An operation that can be safely repeated without changing the result.
Critical for retries. 'Set balance to $100' is idempotent; 'Add $10' is not.
An operation you can safely apply more than once and get the same result: the foundation of every retryable system.
Idempotency keys on every charge request, Stripe-style. Duplicate POSTs return the original response, no double-charge
At-least-once delivery is the default. Consumers must be idempotent, processing the same message twice without effect
Send-notification accepts an idempotency key so a flaky publisher's retries don't spam the user