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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:

```javascript
  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:

```javascript
  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.

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