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Nicolas Favre-Felix commented on CASSANDRA-6412:
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Thanks for the feedback, [~slebresne].
I like your suggestion to use user-defined types, this is definitely better
than the home-made candlestick structure.
I also like that having fixed types with custom resolver makes it easier to
write type-safe code with minimal changes to the Cassandra code base.
As you point out, we can use the same technique as for counter deletion. I
understand that counter deletes are somewhat broken, and that columns with a
custom resolver would suffer from a similar defect (CASSANDRA-2774).
I don't think that there is an easy solution to this problem; only deleting al
CL.ALL would prevent old values from being merged with newer ones.
> 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
>
> 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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