title: Cassandra DB secondary index
body: <p>I am starting to look at the Cassandra DB, something draw my attention in terms of its consistency aspect.
  I notice in order to do the secondary index, it needs to create another column family to store the keys and values reversely, so then the values I would try to search for become the keys  in index purposed column family.</p>
  
  <p>Then the question comes. For example, a column family encapsulating the user name and the user's living city. Its index column family has the city as the key and the underlying value is a list of the row keys of the original "user-city" column family. </p>
  
  <p>What if I change a user's city in the "user-city" column family, will Cassandra automatically update the corresponding index, such as remove/add the row key of the affected records from the corresponding index column family?</p>