[fourspace]

I managed a Google SRE team that helped run Borg for 5 years, so I'm pretty familiar with this. You can certainly run multiple application servers on one machine using virtualization. What isn't easy is automating where the applications are deployed, in an elastic, constantly shifting way that deals with massive numbers of machines and constantly changing hardware.

Cheap hardware crashes. It crashes all the time. Furthermore, the application's needs themselves change all the time, depending on traffic curves and processing needs. The dynamic needs of Google's (and presumably Twitter's) completely heterogeneous application stacks don't lend themselves to simple virtualization and over-the-counter software. This is an incredibly tricky bin packing problem that was never quite solved in my 5 years at Google.

I don't know a lot about the "distributed frameworks" you mentioned, so perhaps they do this too. I kind of doubt it. If it were as easy as you think, I'm sure my friends at Twitter would be using what you mentioned.