How are slots distributed among nodes?

In Redis Cluster, every key is distributed across 16384 hash slots, and those slots are split among the master nodes.

#1. Hash slots

• There are 16384 slots in total (0-16383).

• Every key lands in a slot via this formula:

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  CRC16(key) % 16384

• The slot determines which master holds the key.

#2. Distributing slots among master nodes

• When the cluster is created, slots are spread evenly across all master nodes. An example with 3 masters:

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  master1 → slots 0-5460 master2 → slots 5461-10922 master3 → slots 10923-16383

• If a new master is added, some slots get moved from the existing masters onto it.

#3. Replicas

• Replicas don't receive slots directly.
• They simply copy their master's data.
• So a read request to a replica only works with the data of the master it's subordinate to.

#4. Redistribution during scaling

• Redis Cluster supports resharding:
  • some slots move from an overloaded master to a new one;
  • the data for those slots moves automatically;
  • clients get notified about the slots' new location via MOVED.

#5. Summary

• Slots are a logical partitioning of keys.
• Every master serves a specific range of slots.
• Replicas copy their master's slots.
• Adding new nodes or removing old ones comes with a redistribution of slots to balance the load.

This mechanism provides horizontal scaling, even load, and fault tolerance.