传统的取模方式
例如10条数据,3个节点,如果按照取模的方式,那就是
node a: 0,3,6,9
node b: 1,4,7
node c: 2,5,8
当增加一个节点的时候,数据分布就变更为
node a:0,4,8
node b:1,5,9
node c: 2,6
node d: 3,7
总结:数据3,4,5,6,7,8,9在增加节点的时候,都需要做搬迁,成本太高
一致性哈希方式
最关键的区别就是,对节点和数据,都做一次哈希运算,然后比较节点和数据的哈希值,数据取和节点最相近的节点做为存放节点。这样就保证当节点增加或者减少的时候,影响的数据最少。
还是拿刚刚的例子,(用简单的字符串的ascii码做哈希key):
十条数据,算出各自的哈希值
0:192
1:196
2:200
3:204
4:208
5:212
6:216
7:220
8:224
9:228
有三个节点,算出各自的哈希值
node a: 203
node g: 209
node z: 228
这个时候比较两者的哈希值,如果大于228,就归到前面的203,相当于整个哈希值就是一个环,对应的映射结果:
node a: 0,1,2
node g: 3,4
node z: 5,6,7,8,9
这个时候加入node n, 就可以算出node n的哈希值:
node n: 216
这个时候对应的数据就会做迁移:
node a: 0,1,2
node g: 3,4
node n: 5,6
node z: 7,8,9
这个时候只有5和6需要做迁移
另外,这个时候如果只算出三个哈希值,那再跟数据的哈希值比较的时候,很容易分得不均衡,因此就引入了虚拟节点的概念,通过把三个节点加上ID后缀等方式,每个节点算出n个哈希值,均匀的放在哈希环上,这样对于数据算出的哈希值,能够比较散列的分布(详见下面代码中的replica)
通过这种算法做数据分布,在增减节点的时候,可以大大减少数据的迁移规模。
下面转载的哈希代码,已经将gen_key改成上述描述的用字符串ascii相加的方式,便于测试验证。
[python]
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- 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))
- print “node %s-%s key is %ld” % (node, i, key)
- 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:
- print “string_key %s key %ld” % (string_key, key)
- print “get node %s-%d “ % (self.ring[node], i)
- return self.ring[node], i
- return self.ring[nodes[0]], 0
- def print_ring(self):
- if not self.ring:
- return None, None
- nodes = self._sorted_keys
- for i in xrange(0, len(nodes)):
- node = nodes[i]
- print “ring slot %d is node %s, hash vale is %s” % (i, self.ring[node], node)
- 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)
- “””
- hash = 0
- for i in xrange(0, len(key)):
- hash += ord(key[i])
- return hash
- “””
- memcache_servers = [‘a’,
- ‘g’,
- ‘z’]
- ring = HashRing(memcache_servers,1)
- ring.print_ring()
- server = ring.get_node(‘0000’)
- server = ring.get_node(‘1111’)
- server = ring.get_node(‘2222’)
- server = ring.get_node(‘3333’)
- server = ring.get_node(‘4444’)
- server = ring.get_node(‘5555’)
- server = ring.get_node(‘6666’)
- server = ring.get_node(‘7777’)
- server = ring.get_node(‘8888’)
- server = ring.get_node(‘9999’)
- print ‘———————————————————-‘
- memcache_servers = [‘a’,
- ‘g’,
- ‘n’,
- ‘z’]
- ring = HashRing(memcache_servers,1)
- ring.print_ring()
- server = ring.get_node(‘0000’)
- server = ring.get_node(‘1111’)
- server = ring.get_node(‘2222’)
- server = ring.get_node(‘3333’)
- server = ring.get_node(‘4444’)
- server = ring.get_node(‘5555’)
- server = ring.get_node(‘6666’)
- server = ring.get_node(‘7777’)
- server = ring.get_node(‘8888’)
- server = ring.get_node(‘9999’)