Idempotency

Idempotency, in the context of Kafka consumers, is the ability to process the same event multiple times without producing duplicated side effects — crediting a balance twice, sending two emails, generating two charges. It's a requirement, not a nice-to-have, because at-least-once guarantees (the most common setup in production) imply the same event can be delivered more than once, due to retries, rebalances, or failures between processing and committing the offset. The most common implementation uses a unique event identifier (eventId) and a database uniqueness constraint, checked and applied within the same transaction that executes the business effect.