FIXME! As memcached looses its content e.g cookies on a reboot, we will have to use redis instead!!!

Apache

Running a redundant apache server does not require much from apache itself, but rather the underlying filesystem and a proxy in front of it. The filesystem is important, so the website can be shared between the servers. For this we will use GlusterFS. PHP sessions will have to be shared betwen the webservers too. Apache will be configured to store the sessions using Redis and finally we will put a proxy in front of apache, which will be a combination of Corosync and HA-proxy.

In this setup we will configure 2 apache servers. None of these will be master/slave/primary/secondary. They will just be running with the same configuration. Since the servers are configured the same way it makes sense to share some files.

Software

apt-get install apache2 php php-mysql

Filesystem

Configure your system as a GlusterFS client, so you have the following in your fstab. 

/etc/glusterfs/www.vol /var/www glusterfs defaults,_netdev,rw 0 0

PHP Sessions

Creating a PHP session on one webserver, does not create it on the other. As requests bounces back and forth between the two webservers, this will be a problem for PHP sites. The solution is called memcached.

Add this to redis configuration file whatever its called .... /etc/php/7.0/mods-available/memcache.ini. 

something

In /etc/php/7.0/apache2/php.ini and /etc/php/7.0/cli/php.ini find the entry session.save_handler = files and change it to the following. 

session.save_handler = redis
session.save_path = "tcp://redis01:6379 tcp://redis02:6379 tcp://redis03:6379"

Copy the configuration to all webservers and finally restart the services. 

service apache2 restart

Testpage

Make a simple webpage, that shows something unique about the system and also creates a PHP-session. Use this to test the setup.

index.php 

   1 <?php
   2         header('Content-Type: text/plain');
   3         session_start();
   4         if(!isset($_SESSION['visits']))
   5                 $_SESSION['visits'] = 0;
   6         $_SESSION['visits']++;
   7         echo "client: " . $_SERVER['REMOTE_ADDR'] . "\n";
   8         echo "server: " . $_SERVER['SERVER_ADDR'] . "\n";
   9         echo "hostname: " . gethostname() . "\n";
  10         echo "visits: " . $_SESSION['visits'] . "\n";
  11         echo "cookie: ";
  12         if (array_key_exists('PHPSESSID', $_COOKIE))
  13                 echo $_COOKIE['PHPSESSID'];
  14         echo "\n";
  15 ?>

References