run sinatra on puma application server behind nginx as a reverse proxy

app.rb

# /home/<user>/hello-nginx-sinatra/app.rb

require 'rubygems'
require 'sinatra/base'

class App < Sinatra::Application

  get '/app' do
    "hello world!"
  end
end

config.ru

# /home/<user>/hello-nginx-sinatra/config.ru
require './app'
run App

hello-sinatra.service

# replace user with your user
# we use a binstub (https://github.com/sstephenson/rbenv/wiki/Understanding-binstubs)

[Unit]
Description=Hello Sinatra

[Service]
User=<user>
WorkingDirectory=/home/<user>/hello-nginx-sinatra
ExecStart=/home/<user>/.rbenv/versions/jruby-1.7.16/bin/puma -t 2:16 -p 9000

[Install]
WantedBy=multi-user.target

nginx.conf

worker_processes  1;

events {
    worker_connections  1024;
}

http {

  upstream app_server {
    server 127.0.0.1:9000;
  }

  server {
    listen 80 default deferred;

    gzip on;

    server_name _;

    keepalive_timeout 5;

    location /app {
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_redirect off;
      proxy_pass http://app_server;
    }
  }
}

run

systemctl start nginx
systemctl start hello-sinatra