If you are looking for something pure ruby ββand simple, WEBrick is a good choice. Because it is pure-Ruby, it will not be very fast.
Mongrel is partially implemented in C and Ruby and is better for performance than WEBrick. (Ruby on Rails development mode will use Mongrel to prefer WEBrick if Mongrel is installed.)
If you want your server to scale better than any of WEBrick or Mongrel, then thin is probably the choice - it scales Mongrel together with EventMachine further than other, easier, systems can scale.
Nothing can replace the launch of a full-blown web server like nginx , but integrating your application with a lot more work for you and your users.
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