Consistent Hashing Explained for System Design Interviews

When you shard a cache or database with hash(key) mod N, adding or removing one server remaps almost every key. Consistent hashing limits the damage to roughly 1/N of the keys.

How it works

1. Map servers and keys onto the same ring of hash values (for example 0 to 2^32 - 1).
2. A key belongs to the first server clockwise from its position.
3. When a server joins, it takes keys only from its clockwise neighbour; when one leaves, its keys move to the next server.