Jonathan Willis, software developer by day and superhero by night, asked an interesting question via Twitter on StackOverflow: 

tl;dr Many Rails apps or one Vertx/Play! app?

I've been having discussions with other members of my team on the pros and cons of using an async app server such as the Play! Framework (built on Netty) versus spinning up multiple instances of a Rails app server. I know that Netty is asynchronous/non-blocking, meaning during a database query, network request, or something similar an async call will allow the event loop thread to switch from the blocked request to another request ready to be processed/served. This will keep the CPUs busy instead of blocking and waiting.

I'm arguing in favor or using something such as the Play! Framework or Vertx.io, something that is non-blocking... Scalable. My team members, on the other hand, are saying that you can get the same benefit by using multiple instances of a Rails app, which out of the box only comes with one thread and doesn't have true concurrency as do apps on the JVM. They are saying just use enough app instances to match the performance of one Play! application (or however many Play! apps we use), and when a Rails app blocks the OS will switch processes to a different Rails app. In the end, they are saying that the CPUs will be doing the same amount of work and we will get the same performance.

What do you think? The marketplace has seemingly moved, in the form of node.js, Golang, Akka, and even Java, to the async server model. Does that mean it's the only right way?

Here's my attempt at a response: