systemd .service unit file generator

[jonnor]

We should be able to generate .service unit files for systemd, that can start MsgFlo participant(s).

Similar to to how we can generate Procfile using msgflo-procfile for starting participants on Heroku.

Useful when hosting participants directly on a Linux machine, be it a embedded device or a VM.

[bergie]

Potentially useful also: supervisord example

[jonnor]

systemd has a concept of a .service file template, which multiple instance can be created of. https://fedoramagazine.org/systemd-template-unit-files/
Probably it makes sense that we generate one template service file per MsgFlo component, and then use the instance mechanism to spawn one or more roles. We should also set DefaultUnit=defaultrole though, to make using instances optional.

Because systemd provides both start and stop (and unlike in sysvinit, they are reliable), and these days also supports a --user mode (no root needed to install .service files) - we can potentially use systemd for 'live-coding' with msgflo.
Persistence would be doable by calling systemctl enable ...

[jonnor]

It would also be good to have one unit created for the entire graphs, which pulls in the individual service units for each node to realize the whole system. It is a bit unclear how to do this, but I think the pieces are:

• A .target file to specify the grouping of units. https://www.freedesktop.org/software/systemd/man/systemd.target.html
• In .target use Also stanzas to mention each of the .service templates which should be installed together.
  https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files
• In .target use Requires stanzas to meantion each of the .service instances needed for the target
• Probably also want AllowIsolate=no in the .target, to prevent accidentally killing everything not in the target.
• Have StopWhenUnneeded=yes on each of the .service files, so they get stopped when parent .target does (nothing requires it anymore)
  http://superuser.com/questions/470153/how-to-stop-a-group-of-systemd-custom-services

[jonnor]

Here is a (handwritten) example of the above approach https://github.com/jonnor/rebirth/tree/master/systemd
It seems to work fine except for one oddity systemctl restart rebirth.target does not stop/start the .services, however systemctl stop/start does.
Possibly should use PartOf directive. However not clear if this augments or replaces the Requires. http://unix.stackexchange.com/questions/212438/how-to-stop-all-units-belonging-to-the-same-target