Etcd

etcd is a distributed reliable key-value store developed by CoreOS (now part of Red Hat) and first released in 2013. It is the default coordination backend for Kubernetes and serves as the configuration store and service discovery layer for the entire container orchestration ecosystem. Where ZooKeeper offers a filesystem-like znode tree and a custom protocol (ZAB), etcd exposes a simple HTTP API and implements consensus through the Raft protocol — a deliberate choice that trades ZooKeeper's mature ecosystem for operational simplicity and standard interfaces.

The design philosophy is minimalism. etcd stores small amounts of data — configuration, service endpoints, leader leases — and replicates them across a cluster using Raft's leader-follower model. A write is acknowledged only after it has been persisted by a quorum of nodes, ensuring linearizability. Reads can be served by any node, but etcd provides a 'quorum read' option that routes the request through the leader to guarantee recency. This is a CP system in the CAP theorem framework: it favors consistency over availability during network partitions.

etcd's greatest impact has been as Kubernetes' brain. Every Kubernetes cluster stores its entire state — pods, services, deployments, secrets — in etcd. If etcd fails, the cluster cannot schedule new work, cannot detect failures, and cannot reconcile desired state with actual state. This makes etcd arguably the most consequential distributed system in production today, despite being one of the smallest in scope. It is the ground on which the container orchestration layer stands.

The system's simplicity is also its limitation. etcd does not provide the rich coordination primitives that ZooKeeper offers — no distributed locks, no barriers, no queues out of the box. These must be built on top of its compare-and-swap operations. The tradeoff is explicit: etcd gives you a linearizable key-value store and expects you to build everything else yourself. For Kubernetes, this is sufficient. For more complex coordination problems, it is not.