title: Hadoop reducer output order when output key is NullWritable
body: <p>I am writing a <code>hadoop</code> application whose final output is a <code>bmp</code> image. I change the output format to <code>byte</code>, here is my question: what will the <code>reducer output order</code> be if I set the output key being <code>NullWritable</code>?</p>