Data.Hash.Consistent

Data.Hash.Consistent

A consistent hash is a technique to manage the fair distribution of cacheable entities among hosts. Each host identifier has its crc32 hash calculated and stored in a Vector along with its canonical host name. The host identifier may be differentiated from its canonical host name by a multiplying factor, in our case a simple integer appeneded to the hostname to provide it with a number of entries in the consistent hash, all evenly distributed.

This technique is explained in these links:

http:en.wikipedia.orgwikiConsistent_hashing

http:www.tomkleinpeter.com20080317programmers-toolbox-part-3-consistent-hashing/

Here is a small program illustrating its use:

module Main where
  import qualified Data.Hash.Consistent as CH

main = do
    let hosts  = [hi.example.net,bar.example.net,foo.example.net] :: [CH.Host]
    let n = 2 :: Int
    let ch = CH.new
    print $ show $ ch
    ch <- return $ CH.add hosts n ch
    print $ show $ ch  
    let fh = [head hosts] :: [CH.Host]
    let hh_fh = CH.hashHosts fh n
    print hh_fh
    ch <- return $ CH.del fh n ch
    print $ show $ ch  
    let i = 770931073
    let tgt = CH.targetHost i ch 
    print tgt
    return ()

License info:

The license is a simple BSD3-style license available here: https:www.b7j0c.orgstufflicense.txt

Documentation

textCrc32 :: String -> Hash

search :: Hash -> ConsistentHash -> Int

add :: [Host] -> Int -> ConsistentHash -> ConsistentHash

del :: [Host] -> Int -> ConsistentHash -> ConsistentHash

new :: ConsistentHash

targetHost :: Hash -> ConsistentHash -> HashHost

hashHosts :: [Host] -> Int -> ConsistentHash

type Host = String

type Hash = Word32

type HashHost = (Hash, Host)

type ConsistentHash = Vector HashHost