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

Fri Sep 15 08:02:32 UTC 2017


On 15/09/2017 07:27, Luiz Angelo Daros de Luca wrote:
> 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
>     <mailto: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?
>
both method work... the first one allow to add stuff to the common 
template, the second to override more easily in the case of a VM that is 
totally different from the others...
> 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=.
A generator is probably the way to go... generators can create ANY 
systemd configuration files, including templates, drop-ins and .wants 
directory

You probably already found that, but in case you didn't, you should read 
the DESCRIPTION section of man:systemd.unit It describes all the ways 
you can add files to configure units

In particular, if you want a service started at boot time, you need to 
create a default.target.wants/ directory and add a symbolic link to the 
unit to start in there. Populating such a directory at boot time with a 
generator might be the best approch for you...
>
> 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=?
see my answer above. There is no such thing, but generators can do the 
equivalent by creating a .wants link

(as a side note, I don't know if you could do a "systemctl enable 
--runtime" from a generator. maybe someone else here would know)
> 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.
what would be your use-case ?
status is not "an event" nothing real happens on status
start/stop can propagate stuff based on their parameters, but the 
services themselves are not meant to be dynamic...

That being said, it might be possible to dinamically add dependencies to 
a unit via dbus and/or systemctl set-property, but I have never tried 
it, so you'll need to try and see

>
> 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.
Your requirements seems very complex (and, I have to admit, a bit hard 
to understand through e-mail) but I think that with a good understanding 
of templates and generators it can be solved elegantly

* use templates to define how to start vms (common parts in the 
template, particularities in the instances)
* add all boot-time depedencies via .wants directory via a generator 
(maybe usint systemctl enable if it can be run at generator-time, but 
i'm not sure it's possible)


Hope this helps

Jérémy

>
> Regards,
>
> -- 
>
> Luiz Angelo Daros de Luca
> luizluca at gmail.com <mailto:luizluca at gmail.com>
>

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