Express Idempotency Middleware and Client Retry Semantics

The integration objective is to establish how an Express server should process client-initiated HTTP retries when an idempotency key is missing, expired, or mismatched, thereby preventing duplicate write operations across retry attempts. Express does not include native retry handling, and idempotency middleware typically depends on an external datastore such as Redis to track key lifecycles, creating a coordination point between the application logic and the key store. Express 4.x and 5.x share the same middleware APIs, yet version-specific deprecations or runtime behavior may affect how retries are interpreted.

Critical uncertainty surrounds the server's default action when a retry arrives without a valid key: should the request be rejected, queued, or treated as a new idempotent operation? Additionally, the expiration policy for stored idempotency keys influences whether a retry is accepted or silently dropped, and the availability state of the backing datastore during a retry can change the server's response path without clear guidance in the Express documentation.

• What criteria should an Express endpoint apply to differentiate a legitimate retry from a new request when the idempotency key has expired?
• How should the server semantically and operationally respond when the idempotency datastore is unreachable during a retry attempt?
• Does the selection of idempotency middleware (token-based versus store-based) fundamentally alter the retry safety contract between client and server?