apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/util/ConfigFileUtils.java/ConfigFileUtils/toFilename
class ConfigFileUtils:  public static String toFilename(
      final String appId,
      final String clusterName,
      final String namespace,
      final ConfigFileFormat configFileFormat
  ) {
    final String suffix;
    if (ConfigFileFormat.Properties.equals(configFileFormat)) {
      suffix = "." + ConfigFileFormat.Properties.getValue();
    } else {
      suffix = "";
    }
    return appId + "+" + clusterName + "+" + namespace + suffix;
  }