how to make rackup sinatra as systemd and auto restart on failure

#acc-server.rb
require 'sinatra/base'

class MyApp < Sinatra::Base
  set :bind, '0.0.0.0'
  set :port, 3333

  get '/' do
    "Hello #{params[:name]}"
  end
  run! if app_file == $0
end

require './acc-server'
run MyApp

#!/bin/bash
cd /root/server && /usr/local/bin/rackup -p 3333 -P /root/server/server.pid </dev/null &>>/root/server/log

[Unit]
Description=Prosody http server

[Service]
ExecStart=/root/server/server.sh
Restart=on-failure

[Install]
WantedBy=multi-user.target

systemctl start pserver.service 

systemctl enable pserver.service