Understanding Redis Cluster Hash Slots: Concepts, Calculation, Allocation, and Dynamic Scaling

Redis cluster uses 16,384 hash slots to shard data across nodes, ensuring even distribution and supporting dynamic scaling.

Each key's CRC16 hash modulo 16,384 determines its slot; slots are assigned to nodes, each node managing a specific range.

Hash slots enable data partitioning, dynamic expansion/reduction, fault tolerance, and easier management.

Example allocation: Node A slots 0‑5500, Node B slots 5501‑11000, Node C slots 11001‑16383.

Commands to view slots, set keys, and observe redirection:

127.0.0.1:7001> set hello world
-> Redirected to slot [866] located at 127.0.0.1:7000
OK

Dynamic scaling: add a new node (port 7006) with

redis-cli --cluster add-node 127.0.0.1:7006 127.0.0.1:7000 -a 123456

, then reshard slots using redis-cli --cluster reshard 127.0.0.1:7000 -a 123456 (move 300 slots to node 7006).

After resharding, verify slot distribution with cluster nodes. To shrink, move slots back and delete the node with

redis-cli --cluster del-node 127.0.0.1:7006 624060ec59626a594ecffdb2f98e7d11a969d78d -a 123456

.

The article concludes that mastering hash slots is essential for Redis cluster operation and interview preparation.