什么是好的一致性 Hash 實現
本文轉載自微信公眾號「董澤潤的技術筆記」，作者董澤潤 。轉載本文請聯系董澤潤的技術筆記公眾號。
看到微信群有人問 prd 用什么一致性 hash 算法好，就想起了這篇文章。這是以前做的測試。
那么什么是好的一致性 hash 算法呢?除了性能還要考慮哪些因素呢?根據我自己的經驗簡單聊一下，有不正確的請大家指正 ^^
背景
關于什么是 Consistent Hashing 網上很多，一般用于對于數據狀態有一定要求的緩存場景，比如 web session 數據
節點根據算法，映射到環中。用戶請求時，根據 id 求出 hash 值，按順(逆)時針找到的第一個節點，就是存儲數據節點。一般為了更加均勻些，同時防止一個節點下線，導致過多數據移動，會將節點按 replica (weight) 復制多個虛擬節點映射到環中。
數據不均勻問題
了解 LB 的都知道，lvs 這些負載均衡設備能做到請求均勻，但經典實現的一致性 hash 是做不到的。具體場景，可能差別更大。測試腳本放到了 consistenthash balance test github[1]，感興趣的可以看下，測試場景如下：
- 節點數量：10
- 虛擬系數：3, 10, 50, 100, 200, 400, 600, 800, 1000
- 測試算法：MurMurHash, Crc32, Fnv1, CityHash
- 請求數量：50w
方差可以很好的衡量數據分布程度，可以看到，Crc32 是最差的，Fnv1 在數量很少時也差，MurMurHash 和 CityHash 在虛擬節點超過 100 個后迅速收斂。
diff ratio 指數據分布最多的與最小的差值，除以平均值。從數據分布可以看到， MurMurHash, CityHash 首選的 hash 算法。replica 虛節點數量可以調大一些，畢竟動態增減節點不是常態
另一種方案
冒泡哥提到了一種實現，提前預計算好節點順序，可以減少分布 diff ratio. 他在生產環境，只用 20 replica 就降低到了 5% 當然了前提是，增加減節點，必須符合一致性 hash 的思想，數據盡可能少的移動。
測試數據
- 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
小結
這次分享就這些，以后面還會分享更多的內容，如果感興趣，可以關注并點擊左下角的分享轉發哦(:
參考資料
[1]
測試腳本: https://github.com/dongzerun/consistenthash_balance_test,
