Preventing Duplicate Password-Reset Notifications (Under SMS Timeout and Retry Pressure)

Treat an SMS timeout as an unknown outcome, not a failed send: accept each password-reset event once, persist its expiry and idempotency key before dispatch, and retry only through a worker that can reconcile the original attempt. For a short-lived e-commerce reset token, compliance evidence is the deciding constraint. The system must be able to show what it accepted, what it attempted, when it stopped, and why, without storing the token or message body in an audit log. This changes the shape of the endpoint. A Node.js Express handler may receive the event, but it shouldn't hold the HTTP request open while an SMS provider decides the final delivery state. Return an accepted response after durable admission, then expose status from local state. The Go example below shows the same transport-independent contract because the hard part isn't an Express API call; it's controlling ownership of retries. One event, one logical notification. How should event notifications handle SMS timeout, retry, and duplicate sends? Use two identifiers with different jobs. event_id identifies the business action, such as one password-reset request. idempotency_key identifies the logical notification command. A unique constraint on the key makes two concurrent HTTP requests converge on one stored record; checking memory before an insert is not enough because two processes can pass that check together. A timeout leaves three possible realities: the provider never accepted the request, it accepted the request but the response was lost, or it accepted and sent the message before the caller stopped waiting. Retrying immediately as though the first case were certain is how customers receive two reset messages. Declaring success is no better. The durable record should therefore enter dispatch_unknown , keep the provider's attempt identifier when one exists, and move through reconciliation before another send can be authorized. Status polling serves a different purpose from retry. Polling reads th