• config.ru
• config/puma.rb

install vagrant: https://www.vagrantup.com/downloads.html

vagrant up # will take several minutes if you don't have a xenial32 base box locally
vagrant ssh
cd /vagrant
bundle install

vagrant ssh

1. run rails s in terminal 1
2. run curl http://0.0.0.0:3000 in terminal 2
3. observe correct behavior in both terminals
4. ^c out of rails s in terminal 1
5. edit Gemfile to comment out puma 3.7.1 and activate puma 3.11.0
6. bundle update puma in one of the terminals
7. run rails s in terminal 1
8. run curl http://0.0.0.0:3000 in terminal 2
9. observe incorrect behavior in both terminals