How should i design best caching architecture

Assume that you are developing a web application that shows currency exchange rates for a bank web site. The rates are stored in an IBM Mainframe in the bank and rates are available through web services. Each time a user accesses the rates page, the page makes a request to the mainframe. This generates too much load on the mainframe. Especially most of the time the rates delivered is the same. But at the same time rates may fluctuate within the day and if rates have changed the web page should not display the cached values but make another request to the web service. The rates table at mainframe supports triggers where each update to the rates table can invoke a trigger where the trigger may further call a web service. How would you design such a caching architecture and make sure it is invalidated on rate changes? Please explain by drawing a diagram.