Hi
I think in your case, what you need to do is that let the trigger post only the changed rate. Each changed rate will be in one row. The on-line application would call the web service to extract the changes in a given time period. Using the opening rate, the system would access the change-rate-table and see if that currency has been changed or not. If it has been changed, it would display the new rate. Since the change is infrequent, you may schedule a batch task on the server to refresh the rates every time a change occurs, so you don't have to do the logic in your code. Also, you could schedule another task to run 3 times a day to ensure that the Mainframe rates are in sync with the web server rates. All are extra options you can have.
This is my understanding.