[systemd-devel] Service reactivation with device plugging

[Lennart Poettering]

On Wed, 04.07.12 00:10, Rob Spanton (rspanton at zepler.net) wrote:

> Hi,
> 
> I was recently working on an embedded system that had a daemon that
> depended on the presence of two USB serial devices.  The daemon needed
> to be run when both devices were plugged in.  If any of the devices was
> unplugged, the daemon would exit, and would need to be restarted the
> next time both devices were plugged in.
> 
> I created a service file pretty-much like the one at the end of this
> email.  This kind-of did what I wanted for a short while.  However, if
> the daemon was running, and then I unplugged the serial devices, systemd
> seemed to attempt to continuously restart the daemon despite the serial
> devices not being present.  Furthermore, it seemed to give up doing this
> after some period of time (not sure exactly how long, but somewhere
> around minute or two).  Once it had given up restarting the daemon, the
> daemon was not restarted when I plugged the devices back in again.
> 
> In the end, due to looming deadlines, I added a cron job that just ran
> `systemctl start mydaemon.service` every minute to ensure that systemd
> didn't give up on reactivating the service.
> 
> Am I missing something here?  Is this a valid application of systemd?

Yes it is, absolutely.

Requires= is used for deps that only need to be fulfilled at startup
time of a service. BindTo= is for deps that need to be fulfilled all the
time. So for your case you should be able to simply replace Requires= by
BindTo= and that should be it.

> [Unit]
> Description=...
> Requires=dev-serial-a.device
> Requires=dev-serial-b.device
  ^^^^^^^^ 
Use BindTo= here

Lennart

-- 
Lennart Poettering - Red Hat, Inc.