Fix Version/s: 4.x
Considering that HyperLogLog and its variants have become pretty popular in analytics space and Cassandra has "read-before-write" collections (Lists), I think it would not be too painful to add support for HyperLogLog "collection" type. They would act similar to CQL 3 Sets, meaning you would be able to "set" the value and "add" an element, but you won't be able to remove an element. Also, when getting the value of a HyperLogLog collection column, you'd get the cardinality.
There are a couple of good attributes with HyperLogLog which fit Cassandra pretty well.
- Adding an element is idempotent (adding an existing element doesn't change the HLL)
- HLL can be thought of as a CRDT, since we can safely merge them. Which means we can merge two HLLs during read-repair. But if that's too much work, I guess we can even live with LWW since these counts are "estimates" after all.
There is already a proof of concept at: