Core Vocabulary (First Principles)

• Producer: creates messages and sends to a topic/queue.
• Broker: the server/cluster that persists, routes, and delivers messages.
• Consumer: receives messages; may be part of a consumer group to scale out.
• Ack (acknowledgment): consumer’s signal “processed; you can remove it.” Systems redeliver unacked messages.

Delivery modes: at-least-once (may duplicate), at-most-once (may drop), exactly-once (hard; some platforms offer it with constraints).