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

[Alexander Koch]

Dear [systemd-devel],

imagine you've got multiple services that perform system housekeeping
tasks, all triggered by .timer units. These services all happen to use
a specific resource (e.g. the system package manager) so they must not
be run in parallel, but they all need to be run.

Is there a systemd'ish way of modeling this?

I first thought of using `Conflicts=` but having read the manpages I
understand that queueing one of the services would actively stop any
running instance of any of the others.

`After=` is not an option either as that (unless 'Type=oneshot', which
isn't to be used for long-running tasks) doesn't delay up to completion
but only to initialization. Furthermore I think you'd run into trouble
ordering more than two units using this approach.

Ideally I'd think of something like a 'virtual resource' that can be
specified by service units, like this (real use case on Arch Linux):

     [Unit]
     Description=Pacman sync
     Locks=pacman-db

     [Service]
     ExecStart=/usr/bin/pacman -Sy

     ----

     [Unit]
     Description=Pacman cleanup
     Locks=pacman-db

     [Service]
     ExecStart=/usr/bin/paccache -r -k 0 -u

The value of `Locks=` shall be an arbitrary string describing the
virtual resource the service is requiring exclusive access to. systemd
would then delay the start of a unit if there is another unit with
identical `Locks=` entry currently active.

A nice advantage of this concept is that services depending on the same
virtual resource would not need to know of each other, which simplifies
shipping them via separate packages.

Any thoughts on this? Maybe I'm just too blind to see the obvious
solution to this simple problem.


Best regards,

Alex