How many hash slots does redis have?

There are 16384 hash slots built into the Redis cluster. When a key-value needs to be placed in the Redis cluster, redis first uses the crc16 algorithm to calculate a result for the key, and then calculates the remainder of the result to 16384, so that each key Each will correspond to a hash slot numbered between 0-16383, and redis will map the hash slots to different nodes roughly equally according to the number of nodes.

Redis cluster does not use consistent hashing, but introduces the concept of hash slots.

The Redis cluster has 16384 hash slots. Each key is checked modulo 16384 after CRC16 verification to determine which slot to place. Each node in the cluster is responsible for a part of the hash slot. This structure makes it easy to add or delete nodes, and whether a node is added, deleted, or modified, it will not cause the cluster to become unavailable.

The advantage of using hash slots is that you can easily add or remove nodes.

When you need to add a node, you only need to move some hash slots of other nodes to the new node;

When you need to remove a node, you only need to move the removed node Just move the hash slot to other nodes;

At this point, we don’t need to stop all redis services first when adding or removing nodes in the future.

"The concept of hash slots is used instead of consistent hashing algorithm. Isn't it all hashing? What is the reason for doing this?"

Redis Cluster is a simple hash algorithm of crc16 made by myself, without consistent hashing. The author of Redis believes that the effect of its crc16(key) mod 16384 is already good. Although it is not as flexible as consistent hashing, the implementation is very simple, and it is also very convenient to handle when nodes are added or deleted.

"In order to dynamically add and delete nodes, won't data be lost?"

Not losing data when adding or deleting nodes has nothing to do with the hash algorithm. What is required to not lose data is how many pieces of data are there? copies.

"There are also a total of 2 to the 14th power of the cluster, 16384 hash slots, so what are the keys and values ​​stored in each hash slot?"

When you go to Redis When a Key is added to the Cluster, it will be calculated based on crc16(key) mod 16384 which hash slot the key should be distributed to. There will be many keys and values ​​in a hash slot. You can understand it as table partition. When using redis on a single node, there is only one table, and all keys are placed in this table. After switching to Redis Cluster, 16384 partition tables will be automatically generated for you. When you insert data, it will be based on The above simple algorithm determines which partition your key should exist in. There are many keys in each partition.

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