Design an idempotent job queue that processes background tasks exactly once despite retries and crashes.

Background jobs fail, retry, and get redelivered. If a job sends an email or charges a card, running it twice is a real problem. The design is at-least-once delivery plus idempotent handlers, visibility timeouts, and dead-letter queues. Here is how to make jobs safe.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.