-
The Web Tier
The web tier can run a load-balancer with multiple web servers serving as reverse proxies.
-
The Application Tier
In the application tier any number of application servers can be clustered, and since authoring and delivery applications are separated, these can be individually clustered as well.
-
The Database Tier
The database tier can consist of a single database server, a cluster of database servers, or some master/master or master/slave setup.