API idempotency

API idempotency overview

When issuing requests to an API, there is always a small chance of issues on either side of the connection.

For example, the API may not respond to the request within a reasonable timeframe. Your server will then consider the request to have ‘timed out’. However, your request may still arrive at the API eventually and get executed, despite your server considering it a timeout.

Another example: perhaps you did receive the response correctly, however due to an issue on your side — for example due to a software bug or hardware failure — you failed to store the results from the API correctly.

In most API integrations, this class of issues is usually handled by simply retrying the request. There are cases, however, where this may lead to issues. For example, when executing a recurring payment on the Mollie API, retrying a failed request may lead to the consumer being charged twice.

In the Mollie API, these sorts of problems can be solved in two ways.

  1. If an API request fails, you can actively inspect the API (using its ‘list’ endpoints) to see if the request got executed.
  2. Or, you can send a unique value with every API request via the Idempotency-Key header. If two requests come in with the same value, the second request is considered a duplicate that can be ignored. This makes your requests idempotent.

Relevance for the Mollie API

In the Mollie API, there is in most cases no harm in simply retrying a request without idempotency checks. For example, when setting up a regular payment, setting up the payment twice will just result in one of the two payments eventually expiring.

Retrying can potentially be harmful only in the following cases:

  • Recurring payments. When issuing a payment with sequenceType:recurring, the payment will get executed using a previously established authorization from the consumer. In other words, it gets executed immediately. Issuing these requests twice can lead to double charges.
  • Subscriptions. Similar to recurring payments, creating a subscription will lead to recurring charges on a pre-existing authorization mandate. Creating a subscription twice will lead to double charges during the entire duration of both subscriptions.
  • Partial refunds. A completed payment can be reversed fully or partially, for example if only some parts of the order were returned by the consumer. Since there is a limit to the refund amount, generally executing a full refund twice is not possible. When issuing partial refunds, however, executing the refund twice may lead to two separate partial refunds that will both get executed.

Using an idempotency key

Mollie supports the Idempotency-Key industry standard. When sending a request to the Mollie API, you can send along a header with a unique value. If another request is made with the exact same header value within one hour, the Mollie API will return a cached version of the initial response. This way, your API requests become what we call idempotent.

A simple implementation looks as follows:

  1. You generate a unique value. We recommend using UUID4 to guarantee uniqueness across multiple processes or servers.
  2. When you create your API request, attach the unique value to it using header Idempotency-Key: <value>.
  3. If your request faces for example a network issue, simply retry the same request with the exact same Idempotency-Key value.

A basic example in PHP

$mollie = new \Mollie\Api\MollieApiClient();

$key = generate_uuid4(); // Your own key generating function

$parameters = [
    "amount" => ["currency" => "EUR", "value" => "10.00"],
    "description" => "Order 12345",
    "customerId" => "cst_8wmqcHMN4U",
    "sequenceType" => "recurring",
    "idempotencyKey" => $key,

try {
  $payment = $mollie->payments->create($parameters);
} catch (\Mollie\Api\Exceptions\CurlConnectTimeoutException $e) {
  // Try once more with the same idempotency key
  $payment = $mollie->payments->create($parameters);

All POST endpoints accept idempotency keys. Sending idempotency keys for GET, PATCH, or DELETE requests is not necessary since these API requests are repeatable by nature. These keys will thus always be ignored by the Mollie API.

Keys older than 1 hour will be removed from our cache. If the same key is used again after an hour, it will be considered a new request.

You can recognize whether the Mollie API returned a cached response by the presence of the Idempotent-Replayed: true header.

Handling errors

If you send the same idempotency key for requests to two different endpoints or for requests with different parameters, the API will return a 400 Bad Request HTTP response.

In rare cases, sending the same request with the same idempotency key twice may result in a situation where the second request comes in while the first request is still being processed by the Mollie API. If this is the case, the API will respond to the second request with a 409 Conflict HTTP response.

In the case of a severe network disruption on either side, a single retry may not prove sufficient. When retrying requests we therefore recommend implementing exponential back-off, i.e. adding an exponentially increasing delay between each attempt until the API responds again. This way you can be certain the request will eventually get executed, while preventing flooding the Mollie API with requests and hitting rate-limits.