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Masters and Terms
As discussed in The Device Subsystem, each network device may have direct TCP connections to one or more ONOS instances, and as described in the previous section, ONOS elects one of those controllers to serve as the master for the device. The system invariant is that at any given point in time a device can have one and only one master. If the current master dies, ONOS elects a new master from amongst the controllers that the device can talk to. A mastership term is a per-device monotonically increasing counter (starting at 0) that is bumped each time a new master is elected. The very first time a switch comes online and a new master is elected, the mastership term value is 1. If that master dies and a new master is elected the term is bumped to 2 and so on.
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