Glossary term
What is at-least-once (At-least-once delivery)?
at-least-once stands for At-least-once delivery.
A message may arrive more than once, but it is never lost. So consumers must be idempotent.
Learn it properly · Practical Design ProblemsLesson 5: Design a Notification System (In-App, Push, Email)Polling, long polling, server-sent events and WebSockets, fan-out with queues, read state, user settings, retries and removing duplicates.