[systemd-devel] How to specify dynamic services/requirements

[Luiz Angelo Daros de Luca]

Thanks Jérémy,

That was what I needed. I was afraid to be headed to the wrong way. Now
that I know what to do, I need to know where and when.

> you could create a template (xendomains at .service) and use drop-ins to do
> per-instance overrides (/etc/systemd/system/xendomains at vm1.service.d/append.conf)
> that would allow you to individualize each domain while keeping the common
> parts
>
> I would find a way to prevent a domain with no corresponding to start
> (maybe by not defining a mandatory key in the template) to make sure no
> rogue domains are created...
>

I would use a template like xendomains at .service in order to specify common
stuff. However, the config of which vm are enabled (autostart) is only
available at boot time. Is a systemd generator the way to go? Is is there
another way to define it?

Also, should I use a service patch (like append.conf you mentioned) or
simply directly create /run/.../xendomains at vm1.service?

It would be trivial to define a template xendomains at .service and start
services like "systemctl start xendomains at vm1.service", that would read
/etc/xen/vm/vm1 and launch it. I just don't know how I could define dynamic
dependency for that instance depending on the content of /etc/xen/vm/vm1.
Execstartpre can check requirements and prevent a service to run but not
wait for a resource to be available. It would be something like
ExecRequires=.

Instances seems to be created only when asked directly (systemctl start
xendomains at vm1.service), which might be triggered by something like udev
event. Is there anything like
ExecListStartInstances=, ExecListRunningInstances=?
Generators can create something like that but they are triggered only at
boot and daemon-reload. It would be interesting to somehow run "something
like generators" when start/stop/status is called.

>From what I know about systemd, I'm thinking of something like a generator
that will create services for all autostart and saved vm. The current
script-based xendomains.service will run after all xendomains at .service
instances. This way, a new enabled-after-boot vm would be started with
"systemctl restart xendomains". I could use the existence of
/etc/xen/auto/vm or /var/lib/xen/save/vm as requirement
for xendomains at vm.service, in order to avoid to start a disabled-after-boot
vm. However, this does not look like an elegant solution.

Regards,

> --

Luiz Angelo Daros de Luca
luizluca at gmail.com
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