app.rb

require 'bundler'
Bundler.require

class App < Sinatra::Base
  helpers Sinatra::Streaming

  get '/' do
    subscriber = EM::Hiredis.connect
    subscriber.psubscribe '*'

    stream :keep_open do |out|
      subscriber.on(:pmessage) do |key, channel, message|
        subscriber.punsubscribe '*' if out.closed?

        out << "#{key}, #{channel}, #{message}" unless out.closed?
      end
    end
  end
end

Gemfile

source 'https://rubygems.org'

gem 'sinatra'
gem 'sinatra-contrib', require: 'sinatra/streaming'

group :development do
  gem 'thin'
  gem 'pry-rails'
end

gem 'eventmachine', "= 1.0.0.rc.4"
gem "redis", "~> 3.0"
gem "hiredis", "~> 0.4.5"
gem 'em-hiredis'