Updating a CQL List many times creates many tombstones

We encountered a TombstoneOverwhelmingException in cassandra system.log which caused some of our CQL queries to fail.

We are able to reproduce this issue by updating a CQL List column many times. The number of tombstones created seems to be related to (number of list items * number of list updates). We update the entire list on each update using the java driver. (see attached code for details)

Running nodetool compact does not help, but nodetool flush does. It appears that the tombstones are being accumulated in memory.

For example if we update a list of 100 items 1000 times, this creates more than 100,000 tombstones and exceeds the default tombstone_failure_threshold.