Callback mechanism

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Revision as of 11:34, 8 July 2020 by Stankovicsa (talk | contribs) (Rate limiting)
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Payment callback mechanism (aka. IPN)

Whenever a given payment's state changes, it is expected that the caller system and the Barion database are synchronized. This is accomplished by implementing the callback mechanism (referred to as "Instant Payment Notification" or "IPN" in some terminology) between the two systems.

The callback process

The event flow of the implemented callback mechanism is simple:

  1. The payment gets completed, cancelled, expired, refunded, taken under investigation, or released from investigation
  2. The Barion system sends an HTTP POST request to the merchant's system to the URL defined by the merchant (in the CallbackUrl parameter of the /v2/Payment/Start API call) with the payment's unique identifier, indicating that something has just happened, so the merchant's system should check the payment now
  3. The merchant's system sends a request to the /v2/Payment/GetPaymentState API endpoint with its POSKey and the received payment identifier
  4. Based on the response received, the merchant's system can determine what processing tasks should take place

Structure and processing of the callback request

The callback request contains one parameter, the payment identifier. This is sent in the paymentId field of the POST request body. The merchant's system must send an HTTP 200 OK response (with any content) in order for the callback to be considered successful. The timeout period for answering a callback request is 15 seconds.

If the Barion system does not get an HTTP 200 response, it retries sending the callback for a maximum of 5 times, with exponential back-off timing delay between tries:

  • 2 seconds
  • 6 seconds
  • 18 seconds
  • 54 seconds
  • 2 minutes 42 seconds

So the total time window allocated for successful callback is roughly 4 minutes. If the Barion system fails to get an HTTP 200 response after that, the callback is not sent and the merchant's system automatically gets an e-mail notification about the error.

Every time, when your system receives a callback request from the Barion you have to call the /v2/Payment/GetPaymentState API endpoint to find out the change. First, your system should check the payment's status and after the list of the payment's transactions. For example, if you requested a payment refund or when the reserved payment's time limit has expired the list of payment’s transactions will contain new e-money transactions. It's your system's responsibility to handle changes.


Never rely on the callback function alone. When the callback URL is requested, your application must call the /v2/Payment/GetPaymentState Barion API to get the proper payment state.

Please refer to the Security Measures page for more information and a list of IP addresses.

The IP address of the callback request may change any time for security reasons.

Why you should use it

When a Barion Smart Gateway payment process takes place between the two systems, neither of them can rely purely on explicit user interaction to process things. In such situations, the Barion system must inform the merchant's system that the payment has changed in some way, without involving the user.

Here are some examples:

  • the payment is completed, but the user closed their browser or lose network connection, preventing them from returning to the webshop or application that redirected them to the Barion Smart Gateway
  • the user explicitly cancels the payment, but closes their browser instead of navigating back to the merchant
  • the payment is never completed and expires because of the time window limit
  • the payment is refunded

Implementing callback handling will save you time and pain tracking such "lost" payments. It also eases the troubleshooting with your customers, since you will always have valid data on your side.

Rate limiting

Currently only live in the test environment, production go-live date will be announced soon

When a payment has started we send a callback message about any status change. Then a getpayment request can be sent then and query the status of the payment. We experience some misuse about these getpaymentstate requests.

  • After payment start, mass getpaymentstate requests are sent to the same paymentID about one request/sec or even more, and do not wait for the callback. This request burst cause a great overhead on our systems which is absolutely pointless.
  • If a problem occurs during the payment and it does not finish successfully some getpaymentstate requests to that ID keeps going indefinitely with a 1, 2 or 5 minutes frequency. This frequency itself is not a problem, but as time passes there are more and more phantom requests which in sum can cause a great load.

Because of these two misuse of the getpaymentstate request we implemented rate limiting on calling getpaymentstate:

  • Because of the first problem we accept only two getpaymentstate request (with the same paymentID) in every 5 seconds. So the first two request will be served then every additional request during the 5 seconds interval will be denied with http status code 429 "Too many request". Then two request will be served again, and so on, like that.
  • Because of the second problem we start counting the getpaymentstate requests with the same paymentID. When this count reaches a certain limit during a well-defined time period the further requests with the same ID will be banned with the above http status code. Don't ask for the exact timeframe and limit count, since it is not public.

What NOT to do

Never long-poll the /v2/Payment/GetPaymentState API endpoint and wait for the payment status to suddenly change! If you use a high interval between requests, user experience will deteriorate significantly. If you use a small interval, your requests will get throttled.

If you are having difficulties implementing the proper callback flow, consult our developer support!