title: “Combiner" Class in a mapreduce job
body: <p>A Combiner runs after the Mapper and before the Reducer,it will receive as input all data emitted by the Mapper instances on a given node. then emits output to the Reducers.</p>
  
  <p>And also,If a reduce function is both <strong><em>commutative and associative</em></strong>, then it can be used as a Combiner.</p>
  
  <p>My Question is what does the phrase "<strong><em>commutative and associative</em></strong>" mean in this situation?</p>