On Thu, 14.04.11 16:15, "Jóhann B. Guðmundsson" (johannbg(a)gmail.com) wrote:
On 04/14/2011 03:36 PM, Lennart Poettering wrote:
>> In man systemd.unit
>> >
>> > BindTo=
>> > Configures requirement dependencies, very similar in style
>> > to Requires=, however in addition to this behaviour it also
>> > declares that this unit is stopped when any of the units
>> > listed suddenly disappears. Units can suddenly, unexpectedly
>> > disappear if a service terminates on its own choice, a
>> > device is unplugged or a mount point unmounted without involvement of
>> > systemd.
>> >
>> > ExecStop=-/bin/systemctl stop upsd-device.service
>> > ExecStop=-/bin/systemctl stop upsd-monitor.service
> Why ExecStop= here?
It was meant as an either or.
The BindTo= reference in the man page does not mention that it will take
down with it any service bound to it when the service is stop.
Requires= and BindTo= both do that.
The difference between the two switches is mostly an ordering issue:
Let's say you have a unit A and a unit B. B requires A, and should be
started after A is up. So in B you say:
Requires=A
After=A
now, if you shut down A with "systemctl stop A", this will also stop B,
and it will do so in the inverse starting order. i.e. stop B first, stop
A second. BindTo= would do exactly the same here. The difference now
comes if for some reason A dies independently of anybody running
"systemctl stop A": should we then shut down B retroactviely? The
ordering would normally suggest that B goes down before A, but if A just
goes away on its own, then should we still shut down B? If you use
BindTo= that's what would happen. If you use Requires= it wouldn't.
So where is this interesting? If A is a device and B is a service, then
normally B should start after A is plugged in and be killed before A is
unplugged. By using BindTo= you can also make it be killed if A is
unplugged with B still running.
Lennart
--
Lennart Poettering - Red Hat, Inc.