systemd unit file documentation

[vielmetti]

I'm installing the template [email protected] from the distribution, in order to bring up the system on CoreOS. The docs are a little bit thin; what I'd like to propose is a discussion here, which I'd summarize with a PR for docs/systemd.md that would summarize the instructions and perhaps suggest a best way to deal with that kind of setup, in several ways simple and complex.

To be fair the systemd docs on the use of the @ in a filename to create a variable are also kind of thin.

[kylemanna]

Yes, definitely much needed as you've stumbled across the init script form a long time ago. On the bright side, Ubuntu and Debian have stabilized on systemd so the old upstart init script can probably also be removed.

Few comments while I think aloud:

1. The IPv6 stuff in the current template is a mess because IPv6 in Docker is a mess, inclined to leave it there because it at least works, but I find it distracting.
2. The ExecStartPre=-/usr/bin/docker pull $IMG step will pull updates to the image before running. I think this is handy to resolve security issues as images are pushed , but others may not expect their system to pull new images at init time. Thoughts?

[pieterlange]

Big fan of safe defaults here. If people don't want sudden updates to images they shouldn't be running latest anyway.

[vielmetti]

Is docker pull a safe step if hub.docker.com is down, or network connections are wonky? We had some github down time today so I'm sensitive to overly reliable systems working a little too well so that they fail rarely and spectacularly.

Actually, hub.docker.com has had some issues of late so this is not an idly curious question.

[kylemanna]

Is docker pull a safe step if hub.docker.com is down, or network connections are wonky?

It's setup so that if the docker pull fails, the rest will run anyways:

From systemd.service(5):

If the absolute filename is prefixed with "-", an exit code of the command normally considered a failure (i.e. non-zero exit status or abnormal exit due to signal) is ignored and considered success.

If any of those commands (not prefixed with "-") fail, the rest are not executed and the unit is considered failed.
ExecStart= commands are only run after all ExecStartPre= commands that were not prefixed with a "-" exit successfully.

[pieterlange]

I was just about to link the docs :) https://www.freedesktop.org/software/systemd/man/systemd.service.html

[kylemanna]

Please continue discussion at #204