title: Collision probability of ObjectId vs UUID in a large distributed system
body: <p>Considering that an UUID rfc 4122 (16 bytes) is much larger than a MongoDB ObjectId (12 bytes), I am trying to find out how their collision probability compare.</p>
  
  <p>I know that is something around <strong>quite unlikely</strong>, but in my case most ids will be generated within a large number of mobile clients, not within a limited set of servers. <strong>I wonder if in this case, there is a justified concern</strong>. </p>
  
  <p>Compared to the normal case where all ids are generated by a small number of clients:</p>
  
  <ul>
  <li>It might take months to detect a collision since the document creation</li>
  <li>IDs are generated from a much larger client base</li>
  <li>Each client has a lower ID generation rate</li>
  </ul>