apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/ConfigFileController.java/ConfigFileController/assembleCacheKey
class ConfigFileController:  String assembleCacheKey(ConfigFileOutputFormat outputFormat, String appId, String clusterName,
                          String namespace,
                          String dataCenter) {
    List<String> keyParts =
        Lists.newArrayList(outputFormat.getValue(), appId, clusterName, namespace);
    if (!Strings.isNullOrEmpty(dataCenter)) {
      keyParts.add(dataCenter);
    }
    return STRING_JOINER.join(keyParts);
  }