At-Least-Once Delivery
A messaging guarantee where every message is delivered one or more times. Simpler than exactly-once but requires consumers to handle duplicates via idempotency.
What is At-Least-Once Delivery?
A messaging guarantee where every message is delivered one or more times. Simpler than exactly-once but requires consumers to handle duplicates via idempotency.
At-Least-Once Delivery is a intermediate-level concept that sits in the Messaging & Event Systems area of system design. Engineers reach for it whenever they need to reason about real-world trade-offs in that space — not just for textbook correctness, but because real production systems at companies like Netflix, Amazon, and Google make these decisions every day.
If you want to go deeper than this definition — with diagrams, code, and a quiz to lock it in — work through the "At-Least-Once Delivery" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.
Learn At-Least-Once Delivery in depth
Full interactive lesson with diagrams, code examples, real-world references, and a quiz.
Open the At-Least-Once Delivery lessonSee also
Related glossary terms you might want to look up next.
Exactly-Once Processing
A processing guarantee where each message is processed exactly one time, even in the face of failures. Achieved through idempotent consumers and transactional producers.
Idempotency
An operation that produces the same result whether you run it once or multiple times. Critical for safe retries in distributed systems.
Message Queue
A buffer that stores messages between producers and consumers. Messages are processed one by one, in order. Think of it as a to-do list for your services.