For extremely large sites (more than 5,000,000 active accounts), you can deploy a Static Cluster of Dynamic Clusters. It is essentially the same as a regular Static Cluster with Frontend Servers, but instead of Backend Servers you install Dynamic Clusters. This solves the redundancy problem of Static Clusters, but does not require extremely large Shared Storage devices and excessive network traffic of extra-large Dynamic Clusters:
Frontend Servers in a "Cluster of Clusters" need access to the Directory in order to implement Static Clustering. The Frontend Servers only read the information from the Directory, while the Backend Servers modify the Directory when accounts are added, renamed, or removed. The hostServer attribute of Account directory records contains the name of the Backend Dynamic Cluster hosting the Account (the name of Backend Cluster Servers without the first domain name element).
Frontend Servers can be grouped into subsets for traffic segmentation. Each subset can have its own load balancer(s), and a switch that connects this Frontend Subset with every Backend Dynamic Cluster.
If you plan to deploy many (50 and more) Frontend Servers, the Directory Server itself can become the main site bottleneck. To remove this bottleneck and to provide redundancy on the Directory level, you can deploy several Directory Servers (each Server serving one or several Frontend subsets). Backend Dynamic Clusters can be configured to update only one "Master" Directory Server, and other Directory Servers can use replication mechanisms to synchronize with the Master Directory Server, or the Backend Clusters can be configured to modify all Directory Servers at once.