hash_ring

Implements consistent hashing that can be used when the number of server nodes can increase or decrease (like in memcached). The hashing ring is built using the same algorithm as libketama.

Consistent hashing is a scheme that provides a hash table functionality in a way that the adding or removing of one slot does not significantly change the mapping of keys to slots.

More about hash_ring can be read in a blog post (that explains the idea in greater details):

• Consistent hashing implemented simply in python http://amix.dk/blog/viewEntry/19367

More information about consistent hashing can be read in these articles:

• Web Caching with Consistent Hashing http://www8.org/w8-papers/2a-webserver/caching/paper2.html
• Consistent hashing and random trees http://citeseerx.ist.psu.edu/legacymapper?did=38148

There is also a wrapper MemcacheRing that extends python-memcache to use consistent hashing for key distribution.

Installing

To install hash_ring simply do::

$ sudo easy_install hash_ring

Example

Basic example of usage (for managing memcached instances)::

memcache_servers = ['192.168.0.246:11212',
                    '192.168.0.247:11212',
                    '192.168.0.249:11212']

ring = HashRing(memcache_servers)
server = ring.get_node('my_key')

Example using weights::

memcache_servers = ['192.168.0.246:11212',
                    '192.168.0.247:11212',
                    '192.168.0.249:11212']
weights = {
    '192.168.0.246:11212': 1,
    '192.168.0.247:11212': 2,
    '192.168.0.249:11212': 1
}

ring = HashRing(memcache_servers, weights)
server = ring.get_node('my_key')

How to use MemcacheRing::

from hash_ring import MemcacheRing
mc = MemcacheRing(['127.0.0.1:11212'])
mc.set('hello', 'world')
print mc.get('hello')

The code should be clean and simple. Feel free to contact the author if you detect bugs.

Compatibility

This version of python hash_ring is compatible with:

• nodejs hash_ring (https://github.com/cypreess/node-hash-ring)