= 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.

 * 192.168.1.54 redis (virtual IP-address)
 * 192.168.1.55 redis01 (master)
 * 192.168.1.56 redis02 (replica)
 * 192.168.1.57 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 192.168.1.55 127.0.0.1
}}}

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 192.168.1.55 6379
}}}

Start the replicas again
{{{
services redis-server start
}}}

You should now be able to see the replicas from the master `redis-cli -h 192.168.1.55`.
{{{
192.168.1.55:6379> info replication
# Replication
role:master
connected_slaves:2
slave0:ip=192.168.1.56,port=6379,state=online,offset=2615,lag=0
slave1:ip=192.168.1.57,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 192.168.1.55 127.0.0.1
}}}

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 192.168.1.55 6379 2
}}}

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/