Hash ring

Hash keys and servers into the same circular space. A key belongs to the next server clockwise.

Position:  0────12────────────47────────────78──────────99 ↻
Server:        Saffron         Indigo         Teal
Key:               k1=18          k2=53                 k3=91

k1 → Indigo     k2 → Teal     k3 → Saffron (wraps past 99)

Each server owns the interval after the previous server up to its position. Indigo owns (12, 47].

Add or remove a server

Add a server at position 60:

Before:  ... 47 Indigo ───────────── 78 Teal ...
After:   ... 47 Indigo ── 60 Quartz ── 78 Teal ...

Only keys in (47, 60] move from Teal to Quartz. Removing a server moves its keys to the next server clockwise. All other keys stay put.

With hash(key) % N, adding a server changes N and remaps most keys. Consistent hashing limits movement to nearby intervals, which helps when scaling caches and sharded stores.

Virtual nodes give each physical server several positions on the ring, helping balance ownership.