Consider a problem, where a set of records needs to be assigned to number of servers. An easy way of doing that is to evenly distribute records in each server. For that, we can use
mod operation. For example, for a record we can get the server by
r mod n formula.
Now the issue is if we want to add a new server or if we want to delete a server then nearly all the record keys to be remapped. This brings a great deal of overhead.
In order to solve the above problem, consistent hashing is used because in this technique on average needs to be remapped, where is the number of records and is the number of servers slots. It should be done when:
There are many benefits to using consistent hashing:
The key idea behind consistent hashing is that every record and server is mapped on the unit circle. Each record is then assigned to the first server that appears on the circle in a clockwise direction. This brings even distribution of records.
Here, if a new server is added to the unit circle then the records next to the server need to be updated whereas all the other records maintain the previous assignments. Similarly, when a server is removed from the unit circle, only records that are associated with it needs to be updated.
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