Delivery Guarantees

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Three levels: at-most-once (fire and forget, no retries), at-least-once (retry until acknowledged, may duplicate), and exactly-once (the hardest — requires idempotent consumers or transactional processing). Most systems provide at-least-once and rely on consumer idempotency for effective exactly-once behavior.

Key Properties

• At-Most-Once
• At-Least-Once
• Exactly-Once Semantics

Related

• Dead Letter Queues (handling messages that repeatedly fail)
• Retry with Exponential Backoff (retry strategy for at-least-once)

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distributed-systems messaging