I've realized that this whole thing comes down to the word "most" doing too much duty here. I don't think it's true that most applications just run a single db-and-application node. I've never worked on such an application. You and the author do seem to think this is true. It would be difficult for either of us to support our intuition empirically, so this is where the divide comes from.
> I don't think it's true that most applications just run a single db-and-application node. I've never worked on such an application.
Rather than applications you've worked on (many of us spend years on end working on a narrow range of applications), consider software you use (most of us flip between multiple applications every day spanning the gamut of uses). Ignore, for a moment, whatever you know about their implementation, and focus on the user-facing functionality.
Do even half of them logically communication between application nodes?
(Consider that there is still a rich class of software that is fully capable of running locally on the "client"'s computer!)
Yes I think pretty much all the applications I interact with require fault tolerance and uptime that (to me) seems simpler to implement with separation between application and database nodes.
