Setting Up Redis Sentinel for High Availability: Configuration and Failover Guide

Sentinel is Redis's high‑availability (HA) solution that continuously monitors one or more master servers and their slaves, and automatically promotes a slave to master when a monitored master goes down.

The core functions of Sentinel are:

Monitoring – constantly checks the health of masters and slaves.

Notification – sends alerts via API when a server encounters problems.

Automatic failover – promotes a slave to master and redirects clients to the new master.

0x01: High‑Availability Sentinel Design

Configuration files are created based on redis.conf and sentinel.conf:

cp /usr/local/redis/etc/redis.conf /usr/local/redis/redis-6379.conf
cp /usr/local/redis/etc/redis.conf /usr/local/redis/redis-26379.conf
cp /usr/local/redis/etc/redis.conf /usr/local/redis/redis-26380.conf

Sentinel configuration files:

cp sentinel.conf /usr/local/redis/sentinel-36378.conf
cp sentinel.conf /usr/local/redis/sentinel-36379.conf
cp sentinel.conf /usr/local/redis/sentinel-36380.conf

Master configuration (redis‑6379.conf)

bind 127.0.0.1 192.168.122.1
port 6379
daemonize yes
pidfile /var/run/redis_6379.pid
logfile "/tmp/redis-6379.log"
dbfilename dump-6379.rdb
requirepass new2020

Slave‑1 configuration (redis‑26379.conf)

bind 127.0.0.1 192.168.122.1
port 26379
daemonize yes
pidfile /var/run/redis_26379.pid
logfile "/tmp/redis-26379.log"
dbfilename dump-26379.rdb
requirepass new2020
replicaof 127.0.0.1 6379
masterauth new2020

Slave‑2 configuration (redis‑26380.conf)

bind 127.0.0.1 192.168.122.1
port 26380
daemonize yes
pidfile /var/run/redis_26380.pid
logfile "/tmp/redis-26380.log"
dbfilename dump-26380.rdb
requirepass new2020
replicaof 127.0.0.1 6379
masterauth new2020

Sentinel‑1 configuration (sentinel‑36378.conf)

bind 127.0.0.1 192.168.122.1
protected-mode no
port 36378
daemonize yes
pidfile /var/run/redis-sentinel-36378.pid
logfile "/tmp/redis-36378.log"
dir /tmp/36378
sentinel monitor mymaster 127.0.0.1 6379 2
sentinel auth-pass mymaster new2020
sentinel down-after-milliseconds mymaster 30000
sentinel parallel-syncs mymaster 1
sentinel failover-timeout mymaster 180000
sentinel deny-scripts-reconfig yes

Sentinel‑2 and Sentinel‑3 configurations are identical except for the port numbers (36379, 36380) and corresponding pidfile, logfile, and dir values.

0x02: Starting Redis Services

Start the master:

/usr/local/redis/bin/redis-server /usr/local/redis/redis-6379.conf

Start the two slaves:

/usr/local/redis/bin/redis-server /usr/local/redis/redis-26379.conf
/usr/local/redis/bin/redis-server /usr/local/redis/redis-26380.conf

Start the three sentinel instances:

/usr/local/redis/bin/redis-sentinel /usr/local/redis/sentinel-36378.conf
/usr/local/redis/bin/redis-sentinel /usr/local/redis/sentinel-36379.conf
/usr/local/redis/bin/redis-sentinel /usr/local/redis/sentinel-36380.conf

Verify that all processes are running: ps -ef | grep redis 0x03: Checking Service Status

Use redis-cli info on the master to see role:master and two connected slaves. Run the same command on each slave to see role:slave. On each sentinel, info sentinel shows one master, three sentinels, and the number of slaves.

0x04: Validating Automatic Failover

Stop the master process (e.g., shutdown or kill -9).

Observe that one of the slaves (port 26379) is promoted to master in the sentinel output.

Restart the original master (port 6379) and verify it now appears as a slave using info replication.

Example commands used during the test:

ps -ef | grep redis
kill -9 7973
/usr/local/redis/bin/redis-cli -h 127.0.0.1 -p 36378
info sentinel
/usr/local/redis/bin/redis-cli -h 127.0.0.1 -p 6379
info replication

After the failover, the cluster operates with the new master and the former master functions as a replica, confirming that Redis Sentinel provides reliable automatic failover.