In what way is Ruby on Rails NOT multithreaded?

Question

Disclaimer: I'm a c# ASP.NET developer learning "RoR". Sorry if this question doesn't "get" RoR, any corrections greatly appreciated!

What is multithreading

My understanding of "multithread" ability in web apps is twofold:

1. Every time a web/app server receives a request it can assign a thread to the new request, thus multiple requests can run concurrently.
2. The app runtime + language allows for multiple threads to be used WITHIN a single request (in ASP.NET via "Async" methods and keywords for example).

In this way, IIS7 + ASP.NET can do points 1 AND 2.

I'm confused about RoR

I've read these two articles and they have left me confused:

• Clearing up some things about LinkedIn mobile’s move from Rails to node.js
• How to deploy a multi-threaded Rails app

question one.

I think I understand that RoR doesn't lend itself very well to point number 2 above, that is, having multiple threads within the same request, have I got that right?

question two.

Just to be crystal clear, RoR app/web servers can also do point number 1 above right (that is, multiple requests can run concurrently)? is that not always the case with RoR?

Answer 1

Question 1: You can spawn more Ruby threads in one request if you want, although that seems to be outside the typical use case for Rails. There are uses for it for certain long-running IO or external operations.

Question 2: The limiting factor for Ruby concurrency in general, not just with Rails, is the Global Interpreter Lock. This feature of Ruby prevents more than 1 thread of Ruby from executing at any given time per process. The lock is released whenever there is non-Ruby code executing, such as waiting for disk IO or SQL responses. You can get around this by using a different implementation of Ruby than the default, such as JRuby, but not all.

Phusion Passenger uses process based concurrency to handle a few requests concurrently, so, strictly speaking, is not "multithreaded," but is still concurrent.

This talk from Ruby MidWest 2011 has some good thoughts on getting multithreaded Ruby on Rails going.