Leader election

Where it shows up in real systems

• Scheduler

  Use it for nightly reports, cleanups and retries. A single scheduler is a single point of failure; managed schedulers or leader election avoid running the same job twice.

  In production: Amazon EventBridge Scheduler, Kubernetes CronJob, Celery beat, cron

• Document database

  A replica set elects a new primary when the current one fails. With write concern majority, an acknowledged write survives that failover; with w:1, recent writes can be rolled back. Reading from secondaries scales reads but returns slightly stale data.

  In production: MongoDB Atlas, Amazon DocumentDB, Couchbase, Firestore

• Coordination service

  Runs Raft or ZAB on an odd number of nodes: three nodes survive one failure, five survive two. Use it for small, critical data, never as a general database.

  In production: etcd, Apache ZooKeeper, Consul KV