code: public void commit() {
      if (directory == null)
        return;
  
      try {
        if (reader != null)
          reader.close();
  
        // it turns out that IndexWriter.optimize actually slows
        // searches down, because it invalidates the cache. therefore
        // not calling it any more.
        // http://www.searchworkings.org/blog/-/blogs/uwe-says%3A-is-your-reader-atomic
        // iwriter.optimize();
  
        iwriter.commit();
        openSearchers();
      } catch (IOException e) {
        throw new DukeException(e);
      }
    }
docs: Flushes all changes to disk.