A method of simulating device state changes in an integrated system includes receiving a transaction request from a client device, storing the transaction request as a first event in an event log, transmitting the transaction request to a terminal device, storing the transmission of the transaction request as a second event in the event log, receiving a device response from the terminal device, storing the device response as a third event in the event log, and when the integrated system is under test, a simulator replays the stored events in the integrated system under test.