Redis基础知识（学习笔记5--Redis Cluster）

HASH_SLOT = CRC16(key) mod 16384

Hash tags
There is an exception for the computation of the hash slot that is used in order to implement hash tags. 
Hash tags are a way to ensure that multiple keys are allocated in the same hash slot. 
This is used in order to implement multi-key operations in Redis Cluster.

To implement hash tags, the hash slot for a key is computed in a slightly different way in certain conditions. 
If the key contains a "{...}" pattern only the substring between { and } is hashed in order to obtain the hash slot. 
However since it is possible that there are multiple occurrences of { or } the algorithm is well specified by the following rules:

IF the key contains a { character.
AND IF there is a } character to the right of {.
AND IF there are one or more characters between the first occurrence of { and the first occurrence of }.
Then instead of hashing the key, only what is between the first occurrence of { and the following first occurrence of } is hashed.

Examples:
The two keys {user1000}.following and {user1000}.followers will hash to the same hash slot since only the substring user1000 will be hashed in order to compute the hash slot.
For the key foo{}{bar} the whole key will be hashed as usually since the first occurrence of { is followed by } on the right without characters in the middle.
For the key foo{{bar}}zap the substring {bar will be hashed, because it is the substring between the first occurrence of { and the first occurrence of } on its right.
For the key foo{bar}{zap} the substring bar will be hashed, since the algorithm stops at the first valid or invalid (without bytes inside) match of { and }.
What follows from the algorithm is that if the key starts with {}, it is guaranteed to be hashed as a whole. This is useful when using binary data as key names.