For a network control system that receives, from a user, logical datapath sets that logically express desired forwarding behaviors that are to be implemented by a set of managed switching elements, a controller for managing several managed switching elements that forward data in a network that includes the managed switching elements is described. The controller includes a set of modules for detecting a change in one or more managed switching elements and for updating logical datapath set based on the detected change. The logical datapath set is for subsequent translation into a set of physical forwarding behaviors of the managed switching elements.