title: How does Hadoop run the java reduce function on the DataNode's
body: <p>I am confused on how the Datanode's in a hadoop cluster runs the java code for the reduce function of a job. Like, how does hadoop send a java code to another computer to execute?</p>
  
  <ul>
  <li>Does Hadoop inject a java code to the nodes?  If so, where is the java code located in hadoop?</li>
  <li>Or are the reduce functions ran on the master node not the datanodes?  </li>
  </ul>
  
  <p>Help me trace this code where the Master Node sends the java code for the reduce function to a datanode. </p>