Exactly-Once Delivery

ELI5 — The Vibe Check

Exactly-once delivery is the holy grail of messaging — every message delivered exactly one time, no more, no less. In distributed systems, it's technically impossible without some tricks like transactions or idempotency. It's like promising a package arrives exactly once in a world of unreliable roads.

Real Talk

Exactly-once delivery guarantees each message is processed exactly one time. True exactly-once is impossible in distributed systems (due to the Two Generals Problem), but it can be approximated using idempotent consumers, transactional outbox patterns, or Kafka's exactly-once semantics (EOS) which combines idempotent producers with transactional consumers.

When You'll Hear This

"Kafka's exactly-once semantics use transactional producers and consumer offsets together." / "We achieve effectively-exactly-once by combining at-least-once delivery with idempotency keys."