concurrent_action_method-rails5.rb

# an action method in a Rails controller, Rails5
def show
  futures = 2.times.collect do |n|
    Concurrent::Future.execute do
      # NEED to wrap here, in case _any_ autoloading is going to happen,
      # need to tell Rails so it can be done thread-safely. Or else we might
      # get a deadlock. 
      Rails.application.executor.wrap do
        SomeExpensiveNetworkOrIoWorker.new(n).call
      end
    end
  end
  
  # If any autoloading happened in one or more of those threads, the threads
  # might currently not actually be doing anything, they're just blocking waiting
  # for access to auto-loading, but this 'main' Rails request/response thread
  # may be holding onto the lock, we need to release it to allow the threads to
  # get it and do their work -- making sure not to do anything in _this_ thread
  # that might trigger an auto-load inside the permit_concurrent_loads block. 
  ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
    @results = futures.collect(&:value)
  end
  # render response
end