Why Redis Cluster Has 16384 Slots?

Redis is a popular in-memory data structure store used by many developers worldwide. It is widely known for its speed, simplicity, and versatility in handling various types of data. Redis provides several features that make it a favorite among developers such as caching, messaging, and pub/sub functionality.

One of the critical features that make Redis stand out from other databases is its ability to scale horizontally. Redis can be scaled horizontally by using Redis Cluster, which allows you to distribute your data across multiple nodes in a cluster.

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Redis Cluster uses a concept called slots to distribute data among the nodes. A slot is a range of keys that are mapped to a specific node in the cluster. In Redis Cluster, there are 16384 slots available for use.

You might wonder why Redis Cluster has exactly 16384 slots? The answer lies in how Redis distributes keys among the nodes in the cluster.

When you insert or retrieve a key from Redis Cluster, it uses a hash function to determine which slot the key belongs to. The hash function used by Redis is called CRC16 (Cyclic Redundancy Check 16-bit). The CRC16 hash function returns an integer value between 0 and 65535.

To map the integer value returned by the hash function to one of the available slots, Redis takes the modulo of the integer value with 16384 (the total number of available slots). This ensures that each integer value obtained from the hash function maps uniquely to one of the available slots.

In simple terms, if you have n nodes in your cluster, each node will be responsible for managing approximately 16384/n slots. For example, if you have six nodes in your cluster, each node will manage approximately 2730 slots.

Having exactly 16384 slots also ensures that you can add or remove nodes from your cluster without having to rehash all your keys. When you add or remove a node from your cluster, only a portion of the keys need to be moved to the new node. This is because each node is responsible for managing a specific range of slots.

In conclusion, Redis Cluster has 16384 slots because it provides a unique mapping between the integer value obtained from the hash function and one of the available slots. This ensures that each key is distributed evenly among the nodes in the cluster and allows you to scale horizontally by adding or removing nodes without having to rehash all your keys.