Idempotent Transactions

We recently were talking a lot about reversals this week in the OLS HQ, especially time-out reversals. Andy even mentioned his ever so famous “Refunds are not Reversals” So I was happy we were talking about reversals and not refunds ;)

Situation: What happens if you send a financial transaction to a payment system and we don’t get a response back? You are obligated to reverse it and keep on trying to reverse it (reversals are normally Store-and-Forwared (SAF) until you get a response back that the reversal was accepted.

You would be surprised how many implementations of payment software do not implement this important step, a disaster of not performing this is duplicate charges to cardholders during system or communication issues. This needs be be implemented in each path of a transaction. Terminal to Gateway, Gateway to Switch, Switch to EndPoint. for example. Many applications get-by, by ignoring reversals on Credit product types where cardholder have large open-to-buys and on Authorization Only Transaction Types. Reversal for Debit and and other financial transaction sets are a must.

On our Switch we handle Reversals with Idempotence. Wikipedia defines this as:

Idempotence ( /ˌaɪdɨmˈpoʊtəns/ eye-dəm-poh-təns) is the property of certain operations in mathematics and computer science, that they can be applied multiple times without changing the result beyond the initial application. The concept of idempotence arises in a number of places in abstract algebra (in particular, in the theory of projectors and closure operators) and functional programming (in which it is connected to the property of referential transparency).

Another website describes the problem as:

Problem: Network and server hardware failure can lead to lost messages, resulting in cases where a service consumer receives no response to its request. Attempts to reissue the request message can lead to unpredictable behavior within the service and the service consumer logic.

Solution : Design service capabilities capable of safely supporting repeated message exchanges.

Our implementation of reversals can handle multiple attempts of a reversal, we only process one but will accept any number of them. This is very important, Reversals are not “approved” or “declined” as the endpoint may or may not need to unwind anything. You as a caller don’t know whether the timeout was actually not processed at all, or if it was processed but you just didn’t get the response back.

We have the following ISO8583 v2003 based result codes in OLS.Switch for this so we can note the difference.

4000 Advice Accepted

4999 Advice Accepted – no Action Taken

That also means your logic is very simple – “send this reversal repeatedly on an interval until I get a response”

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