Considerations in Building a Large Infrastructure

Analyze your types of servers for instance Facebook had 5 types of servers:
Web page servers, database computers, data storage systems, news feed servers and something called memcache servers

Possible infrastructure I could imagine being implemented

DNS           Load balancers          Web servers                    Databases and Data Storage
round  --> 1 --------------->      group of web servers                   Session Storage instead of sticky sessions on a web server?
robin            2 --------------->      2nd group of web servers               Databases SQL or NoSQL distributed for failover and also size of data
with Finally Batch Processing and Analytics
Health Check

Good article on load balancing

DNS roundrobin is excellent to increase capacity, by distributing the load across multiple points (potentially geographically distributed). But it does not provide fail-over. You must first describe what type of failure you are trying to cover. A server failure must be covered locally using a standard IP address takeover mechanism (VRRP, CARP, ...). A switch failure is covered by resilient links on the server to two switches. A WAN link failure can be covered by a multi-link setup between you and your provider, using either a routing protocol or a layer2 solution (eg: multi-link PPP). A site failure should be covered by BGP : your IP addresses are replicated over multiple sites and you announce them to the net only where they are available.

http://serverfault.com/questions/101053/is-round-robin-dns-good-enough-for-load-balancing-static-content
http://stackoverflow.com/questions/1472214/load-balancing-dns-round-robin-in-front-of-hardware-load-balancers-how-to-shar