title: Hadoop cluster and MapReduce logic
body: <p>I'm new in hadoop development. I read about hadoop cluster structure and understood that there are one namenode, jobtracker, tasktracker and multiple datanodes. 
  When we write map-reduce programs we implement mapper and reducer. I also understood logic of these clasess. But I don't understand how are they executed in the hadoop cluster. 
  Is mapper executed in the namenode only? 
  Is reducer executed seperatly on the datanodes?
  I need to make a lot of parralel computations and don't want to use HDFS, how can I be sure that each output collection (from mapper) executes seperatly in all datanodes?
  Please explain me the connection between hadoop cluster and map/reduce logic.
  Thanks a lot!</p>