[CASSANDRA-8861] HyperLogLog Collection Type

• Type: New Feature
• Status: Open
• Priority: Normal
• Resolution: Unresolved
• Fix Version/s: 5.x
• Component/s: Legacy/CQL
• Assignee: Unassigned
• Reporter: Drew Kutcharian
• Created: 24/Feb/15 20:58
• Updated: 07/Mar/23 10:54

Description

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:
http://vilkeliskis.com/blog/2013/12/28/hacking_cassandra.html

Issue Links

• depends upon: CASSANDRA-6506 counters++ split counter context shards into separate cells