public boolean isUnfenced(int brokerId) {
        BrokerRegistration registration = brokerRegistrations.get(brokerId);
        if (registration == null) return false;
        return !registration.fenced();
    }
Returns true if the broker is unfenced; Returns false if it is
not or if it does not exist.
metadata/src/main/java/org/apache/kafka/controller/ClusterControlManager.java