Exactly Once
In plain terms
A delivery guarantee that each message is processed precisely once.
Going deeper
Exactly-once semantics ensure a message is neither lost nor processed twice, the hardest delivery guarantee to achieve. It's the gold standard, and the trickiest.
Under the hood
Usually approximated via idempotency plus at-least-once; true exactly-once needs careful coordination (e.g., Kafka transactions).
Examples
Kafka transactions, payment systems, stream processing, deduplication.
Think of it like...
Like guaranteeing a payment is made once, never lost, never doubled.