Consider the following two designs for the elevator problem.

a. The elevator control system, acting as a controller, assigns an elevator to an incoming request by iterating through the elevators, asking each elevator for its current location, direction, and so on (state). It then uses that information to determine which elevator is closest and assigns the request to that elevator.

b. The elevator control system asks each elevator to compute its proximity to a request. Using its location, direction, and so on (state variables), each elevator makes its own determination of the proximity of a request and returns that measure.