US Patent 9432252 Unified replication mechanism for fault-tolerance of state

A network control system that achieves high availability for forwarding state computation within a controller cluster by replicating different levels of table state between controllers of the controller cluster. To build a highly available controller cluster, the tables for storing the forwarding state are replicated across the controllers. In order to take responsibility for a slice, the slave controller of some embodiments performs merging of replicated state on a slice-by-slice basis. The merging is performed in a manner to prevent disruptions to the network state while the slave controller is updated.