使用HashRing实现python下的一致性hash

import md5
class HashRing(object):
    def __init__(self, nodes=None, replicas=3):
        """Manages a hash ring.
        `nodes` is a list of objects that have a proper __str__ representation.
        `replicas` indicates how many virtual points should be used pr. node,
        replicas are required to improve the distribution.
        """
        self.replicas = replicas
        self.ring = dict()
        self._sorted_keys = []
        if nodes:
            for node in nodes:
                self.add_node(node)
    def add_node(self, node):
        """Adds a `node` to the hash ring (including a number of replicas).
        """
        for i in xrange(0, self.replicas):
            key = self.gen_key(‘%s:%s‘ % (node, i))
            self.ring[key] = node
            self._sorted_keys.append(key)
        self._sorted_keys.sort()
    def remove_node(self, node):
        """Removes `node` from the hash ring and its replicas.
        """
        for i in xrange(0, self.replicas):
            key = self.gen_key(‘%s:%s‘ % (node, i))
            del self.ring[key]
            self._sorted_keys.remove(key)
    def get_node(self, string_key):
        """Given a string key a corresponding node in the hash ring is returned.
        If the hash ring is empty, `None` is returned.
        """
        return self.get_node_pos(string_key)[0]
    def get_node_pos(self, string_key):
        """Given a string key a corresponding node in the hash ring is returned
        along with it‘s position in the ring.
        If the hash ring is empty, (`None`, `None`) is returned.
        """
        if not self.ring:
            return None, None
        key = self.gen_key(string_key)
        nodes = self._sorted_keys
        for i in xrange(0, len(nodes)):
            node = nodes[i]
            if key <= node:
                return self.ring[node], i
        return self.ring[nodes[0]], 0
    def get_nodes(self, string_key):
        """Given a string key it returns the nodes as a generator that can hold the key.
        The generator is never ending and iterates through the ring
        starting at the correct position.
        """
        if not self.ring:
            yield None, None
        node, pos = self.get_node_pos(string_key)
        for key in self._sorted_keys[pos:]:
            yield self.ring[key]
        while True:
            for key in self._sorted_keys:
                yield self.ring[key]
    def gen_key(self, key):
        """Given a string key it returns a long value,
        this long value represents a place on the hash ring.
        md5 is currently used because it mixes well.
        """
        m = md5.new()
        m.update(key)
        return long(m.hexdigest(), 16)

from hash_ring import *
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‘)
print server
server = ring.get_node(‘my_keysdfsdf‘)
print server

有了一致性hash，你可以最小程度的减轻因为增加或者是减少服务器带来的大量的缓存MISS

在Memcached、Key-Value Store、Bittorrent DHT、LVS中都采用了Consistent Hashing算法，可以说Consistent Hashing 是分布式系统负载均衡的首选算法