etcd — Distributed Reliable Key-Value Store for Critical Data

What etcd Does

• Distributed KV — reliable storage with strong consistency
• Raft consensus — leader election and log replication
• Watches — subscribe to key changes in real time
• Leases — TTL-based key expiration
• Transactions — atomic multi-key compare-and-swap
• MVCC — multi-version concurrency control, historic reads
• gRPC API — binary protocol for performance
• Dynamic reconfiguration — add/remove members at runtime

Architecture

A cluster of 3 or 5 etcd members runs the Raft protocol. One is elected leader, others are followers. Writes go through the leader and are replicated. Reads can be linearizable (through leader) or serializable (local). Key revisions use MVCC.

Self-Hosting

# 3-node cluster
etcd --name infra1 --initial-advertise-peer-urls http://10.0.1.10:2380 \
     --listen-peer-urls http://10.0.1.10:2380 \
     --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
     --advertise-client-urls http://10.0.1.10:2379 \
     --initial-cluster-token etcd-cluster-1 \
     --initial-cluster infra1=http://10.0.1.10:2380,infra2=http://10.0.1.11:2380,infra3=http://10.0.1.12:2380 \
     --initial-cluster-state new

Key Features

• Strong consistency (Raft)
• Watches for change notifications
• Lease TTLs
• MVCC historical reads
• Transactions (compare-and-swap)
• Role-based access control
• TLS encryption
• Dynamic cluster reconfiguration
• gRPC client protocol

Comparison

FAQ

Q: How many cluster nodes? A: An odd number (3, 5, 7) to form a majority. 3 nodes tolerate 1 failure, 5 tolerate 2. More nodes increase write latency.

Q: etcd vs Consul? A: etcd focuses on KV + strong consistency; Consul is broader (service discovery, health checks, service mesh). K8s uses etcd because it only needs reliable KV storage.

Q: Data size limit? A: 2GB by default, tunable up to 8GB. If you exceed that, split into namespaces or use another solution. etcd is not a general-purpose database.

Sources

• Docs: https://etcd.io/docs
• GitHub: https://github.com/etcd-io/etcd
• License: Apache 2.0