Redis in ProductionLesson 6.3

Redis Cluster: horizontal sharding and how data is distributed

hash slot concept, 16384 slots, CRC16 key hashing, node assignment, CLUSTER INFO, hash tags, MOVED redirect, ASK redirect, minimum 3 primary nodes

Redis Cluster for horizontal scale

Redis Cluster shards data across multiple nodes. Each key is assigned to one of 16,384 hash slots using CRC16. Each node owns a range of slots.

Creating a cluster

# redis.conf on each node
cluster-enabled yes
cluster-config-file nodes.conf
cluster-node-timeout 5000

# Create cluster with 3 primaries + 3 replicas
redis-cli --cluster create \
  127.0.0.1:7000 127.0.0.1:7001 127.0.0.1:7002 \
  127.0.0.1:7003 127.0.0.1:7004 127.0.0.1:7005 \
  --cluster-replicas 1

Key routing

When you write a key, the client computes CRC16(key) % 16384 to find the slot, then sends the command to the owning node. If you hit the wrong node, Redis replies with MOVED and the client retries. Hash tags let you colocate related keys: {user:42}:profile and {user:42}:settings both hash on user:42, landing on the same node.

Multi-key operations

Multi-key commands like MSET only work if all keys share the same hash slot. Use hash tags to guarantee colocation when you need cross-key atomicity in a cluster.