Paxos, is the famous synonym for consistency in the context of distributed system. Unfortunately, consistency alone, is such an overloaded term, it's

Paxos vs. Quorum-based Consistency

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2021-07-17 03:30:05

Paxos, is the famous synonym for consistency in the context of distributed system. Unfortunately, consistency alone, is such an overloaded term, it's often practically meaningless. In this post, I will explain the difference between Paxos-consistency vs. quorum-consistency.

I assume you know what Paxos is and what problem it solves. If you need a quick refresher, or you are in the mood of trying to read about a new way of explaining how Paxos works, click this link.

When I say quorum-based consistency, I mean systems like Dynamo, Cassandra, etc. that claim to support "strong" consistency when you read and write to a quorum of the replica. E.g. if a shard has a replication factor of 3, we say read and write has quorum if every time you write to 2 replicas and read from at least 2 replicas synchronously. It seems straightforward. You always read the latest value – "strong consistency" (hopelessly overloaded). Or really?

If there's a single keyword in distributed system, it's order. Multi-Paxos or Raft provides linearizability – an unambiguous description of one kind of "strong consistency" (applied to simple k/v stores without worrying about transactions).