How to deploy a threadsafe asynchronous Rails app?

Question

I\'ve read tons of material around the web about thread safety and performance in different versions of ruby and rails and I think I understand those things quite well at this p

Answer 1

The right approach for you depends heavily on what your slow method is doing.

In a perfect world, you could use use something like the sinatra-synchrony gem to handle each request in a fiber. You'd only be limited by the maximum number of fibers. Unfortunately, the stack size on fibers is hardcoded, and it is easy to overrun in a Rails app. Additionally, I've read a few horror stories of the difficulties of debugging fibers, due to the automatic yielding after async IO has been initiated. Race conditions are still possible when using fibers, as well. Currently, fibered Ruby is a bit of a ghetto, at least on the front-end of a web app.

A more pragmatic solution that doesn't require code changes is to use a Rack server that has pool of worker threads such as Rainbows! or Puma. I believe Thin's --threaded flag handles each request in a new thread, but spinning up a native OS thread is not cheap. Better to use a thread pool with the pool size set sufficiently high. In Rails, don't forget to set config.threadsafe! in production.

If you're OK with changing code, you can check out Konstantin Haase's excellent talk on real-time Rack. He discusses using the EventMachine::Deferrable class to produce a response outside of the traditional request/response cycle that Rack is built on. This seems really neat, but you have to rewrite the code in an async style.

Also take a look at Cramp and Goliath. These let you implement your slow method in a separate Rack app that is hosted alongside your Rails app, but you will probably have to rewrite your code to work in the Cramp/Goliath handlers as well.

As for your question about the Ruby runtime, it also depends on the work that slow is doing. If you're doing CPU-heavy computation, then you run the risk of the GIL giving you issues. If you're doing IO, then the GIL shouldn't get in your way. (I say shouldn't because I believe I've read about issues with the older mysql gem blocking the GIL.)

Personally, I've had success using sinatra-synchrony for a backend, mashup web service. I can issue several requests to external web services in parallel, and wait for all of them to return. Meanwhile, the frontend Rails server uses a thread pool, and makes requests directly to the backend. Not perfect, but it works well enough right now.