4.1 Redis Cluster

1. What Redis Cluster Solves

Redis Cluster shards a dataset across multiple primary nodes while providing replica-based failover.

This addresses two limits of a single-primary Sentinel topology:

• one node’s memory capacity;
• one primary’s write/CPU throughput.

2. Hash Slots

Redis Cluster divides the key space into 16,384 hash slots.

Conceptually:

slot = CRC16(key) mod 16384

Each primary owns a subset of slots. Cluster-aware clients map slots to nodes.

3. Routing

If a client sends a command to the wrong node, Redis can return a MOVED redirect identifying the node that owns the slot. During resharding, ASK redirects can be used for migrating slots.

Smart clients cache the slot map and update it as topology changes.

4. Hash Tags

For multi-key operations, all involved keys generally need to belong to the same slot.

A hash tag forces only the substring inside {...} to participate in slot hashing:

{user:42}:profile
{user:42}:sessions

Both can land in the same slot.

5. Replication and Failover

Each primary can have replicas. Cluster nodes exchange failure information and can promote an eligible replica when a primary is considered failed and sufficient cluster authorization exists.

Redis Cluster’s failover protocol is not Raft. It provides Redis-specific sharding/failover semantics and does not replicate a single consensus log across all masters.

6. Consistency Limits

Replication is normally asynchronous. Network partitions and failover can therefore lose writes that had been acknowledged by a primary but not replicated before promotion.

Redis Cluster prioritizes practical availability and sharding performance rather than strict linearizability.

7. Multi-Key Constraints

Operations involving multiple keys can execute atomically only when the keys are colocated appropriately. This affects transactions, Lua/functions, set operations, and rename-like commands.

8. Operations

Modern deployments use redis-cli --cluster and managed-service tooling for creation and resharding; older redis-trib.rb instructions are legacy.