Detailed explanation of consistent hashing algorithm implemented by Redis

Consistent Hashing Algorithm is widely used in distributed cache, load balancing and other scenarios, and can effectively improve the performance and scalability of the system. Among them, Redis, as a popular in-memory database, also uses consistent hashing algorithms to achieve data distribution and load balancing. This article will provide a detailed analysis of the consistent hashing algorithm from the perspective of Redis implementation.

1. Introduction to Consistent Hash Algorithm

The consistent hash algorithm was first proposed by David Karger and others. It maps each node to a ring through an algorithm. The data is then mapped to the same ring based on the hash value of its key, and finally the data is assigned to the node closest to it on the ring. In this way, when the number of nodes changes, it will only affect the ownership of a small part of the data on the ring, but not the data ownership of the entire data collection.

At the same time, the consistent hashing algorithm also solves the problem of "hotspot" data sets to a certain extent. Because the distribution of hash values ​​is uniform, the distribution of data is also uniform, which makes the data on any node distributed approximately evenly, thus avoiding the situation where a single node carries too much data.

2. The consistent hash algorithm implemented by Redis

As a high-performance in-memory database, the consistent hash algorithm implemented by Redis is also very efficient and flexible. . Specifically, the consistent hash algorithm implemented by Redis is divided into the following steps:

(1) Initialization ring

First, you need to define a Hash ring to map all nodes to ring. This ring can be implemented using an array or a tree. Redis generally uses a hash ring method, using an ordered linked list to save all nodes. The position of each node in the linked list is determined according to the size of its hash value. In addition, since the number of nodes on the hash ring is generally relatively small, multiple copies can be used to enhance data replication and fault tolerance.

(2) Hash the data

For a piece of data, we need to Hash its key and map it to a certain position on the hash ring. It should be noted here that Redis uses a special Hash algorithm, whose principle is similar to the MD5 algorithm. The purpose of this algorithm is to ensure an even distribution of hash values ​​as much as possible.

(3) Assign nodes to data

After finding the corresponding position of the data on the hash ring, you need to find the node where it is located. This process can be implemented in two ways: clockwise search and skip search. The former searches clockwise along the hash ring starting from the current position until the first node is found. This method is very simple, but may cause node load imbalance. On the contrary, skip search jumps a fixed step size on the ring to find the node. This step size is generally the average hash value distance of the node. Although this method is more complex, it can better balance the node load.

(4) Add/remove nodes

When a node is added/removed from the system, only the data responsible for this node needs to be recalculated. Specifically, if you add a node, you need to move all the data it is responsible for to the new node. If a node is removed, all data it is responsible for needs to be allocated to other nodes. In this process, multi-copy replication is generally used to ensure data consistency and fault tolerance.

3. Summary

The consistent hashing algorithm is an efficient, flexible and scalable algorithm that can be used in distributed cache, load balancing and other scenarios. As a popular in-memory database, Redis also uses consistent hashing algorithms to achieve data distribution and load balancing. Through the analysis and analysis of the consistent hash algorithm implemented by Redis, we can have a deeper understanding of the principle and implementation details of this algorithm.

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