title: Trouble understanding the relation between MongoDB and RocksDB
body: <p>As I am learning to use Parse, I came across a <a href="http://blog.parse.com/announcements/mongodb-rocksdb-parse/" rel="nofollow">announcement from parse</a> saying they were using RocksDB and MongoDB.</p>
  
  <p>Even after research, the relationship between Rocks and MongoDB is not clear cut to me, and what are the benefit to use RocksDB instead of a simple MongoDB.</p>
  
  <p>As I understand it, RocksDB is a key-value store, while MongoDB is a noSQL database.</p>
  
  <p>So, what is the point of adding RocksDB on MongoDB (or is it Mongo on Rocks?), and in that case, where are stored the data?</p>