title: MongoDB standalone vs replica set and how to migrate data from a standalone to a replica set
body: <p>I have a few questions about MongoDB standalone and Replica sets, I don't really get it.</p>
  
  <ul>
  <li>When should I use either of them</li>
  <li>Why all the replica sets tutorials show 3 connections, is there a reason?</li>
  <li>Can I create a replica set for 1 instance only? and in that case how is it different than the standalone mongodb instance?</li>
  <li>How to Migrate data from a standalone instance to replica sets?</li>
  </ul>
  
  <p>All these questions I'm asking because recently I was trying to implement transactions and sessions can only start on "replica sets" I don't really get the difference at all.</p>