Async Processing
Background jobs, workers, idempotency under retries, and backpressure when producers outrun consumers.
Move work that does not have to finish inside the request — image processing, email, reports, AI generation, transcoding — onto a queue consumed by workers, and accept that a job which can be retried will eventually run twice.
A network gives you at-least-once delivery whether you like it or not; idempotency — the same request applied twice has the effect of once — is what turns "at least once" into the behaviour users mean by "exactly once".
When a producer emits 100,000 messages per second and the consumer handles 20,000, the queue grows by 80,000 per second and the oldest message is minutes old within minutes; backpressure is every mechanism that makes the producer feel the consumer's limit before memory, disk or latency does.