4. Gen#3 Web More boom!!, add more web servers but DB is the problem DB Web Web
5. Gen#4 Web Replication could save your life for a while DB Web Web DB Replication WRITE Query READ ONLY
6. Gen#... Memcached: reduce READ query at DB Reverse Proxy: serves static contents DB Web Reverse Proxy DB Replication Memcached Memcached Web Reverse Proxy WRITE Query READ ONLY
7. Finally: Horizontal Scaling masterA Reverse Proxy Replication Reverse Proxy Web (dynamic) Web (dynamic) masterB Global Database - Multimaster Slave 1 Slave 2 UserDB1 UserDB2 UserDB3 UserDB..n Memcached