What is Consistent Hashing?

Consistent Hashing is a distributed hashing scheme that operates independently of the number of servers or objects in a distributed hash table. It offers a significant advantage over conventional hashing techniques, particularly in its ability to efficiently handle the addition or removal of nodes in a system. Consistent Hashing is fundamental in modern distributed systems such as distributed caches and databases, as it ensures minimal data reorganization and reduces the need for data migration when scaling the system.

How does Consistent Hashing work?

In Consistent Hashing, a continuum or hash ring is visualized, and each node or data item in the system is assigned a position on this ring via a hash function. When looking up a particular key, one would move clockwise around the ring until encountering the first server. This approach significantly minimizes the number of keys that need to be reassigned when a node is added or removed - only K/n keys need to be reassigned where K is the total number of keys, and n is the number of slots. This distribution characteristic of Consistent Hashing minimizes the need to rehash and redistribute only a small amount of data, which enhances overall system performance.

Why use Consistent Hashing?

Consistent Hashing can effectively deal with the problem of hot spots in a distributed system. It distributes data evenly among the available nodes, reducing the likelihood of overloading a single node. Its greatest advantage lies in its handling of data re-distribution when the nodes change, which often happens in a large-scale, dynamic environment. Unlike traditional hash functions, which can result in massive data relocation, Consistent Hashing minimizes data movement, thereby saving substantial computational resources.

Use Cases of Consistent Hashing

Distributed Caching: In large-scale web applications where caching is implemented to reduce database load, Consistent Hash helps distribute the cache evenly across multiple cache servers.

Content Delivery Networks (CDNs): CDNs use Consistent Hashing to distribute content efficiently to various servers in different geographical areas. This helps reduce latency and improve response time.

Distributed Databases: Distributed databases use Consistent Hashing to ensure that the database records are evenly distributed across different database servers. This powerful and highly scalable technique becomes critical for applications that handle massive data sets and maintain high availability and performance levels.


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