Event Delivery Semantics & Dead Letter Queues
What does “delivered” guarantee — and what about messages that can't be processed?
Networks drop packets and consumers crash, so async messaging has to define both.
The three semantics
- At-most-once: send & forget; may be lost (fine for metrics).
- At-least-once: retry until acked; may duplicate — the common default.
- Exactly-once: expensive; in practice = at-least-once + an idempotent consumer that dedups by message id, so the duplicate is delivered but its effect happens once.
Ordering
Global ordering is costly; Kafka gives ordering within a partition — route related messages with the same partition key.
Dead Letter Queue
A “poison” message that fails every time would block the queue. After N retries, move it to a DLQ — a side queue for inspection/replay — so the main queue keeps flowing.
Takeaways
- At-least-once + idempotent consumers is the pragmatic default; dedup by message id.
- Ordering is per-partition; a DLQ quarantines poison messages.
Re-authored from-scratch; diagram hand-authored (SVG) for this guide.
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