Well, you are both kindof taking the extreme view. If you build your app such that the webservers can act independently, then by using various methods of load

Well, you are both kindof taking the extreme view. If you build your app such that the webservers can act independently, then by using various methods of load balancing you can just throw hardware at the problem. However, this just usually leaves the database as the single point of contention among all webservers. There are various ways to reduce load on the database server by some constant factor (memcache, query caching), but in the end database load will be roughly linear with site load (requests/time). You can't just add SQL-based DB servers and get linear speedup like (properly thought out) webservers. If you think your site will get so big so fast you won't have time to hire engineers to figure out the scaling problems before they become unmanageable (for example, facebook got huge, but it took a few years, so they had time to raise money and adapt their stuff, although they were pretty slow for a long time), then use appengine datastore or simpledb or some other db that promises to scale really well (this is a really hard problem, which is why there aren't a lot of open source things that do this. Note: couchedb can't scale writes, so I don't count it). My personal thinking is that one big properly designed database can get you pretty far, and database clustering can get you far enough. If you find it pleasant, putting your app in appengine would make you pay the price of scalability a little at a time starting right away, instead of letting you push it off (but with a huge performance wall that you won't be able to get over without basically redesigning much of your apps internals).