Courses/Distributed Systems

Distributed Systems

CS3.401

Prof. Kishore Kothapalli•Monsoon 2025-26•4 credits

Revision Notes/Unit 9 — Raft Consensus

Unit 9 — Raft Consensus

Raft solves the replicated state machine problem under crash failures (not Byzantine). Decomposed into three sub-problems: Leader Election (randomised timeouts, terms), Log Replication (AppendEntries + majority commit), and Safety (election restriction — only most-up-to-date logs can be elected). Production-grade — backs etcd, Consul, CockroachDB.

Leader Election + Log Replication + Safety

16 min

Raft is **Paxos rewritten for understandability** (Ongaro and Ousterhout, 2014). Same correctness as Paxos; simpler decomposition. The cluster keeps a replicated log via three independent mechanisms: random timeouts elect a single leader per term; the leader replicates each new entry to a majority before committing; an 'election restriction' ensures the new leader has every committed entry from prior terms.