Redis Clustering

---
name: redis-clustering
description: Redis Cluster and replication guidance covering hash tags for multi-key operations, avoiding CROSSSLOT errors, and reading from replicas to scale read-heavy workloads. Use when designing keys for a sharded Redis Cluster, debugging CROSSSLOT errors on MGET / SDIFF / pipelines, configuring a multi-key transaction in a cluster, or routing reads to replicas for caches, analytics, or dashboards.
license: MIT
metadata:
  author: Redis, Inc.
  version: "0.1.0"
---

# 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.