Consistent hashing and placement balance

Modulo routing h(key)%N is simple, but changing N relocates many keys. Consistent hashing places tokens and keys on a ring and selects the next clockwise token. Adding a node moves only affected intervals. Virtual nodes improve placement balance and weighting, but do not automatically solve hot-key load.

Rendezvous hashing scores each key-node pair and selects the maximum. A new node moves only the keys it wins. Every participant needs the same stable seed and hash; process-randomised runtime hashes are unsuitable for routing.

Placement does not provide replication consistency. Replication factors, failure domains, membership, migration, tombstones, read repair and quorums need separate design. Balanced key counts do not ensure balanced request load. Not every load balancer uses consistent hashing.

import hashlib
def route(key,nodes):
    return max(nodes,key=lambda node:hashlib.sha256((key+":"+node).encode()).digest())
for i in range(100):
    key=str(i); old=route(key,["A","B","C"]); new=route(key,["A","B","C","D"])
    assert old==new or new=="D"