OptionalLong controlledShutdownOffset(int brokerId) {
        BrokerHeartbeatState broker = brokers.get(brokerId);
        if (broker == null || broker.controlledShutdownOffset == -1) {
            return OptionalLong.empty();
        }
        return OptionalLong.of(broker.controlledShutdownOffset);
    }
The set of active brokers.  A broker is active if it is unfenced, and not shutting
down.
metadata/src/main/java/org/apache/kafka/controller/BrokerHeartbeatManager.java