Distributed caches (Redis/Memcached) scale single-node caches across a cluster.
Consistent Hashing
A mechanism to distribute keys across nodes such that adding/removing a node only disrupts a small fraction (1/N) of keys. It maps nodes and keys to a logical "ring".
Eviction Policies
LRU (Least Recently Used), LFU (Least Frequently Used), and TTL (Time To Live). Redis primarily uses an approximated LRU for performance.