Mastering Redis Sentinel: Setup, Failover, and Multi‑Sentinel Configuration

What Is Redis Sentinel?

Sentinel monitors the health of a Redis deployment. It runs as an independent process and provides two functions:

Monitor whether the master and replica instances are running correctly.

Automatically promote a replica to master when the master fails.

Architecture

Single‑sentinel architecture:

Multiple‑sentinel architecture (sentinels monitor each other):

Test Environment

Example one‑master‑multiple‑replica setup:

Configuring a Sentinel

Create a Sentinel configuration file: vim sentinel.conf Add a monitor line: sentinel monitor taotaoMaster 127.0.0.1 6379 1 Parameters:

taotaoMaster : name of the master to monitor (customizable, may contain letters, digits, “.-_”).

127.0.0.1 : IP address of the master.

6379 : port of the master.

1 : quorum (minimum number of votes required to consider the master down).

Start the Sentinel process: redis-sentinel ./sentinel.conf Console output shows that Sentinel started (ID 9059917216012421e8e89a4aa02f15b75346d2b7), added a monitor for the master, discovered two replicas, and began monitoring them.

Slave Crash and Recovery

After killing the slave process (port 6380), Sentinel logs:

2989:X 05 Jun 20:09:33.509 # +sdown slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379

When the slave is restarted, Sentinel logs:

2989:X 05 Jun 20:13:22.716 * +reboot slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:13:22.788 # -sdown slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379

The slave re‑joins the replication group, and the “-sdown” entry indicates service recovery.

Master Crash and Recovery

When the master (port 6379) goes down, Sentinel logs a series of events leading to failover:

2989:X 05 Jun 20:16:50.300 # +sdown master taotaoMaster 127.0.0.1 6379  # master down
2989:X 05 Jun 20:16:50.300 # +odown master taotaoMaster 127.0.0.1 6379 # quorum 1/1
2989:X 05 Jun 20:16:50.300 # +new-epoch 1
2989:X 05 Jun 20:16:50.300 # +try-failover master taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.304 # +vote-for-leader 9059917216012421e8e89a4aa02f15b75346d2b7 1
2989:X 05 Jun 20:16:50.304 # +elected-leader master taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.304 # +failover-state-select-slave master taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.357 # +selected-slave slave 127.0.0.1:6381 127.0.0.1 6381 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.357 * +failover-state-send-slaveof-noone slave 127.0.0.1:6381 127.0.0.1 6381 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.420 * +failover-state-wait-promotion slave 127.0.0.1:6381 127.0.0.1 6381 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.515 # +promoted-slave slave 127.0.0.1:6381 127.0.0.1 6381 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.515 # +failover-state-reconf-slaves master taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:50.566 * +slave-reconf-sent slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:51.333 * +slave-reconf-inprog slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:52.382 * +slave-reconf-done slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:52.438 # +failover-end master taotaoMaster 127.0.0.1 6379
2989:X 05 Jun 20:16:52.438 # +switch-master taotaoMaster 127.0.0.1 6379 127.0.0.1 6381  # new master
2989:X 05 Jun 20:16:52.438 * +slave slave 127.0.0.1:6380 127.0.0.1 6380 @ taotaoMaster 127.0.0.1 6381
2989:X 05 Jun 20:16:52.438 * +slave slave 127.0.0.1:6379 127.0.0.1 6379 @ taotaoMaster 127.0.0.1 6381

After the failover, port 6381 becomes the new master with 6380 as its replica. When the original master (6379) comes back online, Sentinel logs:

2989:X 05 Jun 20:35:32.172 # -sdown slave 127.0.0.1:6379 127.0.0.1:6379 @ taotaoMaster 127.0.0.1 6381
2989:X 05 Jun 20:35:42.137 * +convert-to-slave slave 127.0.0.1:6379 127.0.0.1:6379 @ taotaoMaster 127.0.0.1 6381

Thus the recovered instance is added as a replica of the new master.

Configuring Multiple Sentinels

To run several Sentinel instances, add multiple monitor lines to the same configuration file:

sentinel monitor taotaoMaster1 127.0.0.1 6381 1
sentinel monitor taotaoMaster2 127.0.0.1 6381 2