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

[林自均]

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?

John
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