Depends on your situation. Dual load balancers serving a virtual IP allows one to fail while still maintaining connectivity. Behind that, you might run a pool

Depends on your situation. Dual load balancers serving a virtual IP allows one to fail while still maintaining connectivity. Behind that, you might run a pool of machines doing static files (or use a CDN where they handle much of the redundancy for you) and machines that run your app along with machines that handle your datastore. Design your application's architecture so that you can pull off pieces to enable easier scaling.