redis-clustering

Redis Clustering

Guidance for designing keys and routing reads in a sharded Redis Cluster (and in standalone primary/replica replication). Covers the two failure modes that bite most new cluster users: CROSSSLOT errors on multi-key operations, and overloading primaries with read traffic.

When to apply

• Designing keys for a Redis Cluster deployment.
• Debugging a CROSSSLOT error on MGET, SDIFF, transactions, or pipelines.
• Implementing transactions / Lua scripts that touch multiple keys.
• Scaling out read traffic without adding shards.

1. Hash tags for multi-key operations

Redis Cluster distributes keys across 16,384 slots by hashing the key name. Any command that touches multiple keys (MGET, SDIFF, SUNIONSTORE, transactions, pipelines, Lua scripts with multiple KEYS[]) requires all keys to live on the same slot — otherwise the server returns a CROSSSLOT error.

Hash tags force this: the part between { and } is the only thing hashed for slot assignment, so two keys sharing a hash tag always land together.

# Same slot — multi-key ops work
redis.set("{user:1001}:profile",  "...")
redis.set("{user:1001}:settings", "...")
redis.lmove("{user:1001}:pending", "{user:1001}:processed", "LEFT", "RIGHT")

# Different keys, no hash tag — CROSSSLOT on multi-key commands in cluster mode
redis.set("user:1001:profile",  "...")
redis.set("user:1001:settings", "...")
pipe = redis.pipeline()
pipe.get("user:1001:profile")
pipe.get("user:1001:settings")
pipe.execute()  # CROSSSLOT error in cluster

Rules of thumb:

• Use a tag scoped to the meaningful entity, e.g. {user:1001}. Avoid bare {1001} — unrelated namespaces (purchase:{1001}, employee:{1001}) would all collide on the same slot.
• Only tag where you actually need multi-key ops. Tagging everything creates hotspots and defeats the point of sharding.
• A single-key command on a hash-tagged key works fine, so adding tags later is incremental — but renaming keys in production is painful, so plan tagging up front for entities you'll group.