When should you use Redis Sentinel instead of Redis Cluster?

The deciding factor is whether the real need is high availability for a dataset that fits comfortably on one node, or horizontal scaling across many nodes because the dataset or write throughput has outgrown a single instance.

Sentinel is the right fit when a single master's capacity (memory and throughput) is genuinely sufficient, and the goal is just automatic failover if that master goes down — it's simpler to operate, has no sharding restrictions (multi-key operations, transactions across arbitrary keys, and all 16 numbered databases work normally), and client configuration is comparatively straightforward.

Redis Cluster is the right fit once the dataset or throughput genuinely needs to span multiple masters — it adds high availability too (each master can have its own replicas, similar to Sentinel's role but built into the cluster itself), but at the cost of the sharding restrictions covered elsewhere (same-slot requirement for multi-key ops, no numbered databases beyond 0, more complex client and operational tooling).

The practical rule: don't reach for Cluster's added complexity just for high availability if a single master would otherwise be plenty — Sentinel solves that specific problem with meaningfully less operational overhead.