HAProxy

Installation

Most Linux distros have HAProxy in their official repositories. To determine the package name for a given distro, I would use Repology.

To use HAProxy, you need to set up a configuration file. On Debian and Ubuntu systems, the format for this file is in the compressed text document /usr/share/doc/haproxy/configuration.txt.gz, and a simple introduction which is good enough for basic use cases can be found at this page on haproxy.com.

Non-HTTP Apps

MySQL/MariaDB

For my Redundancy/High Availability project, I had to use HAProxy to proxy requests between 2 Nextcloud servers and a Galera cluster (i.e. a redundant, distributed MySQL/MariaDB database). There were a few extra steps to this - I had to create haproxy@10.0.6.11 and haproxy@10.0.6.12 users on the Galera cluster, which allowed haproxy to determine which of the servers were alive at any given time. I added nextcloud-user database users to the cluster, with the same password and permissions as those on the nextcloud servers themselves, to allow HAProxy to pass on the requests. I then added the following to the ha1 server's HAProxy configuration, and another entry that was identical save for the name to the ha2 server's configuration.

listen ha1_sql
    bind 10.0.6.10:3306
    balance roundrobin
    mode tcp
    option tcpka
    option mysql-check user haproxy
    server u1 10.0.6.21:3306 check weight 1
    server u2 10.0.6.22:3306 check weight 1
    server u3 10.0.6.23:3306 check weight 1