Consistent hashing

In computer science, consistent hashing is a special kind of hashing technique such that when a hash table is resized, only ${\displaystyle n/m}$ keys need to be remapped on average where ${\displaystyle n}$ is the number of keys and ${\displaystyle m}$ is the number of slots. Consistent hashing evenly distributes cache keys across shards, even if some of the shards crash or become unavailable. 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 is used by Content Delivery Networks because it is useful for distributing requests for content from a rotating population of web servers. Tim Berners-Lee credits consistent hashing algorithms, and Daniel Lewin as their inventor, with solving the slashdotting problem which plagued the World Wide Web in the 1990s.