default Collection<SocketServer> brokerSocketServers() {
        return brokers().values().stream()
                .map(KafkaBroker::socketServer)
                .collect(Collectors.toList());
    }
A collection of all brokers in the cluster. In ZK-based clusters this will also include the broker which is
acting as the controller (since ZK controllers serve both broker and controller roles).
core/src/test/java/kafka/test/ClusterInstance.java