##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
When only one instance should perform a task (cron jobs, coordination), elect a leader. If leader fails, another takes over.
Implementations: Zookeeper ephemeral nodes, etcd leases, Redis SETNX with TTL, database row locking. Leader sends heartbeats. Followers watch for missed heartbeats and trigger election.
