title: How does Hadoop Namenode failover process works?
body: <p>Hadoop defintive guide says - </p>
  
  <blockquote>
    <p><strong>Each Namenode runs</strong> a lightweight failover controller process whose
    job it is <strong>to monitor its Namenode for failures</strong> (using a simple
    heartbeat mechanism) and trigger a failover should a namenode
    fail.</p>
  </blockquote>
  
  <p>How come a namenode can run something to detect its own failure?</p>
  
  <p>Who sends heartbeat to whom?</p>
  
  <p>Where this process runs?</p>
  
  <p>How it detects namenode failure?</p>
  
  <p>To whom it notify for the transition?</p>