How Redis Sentinel Ensures Automatic Failover and High Availability
Redis Sentinel provides a high‑availability solution for Redis by continuously monitoring master and replica servers, automatically promoting a replica to master when the original master fails, and seamlessly handling failover and recovery to keep the system operational without manual intervention.
Sentinel is Redis's high‑availability solution.
It consists of one or more Sentinel instances that monitor any number of master servers and all their replicas. When a monitored master goes offline, Sentinel automatically promotes one of its replicas to become the new master, which then continues to handle client requests.
Example:
Server1 is the master, while server2, server3 and server4 are replicas that replicate server1. Sentinel monitors each server.
If the master server server1 goes offline, the replication processes of server2, server3 and server4 are halted, and the Sentinel system detects that server1 is down.
When server1's downtime exceeds the configured threshold, Sentinel performs a failover:
Sentinel selects one of server1's replicas and promotes it to the new master.
Sentinel sends new replication commands to all remaining replicas, making them follow the new master. The failover completes when all replicas start replicating the new master.
Sentinel continues to monitor the original server1 and, when it comes back online, configures it as a replica of the new master.
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