schemes and different concurrency models like snapshot isolations. Could anybody kindly able to explain these two in regards to their relationship and with few practical examples?

Joarder Kamal
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Snapshot isolation is a strong consistency concurrency criterion in the sense that it totally orders conflicting updates. Total order is equivalent to consensus in distributed systems. Paxos is a solution for consensus in distributed systems with process faults. So the answer to your question is: The relationship between snapshot isolation and Paxos-based replication schemes is that they should be equally hard to implement in a theoretical sense (i.e., will be possible with the same assumptions).

Other examples of strong consistent concurrency criteria are serializability, linearizability, and sequential consistency. In contrast, weak consistent criteria such as causal consistency or eventual consistency don't need consensus and are thus fundamentally different from the Paxos-based replication schemes that you might have seen.

jop
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  • Thanks a lot for your answer. Just to clarify a bit more...I have read in some papers that group communication protocols like uniform reliable multicast can also be used to ensure the total order then how multicast schemes and Paxos differentiate. Or I could say where should we use multicast and where Paxos? Any example will be great ... thanks again – Joarder Kamal Dec 27 '12 at 23:59
  • Don't confuse uniformity with order. Total order (aka atomic) multicast is what solves the same fundamental problem. But it is an abstract specification that must be implemented with a concrete protocol, most commonly with Totem, although Paxos is also an option. To choose among several protocols that solve the same fundamental problem (i.e., total order), you have also to consider practical concerns such as flow control, performance, stability, and so on, so there is no simple answer. – jop Dec 28 '12 at 22:34