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Safely retrying API requests

Lawrence Jones
Written by

Last editedJan 2020

Today we're announcing support for idempotency keys on our Pro API, which make it safe to retry non-idempotent API requests.

Why are they necessary?

Here's an example that illustrates the purpose of idempotency keys.

You submit a POST request to our /payments endpoint to create a payment. If all goes well, you'll receive a 201 Created response. If the request is invalid, you'll receive a 4xx response, and know that the payment wasn't created. But what if something goes wrong our end and we issue a 500 response? Or what if there's a network issue that means you get no response at all? In these cases you have no way of knowing whether or not the payment was created. This leaves you with two options:

  • Hope the request succeeded, and take no further action.
  • Assume the request failed, and retry it. However, if the request did succeed you'll end up with a duplicate payment.

Not an ideal situation.

Idempotency Keys

To solve this, we've now rolled out support for idempotency keys across all our creation endpoints. Idempotency keys are unique tokens that you submit as a request header, that guarantee that only one resource will be created regardless of how many times a request is sent to us.

For example, the following request can be made repeatedly, with only one payment ever being created:

Idempotency-Key: PROCESS-ME-ONCE
  "payments": {
    "amount": 100,
    "currency": "GBP",
    "charge_date": "2015-06-20",
    "reference": "DOLLAR01",
    "links": {
      "mandate": "MD00001EKBQ412"

If the request fails, then it's perfectly safe to retry as long as you use the same idempotency key. If the original request was successful, then you'll receive the following response:

HTTP/1.1 409 (Conflict)
  "error": {
    "code": 409,
    "type": "invalid_state",
    "message": "A resource has already been created with this idempotency key",
    "documentation_url": "",
    "request_id": "5f917bf9-df56-460f-a165-15d9e77414cb",
    "errors": [
        "reason": "idempotent_creation_conflict",
        "message": "A resource has already been created with this idempotency key",
        "links": {
          "conflicting_resource_id": "PM00001KKVGTS0"

It's worth noting that we haven't added support for idempotency keys to our update endpoints, as they're idempotent by nature. For example, trying to cancel the same payment multiple times will have no adverse effect.

We're constantly improving our API to provide a better experience to our integrators. If you have any feedback or suggestions then get in touch, we'd love to hear from you!

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