# Idempotency

Source: https://docs.revoplyai.com/get-started/idempotency/

> How an Idempotency-Key makes flow trigger retries safe, how long keys are remembered, and how to deduplicate our events.

Networks lose answers. A request you retry may already have been accepted, and an event we
retry may already have reached you. Both directions have a way to make a repeat harmless.

## Requests you send to flow triggers [#requests-you-send-to-flow-triggers]

Send an `Idempotency-Key` header with every request to a [flow trigger](/flow-triggers/)
URL. Use an id your system already has for the event, such as
`order-10482-shipped`, and send the same key on every retry of that event.

* A request whose key already started a run of the same flow starts nothing new. It is
  answered `200` with `"status": "duplicate"` and the `id` of the run the first request
  started, whether that run is still going or has ended.
* Keys are scoped to the flow: the same key sent to another flow's URL starts that flow.
  Replacing a flow's URL keeps its keys.
* A key is remembered for as long as the run it started exists. Ended runs are deleted
  90 days after they end, and the key goes with them.
* Only a request that started a run is remembered. Any other answer (`invalid_phone`,
  `flow_off`, `rate_limited`, `service_unavailable`…) leaves nothing behind, so retrying
  with the same key after fixing the cause works.
* The body is not compared. A second request with the same key and a different body is
  answered `duplicate` and its body is ignored.
* Keys are at most 200 characters (`400 idempotency_key_too_long` otherwise), compared
  exactly and case-sensitively after leading and trailing spaces are removed. An empty
  header is the same as none.

Without a key, every accepted request starts a run, including a retry of one that was
already accepted.

## Events we send you [#events-we-send-you]

Webhook delivery is at-least-once: the same event can reach you more than once, after a
retry or a resend. Every delivery of one event carries the same envelope `id`
(`evt_…`). Record the ids you have handled and acknowledge a repeat with `2xx` without
acting on it again.

Keep ids for at least as long as we might send the event again: retries run for about two
days, and any delivery in the 30-day delivery log can be resent by hand. See
[Retries and the delivery log](/webhooks/retries-and-delivery-log/).