The IF statement uses Paxos as underlying implementation and still it may result into an inconsistency if you are using two separate CQL statements. Cassandra does not provide begin ... end construction where everything is atomically committed or rolled back.

You can use Cassandra LWT as a building block for multi-partition transactions, in a similar way like CAS atomic operations are used to build locks, mutexes and monitors and then monitors are used to implement ACID transactions in RDBMS. However, noone says it would be easy or performant, therefore this is probably not a good idea.

Right. Cassandra crawled at 20 TPS when we sent IF cql statements. One thing to note is that Cassandra txns can never be rolled back - they can only be committed/ retrried and so on.

Quorum won't solve consistency problems like this unless you can also guarantee a serialized reader/writer for any given piece of data. The best you can do by pinning queries to primary replicas and enforcing R + W > N is RYOW consistency, and that depends on the aforementioned serialization point.

Quorum reads + writes and applying non-destructive updates only are enough to get linearizabile consistency in Cassandra, even if clocks are not perfectly synchronized.

If you're applying non-destructive updates then you don't even need quorum to achieve consistency. You're just writing immutable data.

If you're applying non-destructive updates without quorum, you would not get monotonic read consistency, because stale reads would be possible. You could write a row, immediately read it back from another replica and find out the row is not there, because that replica didn't get it yet. You'd have only eventual consistency.