1. For a particular controller for managing a network comprising a plurality of managed forwarding elements that forward data in the network, a method for computing forwarding state using inputs from a first controller and a second controller, the method comprising: receiving a first set of inputs from the first controller and a second set of inputs from the second controller; starting to compute a set of outputs using the first set of inputs; after a failure of the first controller, receiving a third set of inputs from the second controller, the third set of inputs and the first or second set of inputs comprising a group of inputs for being processed together and separately from another group of inputs; continuing to compute the set of outputs using the second and third sets of inputs; receiving an indicator from the second controller, the indicator indicating that all inputs of the group of inputs have arrived at the particular controller; and after receiving the indicator and after computing the set of outputs completely, sending the set of outputs to a fourth controller, wherein the fourth controller subsequently processes the set of outputs from the particular controller and sends the processed outputs to a managed forwarding element.