etcd Key-Value Store

etcd is a distributed, reliable key-value store for the most critical data of a distributed system, designed to be highly available, strongly consistent, and fault-tolerant. Originally created by CoreOS (now part of Red Hat) in 2013 by Xiang Li and Kamil Choudhury, with over 48,000 stars as of 2026, etcd is the backbone of Kubernetes cluster state management. Key features include: Raft consensus (Raft consensus algorithm for distributed log replication ensuring strong consistency across cluster nodes with leader election, log replication, and safety guarantees under network partitions), key-value API (gRPC API with Put, Get, Delete, Range, Watch, and Transaction operations, plus REST API gateway for HTTP access), watch API (long-lived watch subscriptions that receive events for key changes in real-time, enabling reactive distributed system design), transactions (atomic compare-and-swap operations with multi-key conditional updates using Txn API, implementing optimistic concurrency control), leases (time-limited key associations with automatic expiration via TTL, used for distributed locks, leader election, and service registration), MVCC (multi-version concurrency control preserving history of all key revisions, enabling consistent reads at any past revision without blocking writes), compaction (history compaction to remove old revisions, reclaiming storage space while preserving the most recent value per key), clustering (dynamic cluster membership with peer discovery via DNS, initial cluster, or discovery service, supporting 3 to 7 node clusters for production), TLS (mutual TLS for client-server and peer-to-peer communication with certificate rotation), authentication (role-based access control with users, roles, and permissions for individual keys and key ranges), and gRPC gateway (HTTP/JSON proxy enabling REST-like access to the gRPC API).