135 lines
5.0 KiB
Python
135 lines
5.0 KiB
Python
# Copyright 2013 Hewlett-Packard Development Company, L.P.
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# All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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import bisect
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import hashlib
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import six
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from nova import exception
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from nova.i18n import _
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# NOTE(jroll) these constants will be config options in Ocata, when the hash
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# ring code is in oslo.
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# Number of partitions per service is 2^PARTITION_EXPONENT.
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# 5 should be fine for most deployments, as an experimental feature.
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PARTITION_EXPONENT = 5
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# This should always be 1 in nova, as two compute daemons handling the same
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# node should not be possible.
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DISTRIBUTION_REPLICAS = 1
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class HashRing(object):
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"""A stable hash ring.
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We map item N to a host Y based on the closest lower hash:
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- hash(item) -> partition
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- hash(host) -> divider
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- closest lower divider is the host to use
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- we hash each host many times to spread load more finely
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as otherwise adding a host gets (on average) 50% of the load of
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just one other host assigned to it.
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"""
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def __init__(self, hosts):
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"""Create a new hash ring across the specified hosts.
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:param hosts: an iterable of hosts which will be mapped.
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"""
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replicas = DISTRIBUTION_REPLICAS
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try:
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self.hosts = set(hosts)
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self.replicas = replicas if replicas <= len(hosts) else len(hosts)
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except TypeError:
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raise exception.Invalid(
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_("Invalid hosts supplied when building HashRing."))
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self._host_hashes = {}
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for host in hosts:
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key = str(host).encode('utf8')
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key_hash = hashlib.md5(key)
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for p in range(2 ** PARTITION_EXPONENT):
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key_hash.update(key)
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hashed_key = self._hash2int(key_hash)
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self._host_hashes[hashed_key] = host
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# Gather the (possibly colliding) resulting hashes into a bisectable
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# list.
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self._partitions = sorted(self._host_hashes.keys())
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def _hash2int(self, key_hash):
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"""Convert the given hash's digest to a numerical value for the ring.
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:returns: An integer equivalent value of the digest.
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"""
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return int(key_hash.hexdigest(), 16)
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def _get_partition(self, data):
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try:
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if six.PY3 and data is not None:
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data = data.encode('utf-8')
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key_hash = hashlib.md5(data)
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hashed_key = self._hash2int(key_hash)
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position = bisect.bisect(self._partitions, hashed_key)
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return position if position < len(self._partitions) else 0
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except TypeError:
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raise exception.Invalid(
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_("Invalid data supplied to HashRing.get_hosts."))
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def get_hosts(self, data, ignore_hosts=None):
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"""Get the list of hosts which the supplied data maps onto.
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:param data: A string identifier to be mapped across the ring.
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:param ignore_hosts: A list of hosts to skip when performing the hash.
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Useful to temporarily skip down hosts without
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performing a full rebalance.
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Default: None.
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:returns: a list of hosts.
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The length of this list depends on the number of replicas
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this `HashRing` was created with. It may be less than this
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if ignore_hosts is not None.
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"""
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hosts = []
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if ignore_hosts is None:
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ignore_hosts = set()
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else:
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ignore_hosts = set(ignore_hosts)
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ignore_hosts.intersection_update(self.hosts)
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partition = self._get_partition(data)
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for replica in range(0, self.replicas):
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if len(hosts) + len(ignore_hosts) == len(self.hosts):
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# prevent infinite loop - cannot allocate more fallbacks.
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break
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# Linear probing: partition N, then N+1 etc.
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host = self._get_host(partition)
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while host in hosts or host in ignore_hosts:
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partition += 1
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if partition >= len(self._partitions):
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partition = 0
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host = self._get_host(partition)
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hosts.append(host)
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return hosts
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def _get_host(self, partition):
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"""Find what host is serving a partition.
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:param partition: The index of the partition in the partition map.
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e.g. 0 is the first partition, 1 is the second.
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:return: The host object the ring was constructed with.
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"""
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return self._host_hashes[self._partitions[partition]]
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