Nicely summarized on the network layer, next you'll want to expand the 'database' box into its components and a storage layer and its components.<p>There is also an interesting layer of networking services which involve routability and validation (certificate checking etc) and then there is the third party API scale so sometimes you're generating traffic back out to things other than a CDN (like Twitter or Facebook or some Google thing)<p>Part 4 should be looking at it from the data center side, which is these things are breaking all the time, building scalable repair systems that give 100% uptime on unreliable hardware.<p>It goes on and on and on ...