Distributed

Exactly-Once Processing

At-least-once delivery combined with idempotent consumers that deduplicate, committing the dedup record with the side effect.

Why interviewers ask about it

Exactly-once delivery is impossible - the sender cannot distinguish a lost message from a lost ack. Saying that precisely is the whole point.

A client-generated unique key stored with a uniqueness constraint so a retry replays the stored response instead of re-executing.

Write the event as a row in the same transaction as the state change; a relay publishes it afterwards.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.