code: public static void flush(ChannelHandlerContext ctx, ChannelFuture future) {
          ctx.sendDownstream(
                  new DownstreamFlushEvent(ctx.getChannel(), future));
      }
docs: Sends a {@code "flush"} request to the
  {@link ChannelDownstreamHandler} which is placed in the closest
  downstream from the handler associated with the specified
  {@link ChannelHandlerContext}.
  
  @param ctx     the context
  @param future  the future which will be notified when the flush
  operation is done