Systemd unit file implementation questions (ypbind)

[Lennart Poettering]

On Thu, 14.04.11 16:15, "Jóhann B. Guðmundsson" (johannbg at 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.