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from bisect import insort
from collections import Counter
from hashlib import md5
class KetamaRing:
"""Implement a ketama compatible consistent hashing ring."""
def __init__(self, replicas=4):
"""Create a new HashRing."""
self._distribution = Counter()
self._keys = []
self._nodes = {}
self._replicas = replicas
self._ring = {}
self._listbytes = lambda x: x
def hashi(self, key, replica=0):
"""Returns a ketama compatible hash from the given key."""
dh = self._listbytes(md5(str(key).encode("utf-8")).digest())
rd = replica * 4
return (dh[3 + rd] << 24) | (dh[2 + rd] << 16) | (dh[1 + rd] << 8) | dh[0 + rd]
def _hashi_weight_generator(self, node_name, node_conf):
"""Calculate the weight factor of the given node and
yield its hash key for every configured replica.
:param node_name: the node name.
"""
ks = (
node_conf["vnodes"] * len(self._nodes) * node_conf["weight"]
) // self._weight_sum
for w in range(0, ks):
w_node_name = f"{node_name}-{w}"
for i in range(0, self._replicas):
yield self.hashi(w_node_name, replica=i)
@staticmethod
def _listbytes(data):
"""Python 2 compatible int iterator from str.
:param data: the string to int iterate upon.
"""
return map(ord, data)
def _create_ring(self, nodes):
"""Generate a ketama compatible continuum/ring."""
_weight_sum = 0
for node_conf in self._nodes.values():
_weight_sum += node_conf["weight"]
self._weight_sum = _weight_sum
_distribution = Counter()
_keys = []
_ring = {}
for node_name, node_conf in self._nodes.items():
for h in self._hashi_weight_generator(node_name, node_conf):
_ring[h] = node_name
insort(_keys, h)
_distribution[node_name] += 1
self._distribution = _distribution
self._keys = _keys
self._ring = _ring
def _remove_node(self, node_name):
"""Remove the given node from the continuum/ring.
:param node_name: the node name.
"""
try:
self._nodes.pop(node_name)
except Exception:
raise KeyError(
"node '{}' not found, available nodes: {}".format(
node_name, self._nodes.keys()
)
)
else:
self._create_ring(self._nodes)