watchdog.rb

require 'socket'
require 'logger'
STDOUT.sync = true

logger           = Logger.new(STDOUT)
watchdog_timeout = (ENV['WATCHDOG_USEC'].to_i / 1_000_000)
sd_notify_socket = Socket.new(Socket::AF_UNIX, Socket::SOCK_DGRAM)
sd_notify_socket.connect(Addrinfo.unix(ENV['NOTIFY_SOCKET']))

loop do
  readable, writable, _ = IO.select([], [], [], watchdog_timeout - 1)
  # handle readable descriptors
  # handle writable descriptors

  logger.info('watchdog')
  sd_notify_socket.write('WATCHDOG=1')
end

watchdog.service

[Unit]
Description=Sample watchdog

[Service]
User=myapp
ExecStart=/srv/myapp/.rubies/ruby-2.3.0/bin/ruby /srv/myapp/watchdog.rb
WatchdogSec=10s
Restart=on-failure

# Uncomment to have no mercy over limit of failures
#
# StartLimitInterval=5min
# StartLimitBurst=4
# StartLimitAction=reboot