Technically, horizontal scaling is the job of the application server runtime. If you can keep server state limited it's possible to use clustered caches. Which

Technically, horizontal scaling is the job of the application server runtime. If you can keep server state limited it's possible to use clustered caches. Which requires a lot more setup than just failing over to another stateless node... A problem with component based frameworks like JSF is they like to put the entire component tree in the session to know which component is associated with a browser control. Complex pages will have a huge overhead to sync the state to a central cache or another node.