How to setup Drupal 7 for a High Traffic Website

The biggest issue with Drupal is that it uses the Database for caching and sessions.
So we change that, in my example I use redis, but all modules here work also with memcached & others.

a) Use Redis for Cache

see http://ixorthings.blogspot.com.es/2016/02/how-to-use-redis-on-all-drupal-7-cache.html

b) Use Redis for Sessions

Warning: Due to Drupals failure of having a open/standard session table the module might break other modules sessions! There is no fix for that. IF you run into issues try the native php management (see session proxy module man) which should work fine no matter what module.

Install session_proxy module

Add this to the settings.php

$conf['session_inc'] = 'sites/all/modules/contrib/session_proxy/session.inc';
$conf['session_storage_force_default'] = FALSE;
$conf['session_storage_class'] = 'SessionProxy_Storage_Cache';
$conf['session_storage_options']['cache_backend'] = 'Redis_Cache';

Pay attention to the module path!