code: private static boolean ifExistsAndFresh(Configuration conf, FileSystem fs,
                                            URI cache, long confFileStamp,
                                            CacheStatus lcacheStatus,
                                            FileStatus fileStatus)
    throws IOException {
      // check for existence of the cache
      long dfsFileStamp;
      if (fileStatus != null) {
        dfsFileStamp = fileStatus.getModificationTime();
      } else {
        dfsFileStamp = getTimestamp(conf, cache);
      }
  
      // ensure that the file on hdfs hasn't been modified since the job started
      if (dfsFileStamp != confFileStamp) {
        LOG.fatal("File: " + cache + " has changed on HDFS since job started");
        throw new IOException("File: " + cache +
            " has changed on HDFS since job started");
      }
  
      if (dfsFileStamp != lcacheStatus.mtime) {
        // needs refreshing
        return false;
      }
  
      return true;
    }
docs: Checks if the cache has already been localized and is fresh