In general, controllers shine when you have complex relationships between the value that you directly control and the measure that you can about. In particular,

In general, controllers shine when you have complex relationships between the value that you directly control and the measure that you can about. In particular, "complex" meaning "well modeled with differential equations". In your scenario, since the rate of database requests is probably a very simple function of number (ie, well modelled by a simple constant multiple) of application requests, you probably don't need a "full controller" to implement the throttling.