[systemd-devel] Why do passive target units have to exist?

[Andrei Borzenkov]

16.10.2017 18:15, 林自均 пишет:
> Hi folks,
> 
> I am reading systemd documents, and I find passive target units a little
> bit confusing.
> 
> Take "network.target" for example:
> 
> "systemd-networkd.service" specifies "Wants=network.target" and
> "Before=network.target". That effectively makes starting
> "systemd-networkd.service" brings up both "systemd-networkd.service" and
> "network.target", and make sure that "network.target" is active after
> "systemd-networkd.service" being active. It also implies that the shutdown
> order is correct: "network.target" will be stopped before
> "systemd-networkd.service".  Everything is fine.
> 
> What if we use an active target unit to achieve all this? Can we specify a
> "WantedBy=network.target" in "systemd-network.target"? So that we can
> enable "systemd-network.service" (which makes a symbolic link in the
> "network.target.wants" directory) and start "network.target" to pull in
> "systemd-networkd.service". That also makes sure "network.target" is active
> after "systemd-networkd.service" because of the target unit default
> dependencies. And shutdown order will be correct too.
> 
> The only difference I can tell is the units to start. With a passive
> "network.target", we start "systemd-networkd.service". With an active
> "network.target", we start "network.target".
> 
> Is there any benefits of passive target units over active target units?
> 

I think it's historical artefact from the very early days when systemd
had to rely on initscripts. initscript implementing networking did
provide $network a.k.a network.target, but of course did not hook itself
into native systemd units. As long as you are sure to use native units
only there is no practical difference.