title: Cassandra and Secondary-Indexes, how do they work internally?
body: <p>How does a Cassandra Secondary-Index work internally? The docs state it is some kind of Hash Index:</p>
  
  <p>Given i have the colum <code>username="foobar"</code> (Column username will be scondary index) in a CF <code>User</code> with <code>RandomOrderingPartitioner</code></p>
  
  <ol>
  <li>Is my asumption correct, that cassandra uses a "Distributed Hash Index" (=so the index is not on one single node=the index is splitted)? </li>
  <li>On how many nodes are the index-parts held (the same amout as the replicatio factor)?</li>
  <li><p>On which nodes are the index-parts held (does Cassandra split the index by the same logic as the key with RandomOrderingPartitioner)?</p></li>
  <li><p>In case the index is hold on only one node (and of course replicated), how does cassandra "determin" the node that is responsible for the 
  index (By hashing the columname and then using  the randompartitioner logik to determine the node)?</p></li>
  <li><p>Is it really true, that this index is optimized for low cardinality? If yes, what is a rough estimate (
  is there a concrete figure that i can use to judge), that I should not use a secondary index (and rather use 
  a seperate CF for the index)? 
  Or said differently how to calculate the cardinality and make the right decision?</p></li>
  </ol>
  
  <p>I am trying to understand this.</p>