Idempotence in the context of Web APIs refers to operations which when repeated provide exactly the same result. In practical terms, if my intention is to create a single item using a remote API and my first attempt provides a network error response, I need to know that when I retry I will not end up creating two or more identical items. At Invopop, we take this very seriously and have designed support for idempotency in all API calls. REST actions like GET and DELETE are inherently idempotent, fetching or deleting the same item twice will simply result in the same response. Update actions, such as PATCH are also usually idempotent. Duplicating calls may be inefficient, but we’ve made sure that in our API update actions are safe to be repeated. Create actions, like POST or PUT however require additional guarantees to ensure data will not be duplicated. The Invopop APIs handle these scenarios by requiring that every model and thus every incoming operation includes either an id field in UUID string format, or often a key property that will be checked to ensure it is unique before processing. Many API endpoints will detect duplicated requests and simply return the same response as expected initially. If however this cannot be guaranteed, the API will return a 409 Conflict response which the client should be able to handle.