Gemfile

source 'https://rubygems.org'

gem 'sinatra'
gem 'puma'
gem 'thin'

puma.rb

# -*- coding: utf-8 -*-

require 'sinatra'
require 'puma'

set :server, :puma

get '/' do
  string = 'にゃーん'
  stream do |out|
    loop do
      out << string += '！'
      out << "\r\n"
      sleep 1
    end
  end
end

thin.rb

# -*- coding: utf-8 -*-

require 'sinatra'
require 'thin'

set :server, :thin

get '/' do
  string = 'にゃーん'
  stream do |out|
    loop do
      out << string += '！'
      out << "\r\n"
      sleep 1
    end
  end
end

$ ruby puma.rb > /dev/null 2>&1 &
$ curl -v http://localhost:4567/
* About to connect() to localhost port 4567 (#0)
*   Trying ::1...
* connected
* Connected to localhost (::1) port 4567 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8y zlib/1.2.5
> Host: localhost:4567
> Accept: */*
> 
< HTTP/1.1 200 OK
< Content-Type: text/html;charset=utf-8
< X-XSS-Protection: 1; mode=block
< X-Content-Type-Options: nosniff
< X-Frame-Options: SAMEORIGIN
< Transfer-Encoding: chunked
< 
にゃーん！
にゃーん！！

$ ruby thin.rb > /dev/null 2>&1 &
$ curl -v http://localhost:4567/
* About to connect() to localhost port 4567 (#0)
*   Trying ::1...
* Connection refused
*   Trying 127.0.0.1...
* connected
* Connected to localhost (127.0.0.1) port 4567 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8y zlib/1.2.5
> Host: localhost:4567
> Accept: */*
> 
< HTTP/1.1 200 OK
< Content-Type: text/html;charset=utf-8
< X-XSS-Protection: 1; mode=block
< X-Content-Type-Options: nosniff
< X-Frame-Options: SAMEORIGIN
< Connection: close
< Server: thin 1.5.1 codename Straight Razor
< 
にゃーん！
にゃーん！！