Exactly-once is a desirable (if not critical) message delivery guarantee and a remarkably complex engineering challenge to solve. In this blog post, we will look at what exactly-once means in the context of distributed pub/sub systems, and the exactly-once guarantees that the Ably realtime pub/sub messaging platform provides. Ably often acts as the broker in data streaming pipelines: publishers send messages to our platform, and we deliver these messages to subscribers.