title: Is there any non-commutative reducer in mapreduce that can be executed in parallel?
body: <p>Some operations like median, and mean, are non-commutative. It seems that there can only be one reducer in this case, as the reducer needs to have a global view. Is there any non-commutative reducer in map-reduce that can be executed in parallel? When encounter non-commutative operations, do people really use map-reduce? Or just run it on some very powerful machine? Is there common ways to break non-commutative operations into commutative operations?</p>
  
  <p>Thanks</p>