title: The output of reduce function in MapReduce
body: <p>I am confused by what the output of <code>reduce</code> should be in MapReduce.</p>
  <p>According to the original Jeff Dean and Sanjay Ghemawat <a href="https://static.googleusercontent.com/media/research.google.com/en//archive/mapreduce-osdi04.pdf" rel="nofollow noreferrer">paper</a> Section 2.2, it should be</p>
  <pre><code>reduce (k2,list(v2)) → list(v2)
  </code></pre>
  <p>But according to the <a href="https://hadoop.apache.org/docs/r1.2.1/mapred_tutorial.html" rel="nofollow noreferrer">Hadoop tutorial</a> Section <code>Inputs and Outputs</code>, and the <a href="https://www.oreilly.com/library/view/hadoop-the-definitive/9781449398644/ch07.html" rel="nofollow noreferrer">oreilly book</a> it is a list of <code>(K3, V3)</code></p>
  <p>So what's the difference and are they essentially the same thing or not?</p>