Handle asynchronous Lambda retries and discarded events as separate outcomes

A successful handler response does not guarantee that an event will never be delivered again.

Source facts

Lambda manages an asynchronous event queue and retries according to the error and configured policy. Duplicate delivery can occur even without a function error because the queue is eventually consistent. Events can age out or exhaust processing attempts. AWS distinguishes dead-letter queues for discarded events from on-failure destinations that retain failed-invocation records.

Source: AWS Documentation.

Applicability

This applies to asynchronous Lambda invocation, not every queue-based event source or synchronous client retry.

DSE recommendation

DSE recommends making the handler safe to repeat before tuning retries. Identify the business key that prevents duplicate external effects and define ownership for discarded events. Review maximum event age, retry behavior and destination permissions with the workload owner. Keep a controlled replay procedure that reuses the same duplicate-protection logic. Monitor capacity as well as handler errors so a growing queue does not hide work that is aging toward discard. Decide whether the retained failure record is enough for the approved replay procedure, and name the person allowed to initiate replay. Keep the business deduplication identifier available to that process so an investigation does not turn a repeated event into a second customer action.

Verification

Proposed verification: send disposable events through a controlled failing handler, inspect retry and failure routing, and replay an event deliberately. Confirm the intended final state without duplicated side effects. Record the actual configuration rather than assuming the documented default policy was retained.

Official reference

AWS Documentation. Reviewed October 20, 2026.