Consistent Hash Rings
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Being able to manage distributed groups enables the utilization of several distributed algorithms. One of them is named consistent hash rings.
A traditional way to spread keys across a distributed system, which is composed of n nodes, is to compute which node should be responsible for a key by using: hash( object) % n. Unfortunately, as soon as n changes, the result of the modulo operation changes and therefore all keys are remapped to a new node. This remapping causes a massive shuffling of data or processing in a cluster.
The point of consistent hashing is to avoid that. By using a different method of computing, when n changes, only K /n of the keys are remapped to the remaining nodes, where K is the number of keys and n the number of nodes handling those keys.
The hash ring
A hash ring is a hashing space that wraps around itself to form a circle. That’s why it is called a ring. Every key computed using the consistent hashing function maps somewhere on this hash space. That means that a key is always in the same place on the ring. The ring is then split into P partitions, where P is a magnitude larger than the number of nodes (a lot more partitions than the nodes). ...