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Database / REDIS

What is Redis Sentinel used for?

Sentinel is a separate, lightweight process (or set of processes) that monitors a Redis master and its replicas, and automatically handles failover if the master becomes unavailable — promoting a replica to master and reconfiguring the rest of the replicas to follow the new master, without a human needing to intervene.

# sentinel.conf
sentinel monitor mymaster 127.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 60000

Running multiple Sentinel instances (typically 3 or 5, spread across different hosts) is standard, since a single Sentinel would itself be a single point of failure for failover detection; Sentinels use a quorum vote among themselves to agree a master is genuinely down before triggering failover, which guards against one Sentinel mistakenly declaring a failure due to its own local network issue. Sentinel is aimed at high availability for a single logical dataset (one master, several replicas); for horizontally scaling data across multiple masters, Redis Cluster is the separate, complementary tool.

What is Sentinel's main job?
Why are multiple Sentinel instances typically run together rather than just one?

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