etcd
Distributed key-value store via Raft consensus. CoreOS, 2013. Go. Powers Kubernetes's control plane (all cluster state stored there). Also used by many service discovery + config store systems.
Strong consistency: all reads see latest write. Raft quorum requires majority — 3 nodes tolerate 1 failure, 5 tolerate 2. Performance recommendation: SSDs are essential (fsync per write), network latency critical (every write = roundtrip to quorum). Watch API: subscribe on key changes — what Kubernetes uses for reconciler loops. Competitor: ZooKeeper (old, Java, more feature-heavy), Consul (HashiCorp, service discovery + KV). etcd v3 is current; v2 deprecated 2020.