Iterator<UsableBroker> usableBrokers() {
        if (heartbeatManager == null) {
            throw new RuntimeException("ClusterControlManager is not active.");
        }
        return heartbeatManager.usableBrokers(
            id -> brokerRegistrations.get(id).rack());
    }
Process an incoming broker registration request.
metadata/src/main/java/org/apache/kafka/controller/ClusterControlManager.java