apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/ConfigController.java/ConfigController/assembleKey
class ConfigController:  private String assembleKey(String appId, String cluster, String namespace, String dataCenter) {
    List<String> keyParts = Lists.newArrayList(appId, cluster, namespace);
    if (!Strings.isNullOrEmpty(dataCenter)) {
      keyParts.add(dataCenter);
    }
    return String.join(ConfigConsts.CLUSTER_NAMESPACE_SEPARATOR, keyParts);
  }