As mentioned in my previous post "Horizontally Scaling Drupal" some very bad experience finally allowed me to get open hands and chance to review options how to horizontally scale some Drupal based web sites.
Easiest option is to split on multiple servers apache, cache servers and MySQL. This option require at least 3 servers but lack ability to add increase resources in any other way except vertically; adding RAM & CPU to every server. Second problem here is each component is single point of failure. One server crashes, complete site is down. So with this we are not far from initial state :-(.
Second option is ability to add multiple server to each component. Since I didn’t know where site will be hosted and what do I have available (most important part is Load Balancer) created solutions required some extra servers for proxy/balancing purpose.