Expanding a Redis Cluster: Adding a Fourth Master and Slave Node

In this guide we demonstrate how to expand an existing Redis cluster from three masters and three slaves to four masters and four slaves by adding a new master node.

2.1 Add master node

2.1.1 Prepare the node – create a copy of an existing configuration file and modify it for the new instance.

# cd /usr/local/redis/cluster/conf
pwd
cp redis-6385.conf redis-6386.conf
more redis-6386.conf

Key parameters in redis-6386.conf include:

bind 0.0.0.0
port 6386
daemonize yes
logfile "/usr/local/redis/cluster/log/redis-6386.log"
dbfilename dump-6386.rdb
appendonly yes
appendfilename "appendonly-6386.aof"
dir /usr/local/redis/cluster/data
requirepass 123456
masterauth 123456
cluster-enabled yes
cluster-config-file nodes-6386.conf
cluster-node-timeout 15000
cluster-announce-ip 192.168.240.8
cluster-announce-port 6386
cluster-announce-bus-port 16386

2.1.2 Start the new node

# ll
# ps -ef | grep redis

(Image showing process list omitted for brevity.)

2.1.3 Add the node to the cluster

# redis-cli -a 123456 --cluster add-node 192.168.240.8:6386 192.168.240.8:6384 --cluster-master-id 6c987380787ed9677a66a1f1ba70c750634a8e4e

The command outputs status messages confirming the addition, slot allocation, and replica assignments, ending with:

[OK] All nodes agree about slots configuration.
[OK] All 16384 slots covered.
[OK] New node added correctly.

After the operation the cluster now has four master nodes, each with its replica, and the slot distribution is balanced.

Conclusion

The master node has been successfully added; further monitoring of the Redis cluster configuration is recommended.

For more Redis tutorials, see the linked articles below:

1. Redis cluster three masters three slaves

2. Sentinel working principle

3. Detailed Redis Sentinel guide

4. Detailed Redis one master two slaves

5. Understanding Redis RDB persistence