What is an idempotency key in payment and write APIs?

Network failures make retries unavoidable. If your API creates money movement or any non-repeatable effect, a naive duplicate POST can double-charge. An idempotency key is a unique value the client sends (header or body) that identifies one logical request. The server records the first response keyed by that value and, on a repeat with the same key, returns the same result without re-running side effects. Providers like Stripe use this model widely. The key must be unique per ‘intent’, stored with TTL, and compared with a hash of the request body in strict designs so the key cannot be reused to mean different payloads.

POST /v1/payments
Idempotency-Key: 550e8400-e29b-41d4-a716-446655440000
Content-Type: application/json

Start simple: try this concept in a tiny project before moving to advanced tools.