SKILL.md
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
CROSSSLOTerror onMGET,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.