为什么Redis集群有16384个槽(上)

一.前言

我在《那些年用过的Redis集群架构（含面试解析）》一文里提到过，现在redis集群架构，redis cluster用的会比较多。 如下图所示

对于客户端请求的key，根据公式HASH_SLOT=CRC16(key) mod 16384，计算出映射到哪个分片上，然后Redis会去相应的节点进行操作！

那大家思考过，为什么有16384个槽么? ps:CRC16算法产生的hash值有16bit，该算法可以产生2^16-=65536个值。换句话说，值是分布在0~65535之间。那作者在做mod运算的时候，为什么不mod65536，而选择mod16384？

其实我当初第一次思考这个问题的时候，我心里是这么想的，作者应该是觉得16384就够了，然后我就开始查这方面资料。

很幸运的是，这个问题，作者是给出了回答的！ 地址如下: https://github.com/antirez/redis/issues/2576

作者原版回答如下: The reason is:

Normal heartbeat packets carry the full configuration of a node, that can be replaced in an idempotent way with the old in order to update an old config. This means they contain the slots configuration for a node, in raw form, that uses 2k of space with16k slots, but would use a prohibitive 8k of space using 65k slots. At the same time it is unlikely that Redis Cluster would scale to more than 1000 mater nodes because of other design tradeoffs. So 16k was in the right range to ensure enough slots per master with a max of 1000 maters, but a small enough number to propagate the slot configuration as a raw bitmap easily. Note that in small clusters the bitmap would be hard to compress because when N is small the bitmap would have slots/N bits set that is a large percentage of bits set.

因此，能看懂上面那段话的读者。这篇文章不用看了，因为作者讲的很清楚了。本文只是对上面那段话做一些解释而已。