D4Consistent hashing

The problem

1 s on screen = 100 ms

You add one more cache server to help. Why does the database get hit harder?

hash(key) % N

Three caches share 150 keys. A router picks each key's cache with hash(key) % N, N being the number of caches.

Live metrics for each version of the scene
hash % N	Hash ring	Change
39%	41%	better: +4%
49%	51%	worse: +6%
0%	0%	—

D4Consistent hashing

Spread keys over N caches with hash % N, and adding one cache moves almost every key. Consistent hashing moves only the keys it has to.