title: Designing of the "mapper" and "reducer" functions' functionality for hadoop?
body: <p>I am trying to design a mapper and reducer for Hadoop. I am new to Hadoop, and I'm a bit confused about how the mapper and reducer is supposed for work for my specific application. </p>
  
  <p>The input to my mapper is a large directed graph's connectivity. It is a 2 column input where each row is an individual edge connectivity. The first column is the start node id and the second column is the end node id of each edge. I'm trying to output the number of neighbors for each start node id into a 2 column text file, where the first column is sorted in order of increasing start node id.</p>
  
  <p>My questions are:</p>
  
  <p>(1) The input is already set up such that each line is a key-value pair, where the key is the start node id, and the value is the end node id. Would the mapper simply just read in each line and write it out? That seems redundant. </p>
  
  <p>(2) Does the sorting take place in between the mapper and reducer or could the sorting actually be done with the reducer itself? </p>