Distributed cron: once, on schedule, while servers die

Why a scheduled job runs three times on three servers, why locks with and without expiry both fail, and how leader election with leases, fencing tokens, heartbeats, stuck-job detection and idempotent, resumable jobs get you effectively-once execution. Then the leader dies mid-job, the real-world tools, and every Week 2 pattern inside one order-processing job.