1. For a particular controller that computes forwarding state for managing managed forwarding elements that forward data in a network, a method for computing the forwarding state using a set of inputs from a first controller and a second controller that is a back up controller for the first controller, the method comprising: receiving a first subset of the set of inputs from the first controller at the particular controller, wherein the particular controller is a master of a set of the managed forwarding elements; after failure of the first controller, receiving a second subset of the set of inputs from the second controller at the particular controller, at least one input of the second subset of the set of inputs being duplicative of an input in the first subset; computing forwarding state using the first and second subsets of the set of inputs but without using the duplicative input; and distributing the computed forwarding state from the particular controller to the set of managed forwarding elements.