C. Scott Andreas updated CASSANDRA-6412:
    Component/s: Core

> Custom creation and merge functions for user-defined column types
> -----------------------------------------------------------------
>                 Key: CASSANDRA-6412
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6412
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>            Reporter: Nicolas Favre-Felix
>            Priority: Major
> This is a proposal for a new feature, mapping custom types to Cassandra 
> columns.
> These types would provide a creation function and a merge function, to be 
> implemented in Java by the user.
> This feature relates to the concept of CRDTs; the proposal is to replicate 
> "operations" on these types during write, to apply these operations 
> internally during merge (Column.reconcile), and to also merge their values on 
> read.
> The following operations are made possible without reading back any data:
> * MIN or MAX(value) for a column
> * First value for a column
> * Count Distinct
> * HyperLogLog
> * Count-Min
> And any composition of these too, e.g. a Candlestick type includes first, 
> last, min, and max.
> The merge operations exposed by these types need to be commutative; this is 
> the case for many functions used in analytics.
> This feature is incomplete without some integration with CASSANDRA-4775 
> (Counters 2.0) which provides a Read-Modify-Write implementation for 
> distributed counters. Integrating custom creation and merge functions with 
> new counters would let users implement complex CRDTs in Cassandra, including:
> * Averages & related (sum of squares, standard deviation)
> * Graphs
> * Sets
> * Custom registers (even with vector clocks)
> I have a working prototype with implementations for min, max, and Candlestick 
> at https://github.com/acunu/cassandra/tree/crdts - I'd appreciate any 
> feedback on the design and interfaces.

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