A method comprising performing, by a computer system: receiving, from a requesting server computer over a first network, a request message that indicates a request for access to a resource at a front-end computing device, the front-end computing device executing a first version of a software code; generating, by the front-end computing device, a request payload using the request message, the request payload having a plurality of request-payload fields; transmitting, by the front-end computing device, the request payload to a processor computer over a second network; receiving, by the front-end computing device, from the processor computer over the second network, a response payload that indicates a granting or denial of the request in the request message; generating, by the front-end computing device, a reply message using the response payload for sending to the requesting server computer, the reply message having a plurality of reply-message fields; providing, by the front-end computing device to a validation computing device executing in coordination with the front-end computing device, the request message, the request payload, and the response payload, the validation computing device executing a second version of the software code; generating, by the validation computing device, a modified request payload using the request message, the modified request payload having a plurality of modified request-payload fields; comparing the plurality of modified request-payload fields of the modified request payload with the plurality of request-payload fields of the request payload to identify discrepancies between the modified request payload and the request payload, the discrepancies corresponding to potential errors in the second version of the software code; generating, by the validation computing device, a modified reply message using the response payload, the modified reply message having a plurality of modified reply-message fields; and comparing the plurality of modified reply-message fields with the plurality of reply-message fields to identify discrepancies between the modified reply message and the reply message, the discrepancies corresponding to potential errors in the second version of the software code. receiving, from a requesting server computer over a first network, a request message that indicates a request for access to a resource at a front-end computing device, the front-end computing device executing a first version of a software code; generating, by the front-end computing device, a request payload using the request message, the request payload having a plurality of request-payload fields; transmitting, by the front-end computing device, the request payload to a processor computer over a second network; receiving, by the front-end computing device, from the processor computer over the second network, a response payload that indicates a granting or denial of the request in the request message; generating, by the front-end computing device, a reply message using the response payload for sending to the requesting server computer, the reply message having a plurality of reply-message fields; providing, by the front-end computing device to a validation computing device executing in coordination with the front-end computing device, the request message, the request payload, and the response payload, the validation computing device executing a second version of the software code; generating, by the validation computing device, a modified request payload using the request message, the modified request payload having a plurality of modified request-payload fields; comparing the plurality of modified request-payload fields of the modified request payload with the plurality of request-payload fields of the request payload to identify discrepancies between the modified request payload and the request payload, the discrepancies corresponding to potential errors in the second version of the software code; generating, by the validation computing device, a modified reply message using the response payload, the modified reply message having a plurality of modified reply-message fields; and comparing the plurality of modified reply-message fields with the plurality of reply-message fields to identify discrepancies between the modified reply message and the reply message, the discrepancies corresponding to potential errors in the second version of the software code.