Repeated input and repeated effect are different problems#
Deduplication detects a repeated event ID or business key. Idempotency ensures repeating an operation does not create an unintended second effect. A service may dedupe within a time window yet still need a unique business constraint.
| Technique | Strength | Cost |
|---|---|---|
| Event-ID table | Explicit message identity | Storage and atomicity |
| Business key | Protects domain invariant | Requires correct semantic key |
| Unique constraint | Race-safe rejection | Must share the side-effect store |
| TTL record | Bounds storage | Duplicates outside window |
Related practice problems
Use the manifest-backed prompts below to apply this lesson in a complete design.
- Kafka design and delivery semanticsApache Kafka Documentation
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