什么是好的一致性 hash 实现

看到微信群有人问 prd 用什么一致性 hash 算法好，就想起了这篇文章。这是以前做的测试。

那么什么是好的一致性 hash 算法呢？除了性能还要考虑哪些因素呢？根据我自己的经验简单聊一下，有不正确的请大家指正 ^^

背景

关于什么是 Consistent Hashing 网上很多，一般用于对于数据状态有一定要求的缓存场景，比如 web session 数据

节点根据算法，映射到环中。用户请求时，根据 id 求出 hash 值，按顺(逆)时针找到的第一个节点，就是存储数据节点。一般为了更加均匀些，同时防止一个节点下线，导致过多数据移动，会将节点按 replica (weight) 复制多个虚拟节点映射到环中。

数据不均匀问题

了解 LB 的都知道，lvs 这些负载均衡设备能做到请求均匀，但经典实现的一致性 hash 是做不到的。具体场景，可能差别更大。测试脚本放到了 consistenthash balance test github[1]，感兴趣的可以看下，测试场景如下：

节点数量：10
虚拟系数：3, 10, 50, 100, 200, 400, 600, 800, 1000
测试算法：MurMurHash, Crc32, Fnv1, CityHash
请求数量：50w

方差可以很好的衡量数据分布程度，可以看到，Crc32 是最差的，Fnv1 在数量很少时也差，MurMurHash 和 CityHash 在虚拟节点超过 100 个后迅速收敛。

diff ratio 指数据分布最多的与最小的差值，除以平均值。从数据分布可以看到， MurMurHash, CityHash 首选的 hash 算法。replica 虚节点数量可以调大一些，毕竟动态增减节点不是常态

另一种方案

冒泡哥提到了一种实现，提前预计算好节点顺序，可以减少分布 diff ratio. 他在生产环境，只用 20 replica 就降低到了 5% 当然了前提是，增加减节点，必须符合一致性 hash 的思想，数据尽可能少的移动。

测试数据

MurMur replica:3 var:43930 max:72370 min:35411 diff:36959 ratio:73.918000
MurMur replica:10 var:69441 max:85238 min:19257 diff:65981 ratio:131.962000
MurMur replica:50 var:20520 max:59708 min:40669 diff:19039 ratio:38.078000
MurMur replica:100 var:12556 max:57515 min:42045 diff:15470 ratio:30.940000
MurMur replica:200 var:11539 max:55858 min:41998 diff:13860 ratio:27.720000
MurMur replica:400 var:7161 max:53385 min:45323 diff:8062 ratio:16.124000
MurMur replica:600 var:5586 max:51697 min:45913 diff:5784 ratio:11.568000
MurMur replica:800 var:4101 max:52118 min:48041 diff:4077 ratio:8.154000
MurMur replica:1000 var:4369 max:52888 min:48356 diff:4532 ratio:9.064000
Crc32 replica:3 var:76353 max:97097 min:23872 diff:73225 ratio:146.450000
Crc32 replica:10 var:30219 max:61334 min:31891 diff:29443 ratio:58.886000
Crc32 replica:50 var:12231 max:55475 min:43926 diff:11549 ratio:23.098000
Crc32 replica:100 var:22105 max:58828 min:37776 diff:21052 ratio:42.104000
Crc32 replica:200 var:28465 max:59481 min:35807 diff:23674 ratio:47.348000
Crc32 replica:400 var:25440 max:58781 min:37059 diff:21722 ratio:43.444000
Crc32 replica:600 var:33926 max:61694 min:35887 diff:25807 ratio:51.614000
Crc32 replica:800 var:33978 max:61327 min:36914 diff:24413 ratio:48.826000
Crc32 replica:1000 var:27789 max:60515 min:37709 diff:22806 ratio:45.612000
Fnv1 replica:3 var:348461 max:377986 min:5997 diff:371989 ratio:743.978000
Fnv1 replica:10 var:200673 max:231641 min:14451 diff:217190 ratio:434.380000
Fnv1 replica:50 var:41072 max:84924 min:38135 diff:46789 ratio:93.578000
Fnv1 replica:100 var:11567 max:59547 min:47254 diff:12293 ratio:24.586000
Fnv1 replica:200 var:6465 max:53451 min:46127 diff:7324 ratio:14.648000
Fnv1 replica:400 var:5126 max:52454 min:47757 diff:4697 ratio:9.394000
Fnv1 replica:600 var:3748 max:52520 min:48394 diff:4126 ratio:8.252000
Fnv1 replica:800 var:6089 max:52003 min:46211 diff:5792 ratio:11.584000
Fnv1 replica:1000 var:5881 max:53028 min:46902 diff:6126 ratio:12.252000
Cityhash replica:3 var:77979 max:99378 min:11576 diff:87802 ratio:175.604000
Cityhash replica:10 var:60194 max:84045 min:25002 diff:59043 ratio:118.086000
Cityhash replica:50 var:26496 max:67141 min:38121 diff:29020 ratio:58.040000
Cityhash replica:100 var:11862 max:56008 min:44075 diff:11933 ratio:23.866000
Cityhash replica:200 var:9921 max:54711 min:44592 diff:10119 ratio:20.238000
Cityhash replica:400 var:5542 max:53629 min:47391 diff:6238 ratio:12.476000
Cityhash replica:600 var:6823 max:53417 min:46261 diff:7156 ratio:14.312000
Cityhash replica:800 var:5100 max:51615 min:46577 diff:5038 ratio:10.076000
Cityhash replica:1000 var:4884 max:52151 min:47166 diff:4985 ratio:9.970000

小结

这次分享就这些，以后面还会分享更多的内容。

参考资料

[1]测试脚本: https://github.com/dongzerun/consistenthash_balance_test,