[systemd-devel] Mutually exclusive (timer-triggered) services

[Alexander Koch]

> exactly for this you would use a flock(1) in your .service.

Thanks for the hint, didn't have that in mind. So you're suggesting 
something
like this?

     # service-a.service
     # (...)
     [Service]
     ExecStart=/usr/bin/flock -F /var/lock/resource.lock 
/usr/bin/service-a

     # service-b.service
     # (...)
     [Service]
     ExecStart=/usr/bin/flock -F /var/lock/resource.lock 
/usr/bin/service-b


I see some drawbacks of this approach:

* Wrapping the actual command in an flock call seems pretty ugly in my 
eyes,
   i.e. looking at the `ExecStart=` lines above I find it very hard to 
see where
   the former begins, at first glance. This is subjective, of course.

* flock leaves the lock file behind so you'd need some type of cleanup 
in case
   you really want the jobs to be trace-free. This is not as trivial as 
it might
   seem, e.g. you cannot do it from the service units themselves in
   `ExecStartPost=` or similar.

* /var/lock is very likely not user-writable so you'd need a different 
lock file
   directory for user services. You could of course use /tmp for that.

* File-based locking makes it impossible to use private filesystem 
namespaces
   for the service units (think of `PrivateTmp=yes` and lock files in 
/tmp).


As systemd (or service managers in general) are all about queueing, 
ordering and
controlling services and their dependencies I had hoped there would be a 
cleaner
solution using systemd internals rather than solving it using external 
tools,
which mixes up the service call itself with its requirements (exclusive 
resource
access).

Nevertheless I think I will go with your proposal and see how it works 
out.


Best regards,

Alex