DISCLAIMER: This is running in Buster

Redis

Redis is an in memory data structure storage. It will be used to share PHP-sessions between the apache servers. To guarantee a robust deployment three redis servers will be configured.

• 192.168.1.59 redis01
• 192.168.1.60 redis02
• 192.168.1.61 redis03

Software

apt-get install redis-server redis-sentinel

Configuration

Redis Server

Change the contents of the redis configuration file in /etc/redis/redis.conf.

Add the IP-address for each of the servers. Each of the redis servers needs to listen on its own IP-address. Due to a bug in redis, the added IP-address will have to be in the beginning of the line, right next to bind. If the IP-address is added in the end of the line, then the cluster will not initialize itself, and a Waiting for the cluster to join... message will continuously write dots.

bind 192.168.1.58 127.0.0.1 ::1

Enable redis to listen on the network, instead of only listening on the loopback device.

protected-mode no

Redis stores its data in memory only, hence its speed. To make sure redis does not loose its state, in case of a hardware failure, this will enable it to store the state as it changes in a file on disk.

appendonly yes

Activate redis cluster features

cluster-enabled yes
cluster-config-file nodes-6379.conf
cluster-node-timeout 15000

Restart redis on all hosts.

service redis-server restart

Next we will need to configure the servers to run as a cluster

redis-cli --cluster create 192.168.1.59:6379 192.168.1.60:6379 192.168.1.61:6379

You should get output similar to this

>>> Performing hash slots allocation on 3 nodes...
Master[0] -> Slots 0 - 5460
Master[1] -> Slots 5461 - 10922
Master[2] -> Slots 10923 - 16383
M: 78252519c58c5e7651554b3c82ba5800e4c11c41 192.168.1.59:6379
   slots:[0-5460] (5461 slots) master
M: b26d5a1ebe4891c68acf1d6b7cea5cc19057849f 192.168.1.60:6379
   slots:[5461-10922] (5462 slots) master
M: 0db96b401cca249b61660ed4d2945d658dcdc6e1 192.168.1.61:6379
   slots:[10923-16383] (5461 slots) master
Can I set the above configuration? (type 'yes' to accept): yes
>>> Nodes configuration updated
>>> Assign a different config epoch to each node
>>> Sending CLUSTER MEET messages to join the cluster
Waiting for the cluster to join

>>> Performing Cluster Check (using node 192.168.1.59:6379)
M: 78252519c58c5e7651554b3c82ba5800e4c11c41 192.168.1.59:6379
   slots:[0-5460] (5461 slots) master
M: 0db96b401cca249b61660ed4d2945d658dcdc6e1 192.168.1.61:6379
   slots:[10923-16383] (5461 slots) master
M: b26d5a1ebe4891c68acf1d6b7cea5cc19057849f 192.168.1.60:6379
   slots:[5461-10922] (5462 slots) master
[OK] All nodes agree about slots configuration.
>>> Check for open slots...
>>> Check slots coverage...
[OK] All 16384 slots covered.

References

• https://redis.io/topics/sentinel

• https://redis.io/topics/replication

• https://www.haproxy.com/blog/haproxy-advanced-redis-health-check/

• https://medium.com/@amila922/redis-sentinel-high-availability-everything-you-need-to-know-from-dev-to-prod-complete-guide-deb198e70ea6

• https://blog.usejournal.com/first-step-to-redis-cluster-7712e1c31847

• https://www.linode.com/docs/applications/big-data/how-to-install-and-configure-a-redis-cluster-on-ubuntu-1604/

• https://www.willandskill.se/en/setup-a-highly-available-redis-cluster-with-sentinel-and-haproxy/

• https://www.tecmint.com/setup-redis-replication-in-centos-8/

• https://www.tecmint.com/setup-redis-high-availability-with-sentinel-in-centos-8/

• https://www.tecmint.com/setup-redis-cluster-in-centos-8/

• https://stackoverflow.com/questions/39568561/how-to-solve-redis-cluster-waiting-for-the-cluster-to-join-issue