code: public void flush(IoSession session) {
          try {
              internalFlush(session.getFilterChain().getNextFilter(this),
                      session, buffersMap.get(session));
          } catch (Throwable e) {
              session.getFilterChain().fireExceptionCaught(e);
          }
      }
docs: Flushes the buffered data.
  
  @param session the session where buffer will be written