Improve data storing on Cassandra

[VincenzoFerme]

We should check that we take care of different aspect of the Cassandra design.

As reported on http://www.ipponusa.com/blog/10-tips-and-tricks-for-cassandra/, we should:

Don’t use PreparedStatement if you insert empty columns
If you have an empty column in your PreparedStatement, the CQL driver will in fact insert a null value in Cassandra, which will end up being a tombstone.

This is a very bad behavior, as:

• Those tombstones of course take up valuable resources.
• As a result, you can easily reach the tombstone_failure_threshold (by default at 100,000 which is in fact quite a high value).

The only solution is to have one PreparedStatement per type of insert query, which can be annoying if you have a lot of empty columns! But if you have multiple empty columns, shouldn’t you have used a Map to store that data in the first place?

Some other interesting references:

• frozen: when we need to have data available in an entity that won't change
• https://www.oreilly.com/ideas/apache-cassandra-for-analytics-a-performance-and-storage-analysis