"Rails applications are now threadsafe by default as long as both config.cache_classes and config.eager_load are set to true." --> page 43 https://github.com/alindeman/upgradingtorails4/releases/download/v1.1.0/upgradingtorails4.pdf ... unless I'm reading that wrong, it seems important to point out that threadsafe isn't turned on when either eager_load and cache_classes isn't set to true.