Accord Consensus Algorithm

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... called Accord (pdf) from a team at Apple and the University of Michigan. 

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Accord addresses two problems that aren’t solved in previous consensus protocols: How can we have a globally available consensus and achieve it in one round trip? The first novel mechanism is the reorder buffer.

Assuming commodity hardware is in use, differences in clocks between nodes are inevitable. The reorder buffer measures the difference between nodes in addition to the latency between them. Each replica can use this information to correctly order data from each node and account for the differences, guaranteeing one round-trip consensus with a timestamp protocol.

The other mechanism is fast-path electorates. Failure modes can create latency when electing a new leader before resuming. Fast-path electorates use pre-existing features in Cassandra with some novel implementations to maintain a leaderless fast path to quorum under the same level of failure tolerated by Cassandra.