A standby node only helps if you know how to switch to it and that the switch works. Failover is a decision, a procedure, and a rehearsal. Delegating it to a switch you have never thrown is how a graceful handoff becomes an outage. ...
Redis Sentinel supervises a non-Cluster primary/replica deployment and publishes the current primary’s address. It does not store the application’s dataset or proxy its commands. Automatic promotion can restore write availability after a primary fails, but discovery, replica freshness, client reconnection, and protection from stale writers remain separate parts of the system. ...