Suggest an editImprove this articleRefine the answer for “What does "idempotency" mean in the context of events?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**Idempotency** in the context of events means that processing the same event several times produces the same result, without repeated side effects. **Key point:** it protects an event-driven architecture from duplicate messages, guaranteeing that even with redelivery, the result stays correct.Shown above the full answer for quick recall.Answer (EN)Image**Idempotency** in the context of events means that **processing the same event several times produces the same result**, without repeated side effects. ### Why this is needed In event-driven systems the same message can be delivered again, for example because of a network failure, a timeout, or a re-publication by the broker. If the consumer is not idempotent, a duplicate event can: - create the order twice, - charge the money twice, - send the notification twice. ### How it is achieved 1. **A unique event identifier** (`event_id`). Before processing, the system checks whether it was already processed. 2. **Storing a history of processed events** (a "processed_events" table). 3. **Checking the state before acting**, for example "if the order is already paid, do not repeat it". 4. **Idempotent database operations**: `INSERT ... ON CONFLICT DO NOTHING`, `UPSERT`, `SET` instead of `ADD`. ### Example The `PaymentReceived` event arrives twice. The service checks `payment_id` in the table: if it was already processed, it simply ignores the duplicate. **Summary:** > **Idempotency** protects an event-driven architecture from duplicate messages. > It guarantees that even with redelivery, the result stays correct and the system stays consistent.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.