*threadsafe_rails.md

Running rails in threadsafe mode

production.rb

  # Enable threaded mode. The line you should uncomment out to enable Rails to run 
  # in threaded mode. Apparently this isn't actually a big deal, though
  # is scary to turn on: http://tenderlovemaking.com/2012/06/18/removing-config-threadsafe.html
  config.threadsafe!
  
  # Lazy/Autoloading is not threadsafe, so you need to tell Rails to eagerly load
  # all necessary libraries when it starts up. These settings are NOT inherited
  # from your autoload_path settings (seems like they should, huh?) so you need
  # to duplicate them (or have a better config management strategy)
  # Some confusion: https://github.com/rails/rails/issues/3148#issuecomment-3210973
  config.eager_load_paths += Dir["#{config.root}/lib", "#{config.root}/lib/**/"]
  
  # BUT, rake tasks don't eager-load properly (not sure if defect or by design), 
  # so you need to re-enable lazy/autoloading when running rake tasks...
  # which I guess means your rake tasks aren't threadsafe... but I guess that's ok.
  config.dependency_loading = true if $rails_rake_task

I use:

config.threadsafe! unless $rails_rake_task

which seems to work well for me.