Redis Cluster

Introduction

Redis Cluster shards data across masters using 16384 hash slots — CRC16(key) mod 16384 assigns each key to a slot owned by one master. Horizontal scale when one node's RAM or QPS ceiling is hit.

Clients must be cluster-aware (MOVED/ASK redirects) or use a proxy. Multi-key operations require same slot — use hash tags {user}:session and {user}:cart to co-locate.

Minimum six nodes (3 masters + 3 replicas) recommended for production fault tolerance.

Understanding the topic

Key concepts

• 16384 slots partitioned among masters.
• MOVED redirect — permanent slot owner.
• ASK — importing slot during resharding.
• Hash tag {foo} forces same slot.
• Replica per master for HA within cluster.
• CLUSTER FAILOVER for manual promotion.

flowchart LR
  App -->|CRC16 slot| NodeA
  App -->|MOVED| NodeB
  NodeA --- NodeC

Step-by-step explanation

1. Client computes slot from key.
2. Routes command to node owning slot.
3. Wrong node returns MOVED new host:port.
4. Smart client caches slot map.
5. Gossip propagates topology changes.

Syntax reference

Common commands

• -c enables cluster redirect follow.
• Hash tag {user} in key name.
• CROSSSLOT error — keys in different slots in one transaction.

CLUSTER INFO
CLUSTER NODES
CLUSTER KEYSLOT mykey
redis-cli -c -h node1 CLUSTER SLOTS

Informative example

Cluster-aware session + cart keys with hash tag:

# Both keys land in same slot due to {42}
SET user:{42}:session "data" EX 3600
HSET user:{42}:cart sku1 2

redis-cli -c CLUSTER KEYSLOT user:{42}:session

Without hash tag, MULTI touching session and cart may CROSSSLOT fail.

Real-world use

Real-world use cases

• Dataset exceeds single node RAM.
• Write QPS beyond one event loop.
• Multi-tenant SaaS shard by tenantId tag.
• Global session store at Netflix scale.
• Replace client-side consistent hashing with native Cluster.

Best practices

• Use hash tags for related multi-key ops.
• Cluster-aware client (Lettuce cluster mode).
• Balance slots across masters — rebalance tool.
• Monitor cluster_state:ok in CLUSTER INFO.
• Avoid big multi-key transactions cross-slot.
• Test resharding in staging before prod.

Common mistakes

• Non-cluster client ignoring MOVED.
• Multi-key ops without hash tags.
• Too few nodes — no replica quorum.
• Hot slot from poor key design.

Advanced interview questions

Summary

Cluster = 16384 slots across masters.