ruby - Are rails controllers multithreaded? Thread.exclusive in controllers

Question

Are Rails controllers multithreaded?

If so, can I protect a certain piece of code (which fires only once every ten minutes) from being run from multiple threads by simply doing

require 'thread'
Thread.exclusive do
     # stuff here
end

on do I need to somehow synchronize on a monitor?

Answer 1

Running rake middleware on a basic rails app gives the following:

use Rack::Lock
use ActionController::Failsafe
use ActionController::Reloader
use ActiveRecord::ConnectionAdapters::ConnectionManagement
use ActiveRecord::QueryCache
use ActiveRecord::SessionStore, #<Proc:0x017fb394@(eval):8>
use ActionController::RewindableInput
use ActionController::ParamsParser
use Rack::MethodOverride
use Rack::Head
run ActionController::Dispatcher.new

The first item on the rack stack is Rack::Lock. This puts a lock around each request, so only one request is handled at a time. As such a standard rails app is single threaded. You can however spawn new threads within a request that would make your app multi threaded, most people never encounter this.

If you are having issues…

require 'thread'
Thread.exclusive do
     # stuff here
end

… would ensure that stuff inside the block is never run in parallel with any other code. Creating a shared Mutext between all threads (in a class variable or something, but this could be wiped when reloaded in dev mode, so be careful), and locking on it as Rack::Lock#call does is to be preferred if you just want to ensure no two instances of the same code is executed at the same time.

Also, for the record, each request creates and dereferences one controller in each request cycle. No two requests should see the same instance, although they may see the same class.

Setting config.threadsafe! voids almost everything I said. That removes Rack::Lock from the stack, and means you will need to set a mutex manually to prevent double entry. Don't do it unless you have a really good reason.

Even without Rack::Lock you will still get one controller instance per request. The entry point to your controller ensures that, notice the call to new in process.