Consistent hash ring
Distribute keys across N nodes so that adding/removing a node only moves 1/N of the keys — instead of all of them with a plain hash % N.
David Karger et al. (1997). Place nodes and keys on a circular hash ring (0 to 2³²-1). Each key belongs to the next node clockwise. Adding a node ⇒ only the keys between the predecessor and the new node move. Trick: virtual nodes — every physical node is placed 100–200 times on the ring for an even distribution. The driving technique in Memcached clusters, Cassandra, DynamoDB, Riak, Akamai's CDN. Competitors: Rendezvous hashing, Jump consistent hash (faster but requires knowing N), Maglev (Google).