apollo-client/src/main/java/com/ctrip/framework/apollo/util/ConfigUtil.java/ConfigUtil/initCluster
class ConfigUtil:  private void initCluster() {
    //Load data center from system property
    cluster = System.getProperty(ConfigConsts.APOLLO_CLUSTER_KEY);

    //Use data center as cluster
    if (Strings.isNullOrEmpty(cluster)) {
      cluster = getDataCenter();
    }

    //Use default cluster
    if (Strings.isNullOrEmpty(cluster)) {
      cluster = ConfigConsts.CLUSTER_NAME_DEFAULT;
    }
  }