> ## Documentation Index
> Fetch the complete documentation index at: https://developers.dwolla.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Idempotency Key

> Generate unique keys to prevent accidental duplicate transactions and ensure predictable API behavior. Build robust and reliable financial applications with confidence.

## Overview

To prevent an operation from being performed more than once, Dwolla supports passing in an `Idempotency-Key` header with a unique key as the value. Multiple `POST`s with the same idempotency key and request body won't result in multiple resources being created. It is recommended to use a random value for the idempotency key, like a UUID (i.e. - `Idempotency-Key: d2adcbab-4e4e-430b-9181-ac9346be723a`).

For example, if a request to [initiate a transfer](/docs/api-reference/transfers/initiate-a-transfer) fails due to a network connection issue, you can reattempt the request with the same idempotency key to guarantee that only a single transfer is created.

If you reattempt a `POST` request with the same value for the `Idempotency-Key`, rather than creating new or potentially duplicate resources, you will receive a `201 Created`, with the original response of the created resource. If the Dwolla server is still processing the original `POST` request, you will receive a `409 Conflict` error response on the subsequent request. Multiple `POST`s with the same idempotency key including an **exact match** request body won't result in multiple resources being created. Idempotency keys are intended to prevent conflicts over a short period of time, therefore keys which are paired with a request body expire after 24 hours.

<Info>
  To prevent resources from being created more than once, we highly recommend making all requests idempotent.
</Info>

### Example transfer using an Idempotency Key

```bash theme={"dark"}
POST https://api-sandbox.dwolla.com/transfers
Accept: application/vnd.dwolla.v1.hal+json
Content-Type: application/vnd.dwolla.v1.hal+json
Authorization: Bearer pBA9fVDBEyYZCEsLf/wKehyh1RTpzjUj5KzIRfDi0wKTii7DqY
Idempotency-Key: 19051a62-3403-11e6-ac61-9e71128cae77
{
    "_links": {
        "destination": {
            "href": "https://api-sandbox.dwolla.com/funding-sources/04173e17-6398-4d36-a167-9d98c4b1f1c3"
        },
        "source": {
            "href": "http://api-sandbox.dwolla.com/funding-sources/707177c3-bf15-4e7e-b37c-55c3898d9bf4"
        }
    },
    "amount": {
        "currency": "USD",
        "value": "1337.00"
    }
}
...
HTTP/1.1 201 Created
Location: https://api-sandbox.dwolla.com/transfers/74c9129b-d14a-e511-80da-0aa34a9b2388

```