title: Why do we need an 'arbiter' in MongoDB replication?
body: <p>Assume we setup a MongoDB replication without arbiter, <a href="http://docs.mongodb.org/manual/core/replication-introduction/">If the primary is unavailable, the replica set will elect a secondary to be primary.</a> So I think it's kind of <strong>implicit</strong> arbiter, since the replica will elect a primary automatically.</p>
  
  <p>So I am wondering why do we need a dedicated arbiter node? Thanks!</p>