Consistent hashing

Consistent hashing is a special kind of hashing such that when a hash table is resized, only ${\displaystyle K/n}$ keys need to be remapped on average, where ${\displaystyle K}$ is the number of keys, and ${\displaystyle n}$ is the number of slots. In contrast, in most traditional hash tables, a change in the number of array slots causes nearly all keys to be remapped because the mapping between the keys and the slots is defined by a modular operation.

Consistent hashing achieves some of the same goals as rendezvous hashing (also called HRW Hashing). The two techniques use different algorithms, and were devised independently and contemporaneously.

The term "consistent hashing" was introduced by Karger et al. at MIT for use in distributed caching. An academic paper from 1997 introduced the term "consistent hashing" as a way of distributing requests among a changing population of Web servers. Each slot is then represented by a node in a distributed system. The addition (joins) and removal (leaves/failures) of nodes only requires ${\displaystyle K/n}$ items to be re-shuffled when the number of slots/nodes change.

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