What’s the difference between Fault tolerance, Replication, Consistency & Consensus?

These four concepts are tightly coupled and form a causal chain.

  1. Fault tolerance is the goal: keep serving correctly even when machines crash or the network breaks.
  2. Replication is the mechanism: to not lose data when one machine dies, keep copies on several machines.
  3. Consistency is the problem replication creates: once you have copies, they can disagree. What is a client allowed to observe? (Always the latest write? Maybe a stale read? Never a “torn” state?)
  4. Consensus is the algorithm that solves it: getting N machines to agree on one value / one ordering of operations, despite some of them failing or messages being lost.