Job scheduler: on time, and harmless when twice

One strong interview answer to cron as a service: 100 million jobs in 1,024 leased partitions, a fenced conditional claim and an outbox, a timing wheel for the next minute, look-ahead, jitter and fair queues for the top-of-the-hour spike, and an honest at-least-once guarantee made harmless by leases, attempt-number fencing, run keys and misfire policies.