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Cassy - High Level Cassandra Client for Haskell


The intent is to develop a high quality, high level driver similar to pycassa.

There are currently two modules, one for basic/lower level (but still much more pleasant than thrift) operation, and one for some experimental JSON serialization support.

API Documentation

Please see the Haddocks hosted on HackageDB:


Database.Cassandra.Basic Usage

This module offers straightforward functionality that is still much more pleasant than using Thrift directly.

import Database.Cassandra.Basic

test :: IO ()
test = do
  pool <- createCassandraPool defServers 3 300 "TestKeySpace"
  insert pool "testCF" "key1" QUORUM [col "col1" "value1"]
  getCol pool "testCF" "key1" "col1" QUORUM

Database.Cassandra.JSON Usage

This module does two things in addition to basic functionality:

  • Row and column keys are polymorphic so that you can use any type that is a member of the CKey typeclass. By default, we provide instances for String, ByteString and Text.

  • Values are automatically marshalled to/from JSON.

Example usage:

  import Database.Cassandra.JSON
  import Data.Aeson

  type Name = String
  type Age = Int
  data Person = Person Name Age

  -- Define JSON serialization for our data structure

  instance ToJSON Person where
      toJSON (Person nm age) = toJSON (nm,age)

  instance FromJSON Person where
      parseJSON v = do
          (nm, age) <- parseJSON v
          return $ Person nm age

  test :: Person -> IO ()
  test p@(Person nm age) = do
    pool <- createCassandraPool defServers 3 300 "TestKeySpace"

    -- I can use any string-like key and don't have to explicitly
    -- convert person to ByteString.
    insertCol pool "testCF" "people" nm QUORUM p

    -- Automatically de-serialized back to a datatype
    res <- getCol pool "testCF" "people" nm QUORUM
    case res of
        Just (Person nm age) -> return age
        Nothing -> error "Oh NO!!!"

Release Notes

Version 0.4

This version packs a fairly large changeset. It will almost definitely break your code, although the fix/adjustment is likely to be minor.

  • Vastly enhanced the Database.Cassandra.Pack module to represent types that Cassandra can sort and validate.
  • Added CasType typeclass that offers encodeCas and decodeCas methods that handle conversions to/from the binary ByteString representation.
  • Composite columns are now supported through tuples. Just pick two or more CasType instances and put them in a tuple to automatically trigger composite column conversion. Keep in mind that your ColumnFamily schema must be configured right or else you'll get runtime exceptions.
  • Added a bunch of newtype wrappers to directly map to types Cassandra knows. These include TAscii, TBytes, TInt, TUtf8 and some others.
  • Changed several methods in Basic and JSON modules to expect CasType column key values instead of concrete ByteString.
  • Added the useful packCol and unpackCol functions to smoothly handle column key type conversions when working with the Basic module.
  • Made the Cas monad a simple type synonym for ReaderT CPool.
  • Added the get_ metho to JSON to make it easier to discard key names and just get the column contents.
  • Numerous fixes and minor tweaks.

Version 0.3

  • Added MonadCassandra typeclass, which is now used by all operations by default.
  • Added a default Cas moand that instantiates MonadCassandra for convenience.
  • All Basic module ops now return results directly instead of an Either wrapper. Each operation may raise a CassandraException.
  • Connection pooling now builds on top of the resource-pool library to initiate connections to multiple servers in round-robin fashion.
  • Basic.insert now knows how to insert a SuperColumn.


  • Add support for counters and batch mutators
  • Add support for composite columns
  • Add support for database/admin operations


Would love to get contributions, bug reports, suggestions, feedback from the community.

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