title: Cassandra - unique constraint on row key
body: <p>I would like to know whenever it is possible in Cassandra to specify unique constrain on row key. Something similar to SQL Server's <code>ADD CONSTRAINT myConstrain UNIQUE (ROW_PK)</code></p>
  
  <p>In case of insert with already existing row key, the existing data will be not overwritten, but I receive kind of exception or response that update cannot be performed due to constrain violation.</p>
  
  <p>Maybe there is a workaround for this problem - there are counters which updates seams to be atomic.</p>