Database Internals

Chapter 14. Consensus

We’ve discussed quite a few concepts in distributed systems, starting with basics, such as links and processes, problems with distributed computing; then going through failure models, failure detectors, and leader election; discussed consistency models; and we’re finally ready to put it all together for a pinnacle of distributed systems research: distributed consensus.

Consensus algorithms in distributed systems allow multiple processes to reach an agreement on a value. FLP impossibility (see “FLP Impossibility”) shows that it is impossible to guarantee consensus in a completely asynchronous system in a bounded time. Even if message delivery is guaranteed, it is impossible for one process to know whether the other one has crashed or is running slowly.

In Chapter 9, we discussed that there’s a trade-off between failure-detection accuracy and how quickly the failure can be detected. Consensus algorithms assume an asynchronous model and guarantee safety, while an external failure detector can provide information about other processes, guaranteeing liveness [CHANDRA96]. Since failure detection is not always fully accurate, there will be situations when a consensus algorithm waits for a process failure to be detected, or when the algorithm is restarted because some process is incorrectly suspected to be faulty.

Processes have to agree on some value proposed by one of the participants, even if some of them happen to crash. A process is said to be correct if hasn’t ...