Rails 5.1.4 not starting and giving me ERROR: worker mode not supported on JRuby or Windows Exiting

Question

I tried starting my rails server but got the error message below, how can i fix it please

C:\Sites\sample_app>rails s
=> Booting Puma
=> Rails 5.1.4 application starting in development
=> Run `rails server -h` for more startup options
  Please add the following to your Gemfile to avoid polling for changes:
    gem 'wdm', '>= 0.1.0' if Gem.win_platform?
  Please add the following to your Gemfile to avoid polling for changes:
    gem 'wdm', '>= 0.1.0' if Gem.win_platform?
ERROR: worker mode not supported on JRuby or Windows
Exiting

Answer 1

根据此 Puma的问题页面，可以通过注释掉puma.rb的以下行来解决此问题：

workers Integer(ENV['WEB_CONCURRENCY'] || 2)

Answer 2

According to the same issue page for Puma, for the newer versions of rails, it can be resolved by commenting out the following line in puma.rb :

# workers ENV.fetch("WEB_CONCURRENCY") { 4 }
# workers ENV.fetch("WEB_CONCURRENCY") { 2 }