etcd Distributed Key-Value Store

etcd is a strongly consistent, distributed key-value store designed for reliably storing configuration data and coordinating distributed systems. Originally created at CoreOS in 2013 and now maintained by the Cloud Native Computing Foundation as a graduated project, it serves as the primary backing store and coordination service for Kubernetes clusters worldwide. The system is written in Go and uses the Raft consensus algorithm to ensure strong consistency and fault tolerance across a cluster of typically 3 or 5 nodes. Key features include a gRPC based API with watch capabilities for real-time change notifications, time-limited leases for distributed locks and leader election, multi-key transactions with compare and swap semantics, and automatic history compaction to control storage growth. Version 3.5 provides read consistency guarantees, defragmentation tools, and built-in snapshot and restore for disaster recovery. The store can handle millions of keys per instance and supports TLS encryption, role-based access control, and authentication for secure multi-tenant deployments. Production deployments at Google, AWS, and Microsoft Azure rely on it for critical cluster state management. The Apache 2.0 licensed project integrates with tools like Patroni for PostgreSQL high availability, Rook for storage orchestration, and CoreDNS for service discovery, making it a foundational building block of modern cloud native infrastructure.