第一节：Redis cluster 槽

为什么 Redis cluster 要设计 16384 个槽

Redis Cluster 采用数据分片机制，定义了 16384个 Slot槽位，集群中的每个Redis 实例负责维护一部分槽以及槽所映射的键值数据。

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！

然而，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。
那在做mod运算的时候，为什么不mod 65536，而选择mod 16384？

redis_slot_16384

这个问题，作者给出了回答， 地址如下: https://github.com/antirez/redis/issues/2576。

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.