When is a unit "loaded"?

[filbranden@xxxxxxxxxx (Filipe Brandenburger)]

Hey Daniel!

On Wed, Jul 11, 2018 at 5:16 PM Daniel Wang <wonderfly at google.com> wrote:
> I have a unit, say foo.service, on my system that's in /usr/lib/systemd/system, but disabled by preset.

Not that it matters, but presets don't really matter here. The unit is
disabled, period.

> On system boot, it doesn't show as "loaded" per `systemctl --all | grep foo`.

Because there's no reference to it in units systemd sees, so systemd
doesn't need to load it.

At bootup, systemd will simply start with default.target and then
recursively load the units it needs.

See this link for more details:
https://www.freedesktop.org/software/systemd/man/bootup.html#System%20Manager%20Bootup

`systemctl --all` will only show the units in memory, so foo.service
won't be listed since it's not loaded.

> So if I override it with a file with the same name but under /etc/systemd/system, `systemctl cat foo.service` will show the one under /etc without the need for a `systemctl daemon-reload`.

Yes, because it's not loaded.

> If I create another service unit, bar.service, which has a After= dependency on foo.service, and start bar, foo.service will show as loaded. And then if I try to override it, `systemctl cat foo.service` will print a warning saying a daemon-reload is needed.

Yes. If systemd sees a reference for that unit (even if it's an
After=), it will need to load it, since it needs to record the
dependency between the units in the internal memory structures, so it
needs a reference to the unit, and it loads it to have a complete
reference to it...

> Nothings seems incorrect, but I have a few questions:
> - Which units are loaded on-boot and which are not?

Only default.target and recursively any unit referred to by the loaded units.

> - Is the After= dependency alone enough to have systemd load a unit? Are there any other dependency directives that will result in the same effect?

Yes, I believe any reference to units will trigger the unit to be
loaded. As I mentioned, systemd wants to keep the state in memory, so
loading the unit will allow it to keep complete state.

An exception (haven't checked it, but I expect it) is references in
the [Install] section (such as Also=) since those are only used by
`systemctl enable` and are not really loaded into memory as far as I
can tell (but I might be wrong here, and these might trigger the unit
to load as well.)

I hope this helps!

Cheers,
Filipe
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