Consistent Hashing and Sharding

Sharding spreads data across nodes so no single machine holds everything, but naive modulo hashing remaps almost every key when a node joins or leaves. Consistent hashing places nodes and keys on a hash ring so that adding or removing a node only reshuffles the keys near it, roughly K/N keys instead of all of them. Virtual nodes even out load imbalance. AI, ML, and GenAI engineer interviews probe it because vector indexes, KV caches, and feature stores are all sharded, and rebalancing cost is the difference between a rolling deploy and an outage.