I think the original comment was talking about the need to scale out application servers to take on additional load. Each application sever still communicates w

I think the original comment was talking about the need to scale out application servers to take on additional load. Each application sever still communicates with (usually) the one database. Normally people talk about that as a way to increase the throughput of requests that your site can respond to, but it has another benefit: more time spent performing data validation, string munging, date checking, etc adds up to less time spent in the database on the same thing. Stack Overflow may run off of one relational database, but it solves the scaling problem through liberal application-level caching and reverse-proxy caching, all using about a dozen servers that connect to the one database.