machinectl + systemd-machined

systemctl -M,
systemctl -r,
systemctl list-machines
loginctl -M
journalctl -M
journalctl -m

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Blog post linked from the wiki

man sd_notify
sd_notify (c call with man page)
  NOTIFY_SOCKET=socketno
  send datagram to socket when ready (looks like environment variable block, newline deliniated)

socket activation
  inet style
  LISTEN_FDS=number (starting at 3)

man 5 systemd.service

  WatchdogSec=
  sets WATCHDOG_USEC= environment variable
  client should sent WATCHDOG=1 every 1/2 interval.

  FDSTORE= pool of file descriptors for services that want to restart
    implemented using AF_UNIX socket

  on relaunch, LISTEN_FDS= gives multiple sockets, then FDSTORE sockets.
    Order returned is the order you sent. Order is preserved.
      - except it filters out duplicates
      - functions to identify FD type

man 3 sd-daemon
  demons started with stdin/stdout connected to syslog
  plus dmesg-style prefixes (printk logic, implies INFO)

header file /usr/include/systemd/sd-daemon.h
  
There's a python reimplementation, python-systemd in source

http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/

http://www.freedesktop.org/wiki/Software/systemd/InitrdInterface/