Thank you for the great link!
The mentioned solution is using locking but I would prefer some optimistic strategy (because the conflicts are rare in my situation) but I'm afraid that this is really the best solution...

So the solution is probably to use some kind of 

2010/9/6 Reza Lesmana <lesmana.reza@gmail.com>
I read an article about using CAGES with Cassandra to achieve locking
and transaction...

Here is the link :

http://ria101.wordpress.com/2010/05/12/locking-and-transactions-over-cassandra-using-cages/

On 9/5/10, Michal Augustın <augustyn.michal@gmail.com> wrote:
> Hello,
>
> we can read everywhere that Cassandra (and similar NoSQL solutions) doesn't
> support full ACID and (when we want to have ACID) we have to implement ACID
> in higher layers of our application. Are there some good resources on how to
> implement ACID on higher layers? I.e. how to implement repository
> pattern/DAO with ACID support when Cassandra is the database.
>
> I'm sure that some pessimistic solution (locks) is absolutely unsuitable for
> Cassandra so the solution probably would deal with optimistic concurrency...
>
> Thank you!
>
> Augi
>