1. A method for upgrading a controller cluster comprising a plurality of controller nodes, the controller cluster for managing a plurality of forwarding elements, each controller node executing a version of a network control application, the method comprising: upgrading a first subset of the plurality of controller nodes to execute a newer version of the network control application while managing the forwarding elements with a second subset of the plurality of controller nodes that execute an older version of the network control application; upon completion of the upgrade of the first subset of controller nodes, upgrading a decisive controller node in the second subset of controller nodes to signal the controller cluster to manage the forwarding elements with the first subset of controller nodes; upgrading the second subset of controller nodes to the newer version of the network control application while managing the forwarding elements with the first subset of controller nodes and the decisive controller node; and upon completion of the upgrade of the second subset of controller nodes, managing the forwarding elements with the first and second subsets of controller nodes.