Upstart is Ubuntu's default service manager. One of the goodies it provides is the possibility to restart the server should it crash. Adding a HTTP service that is started using library(http/http_unix_daemon) is quite simple. Here is some example code, which assumes a file daemon.pl that starts the server. The server is started on port 80, which is a good setup if you start the server in a virtualized environment such as Linux containers.

# demo - SWI-Prolog demo server
#
# The SWI-Prologdemo server

description	"SWI-Prolog demo server"

start on runlevel [2345]
stop on runlevel [!2345]

respawn
respawn limit 5 60
umask 022

console log
chdir /home/swipl/src/demo

script
  export LANG=en_US.utf8
  ./daemon.pl --no-fork --port=80 --user=www-data --pidfile=/var/run/swipl.pid --workers=16 --syslog=swipl
end script