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.

• 192.168.1.58 redis (virtual IP-address)
• 192.168.1.59 redis01 (master)
• 192.168.1.60 redis02 (replica)
• 192.168.1.61 redis03 (replica)

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. Make sure to add its IP-address in the beginning on the line. (running as a cluster currently requires this due to a bug, so we will do this although we are not setting up a cluster).

bind 192.168.1.59 127.0.0.1

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

protected-mode no

For better dataintegrity enable this.

appendonly yes

Now restart the redis master

service redis-server restart

And stop the replicas

service redis-server stop

On each of the replicas, set them up af a replica of the master. (This was earlier called slaveof)

replicaof 192.168.1.59 6379

Start the replicas again

services redis-server start

You should now be able to see the replicas from the master redis-cli -h 192.168.1.59.

192.168.1.59:6379> info replication
# Replication
role:master
connected_slaves:2
slave0:ip=192.168.1.60,port=6379,state=online,offset=2615,lag=0
slave1:ip=192.168.1.61,port=6379,state=online,offset=2615,lag=1
master_replid:79a21f08669303e71990ea8819830f30c94384c6
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:2615
second_repl_offset:-1
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:1
repl_backlog_histlen:2615

Redis Sentinel

Stop the sentinel

service redis-sentinel stop

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

Add the IP-address for each of the servers. Each of the redis servers needs to listen on its own IP-address.

bind 192.168.1.59 127.0.0.1

Configure each of the hosts to monitor the redis master server, and at least 2 sentinels should agree to change the master.

sentinel monitor mymaster 192.168.1.59 6379 2

Set the time the redis master can be down

sentinel down-after-milliseconds mymaster 5000

Set the number of replicas who can change master at the same time. Setting this too high may cause none of the replicas to respond.

sentinel parallel-syncs mymaster 1

Finally set the timeout before the failover happens

sentinel failover-timeout mymaster 10000

Start the sentinel

service redis-sentinel start

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

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