How to run a FastCGI process as a service under Systemd, based on http://redmine.lighttpd.net/projects/spawn-fcgi/wiki/Systemd/2

systemd-spawn-fcgi.sh

#!/bin/bash

set -e

if [ "${LISTEN_PID}" != $$ ]; then
	echo >&2 "file descriptors not for us, pid not matching: '${LISTEN_PID}' != '$$'"
	exit 255
fi

if [ "${LISTEN_FDS}" != "1" ]; then
	echo >&2 "Requires exactly one socket passed to fastcgi, got: '${LISTEN_FDS:-0}'"
	exit 255
fi

unset LISTEN_FDS

# move socket from 3 to 0
exec 0<&3
exec 3<&-

# spawn fastcgi backend
exec "$@"

webapp.service

[Unit]
Description=WebApp service
After=network.target postgresql.service mysql.service
Wants=postgresql.service mysql.service

[Service]
User=webappuser
Group=webappgroup
StandardOutput=null
StandardError=syslog
ExecStart=/usr/local/sbin/systemd-spawn-fcgi.sh /home/webappuser/webapp/runfcgi

[Install]
WantedBy=multi-user.target

webapp.socket

[Unit]
Description=WebApp Listen Socket

[Socket]
SocketUser=www-data
SocketGroup=www-data
SocketMode=0600
ListenStream=/run/webapp.sock

[Install]
WantedBy=sockets.target