Adaptive Strategies for Speeding Up Sequences of Consensus

The Paxos-MIC protocol allows to solve a sequence of consensus instances in an unreliable asynchronous system. It follows the basic principles of Paxos and uses two optimizations of this protocol: a safe one, So that is always activated and a risky one, Ro. The paper focuses on the interest of Ro that has been introduced by Lamport in the Fast Paxos protocol. We study the optimization Ro in favorable and unfavorable scenarios where it may lead to an additional cost. Paxos-MIC is adaptive as it tries to obtain the best performance gain depending on the current context. Indeed, between two consecutive consensus instances, the leader determines if Ro has to be triggered or not. In the particular context of a secure Web architecture whose design relies on a consensus service, we use a trace that contains all the HTTP requests addressed to a real Web site during a period of 16 days. We propose different triggering criteria and we analyze their accuracy to predict collisions when proposed values are produced at the rate observed in this trace.