Methods and apparatus are provided to combine responses to a plurality of user requests in a system that validates transactions. The system includes a plurality of nodes, a transaction response combiner (TRC), and a plurality of queues located in the TRC. Each queue is associated with one of the plurality of nodes. Each node executes an instance of the same application, and each node receives each one of the plurality of user requests. The application produces a response for each one of the plurality of user requests, and each response includes response details. In operation, the TRC receives a response from each of the nodes for each of the user requests. Each response is tagged with an identifier that corresponds with the respective user request. At least some of the tagged responses for one of the plurality of nodes is inserted into a queue. It is then detected when a response received at the TRC from one of the nodes has one or more corresponding responses in one or more of the queues by searching for responses that have the same tag. Rules of a rules engine are applied to combine the responses generated by the applications on the plurality of nodes for each of the user requests into a final response. The rules of the rules engine are applied to the corresponding responses with the same tag and the response details for the response that shares the same detected tag. The final response is then outputted by the TRC.