Queues and background jobs

Instead of doing everything inside the request, the app adds a job to a queue ('send the receipt for order 42') and responds at once. Worker processes take jobs off the queue and run them, retrying with growing delays when something fails and moving jobs that keep failing to a dead-letter queue for someone to inspect.

Common tools include BullMQ (Node.js on Redis), Celery (Python), Laravel's queues, Amazon SQS, RabbitMQ and Cloudflare Queues, plus HTTP-based services such as QStash, Inngest and Trigger.dev that suit serverless apps. Most deliver each job at least once, so a job can occasionally run twice and its handler should be idempotent.