8

I know that systemd provides an excellent mechanism to override a package-provided unit file to influence the service configuration/behavior. This is typically done by using the below command

sudo systemctl edit <unitfile>

to create an override conf file at

/etc/systemd/system/<unitfile.d>/

Systemd also provides a separate mechanism to define a template unit file, and have it instantiated to create instance-specific units at runtime. This requires naming the template file as

<servicename>@.service

and then instantiating it as

systemctl start <servicename>@<instancename>

Now, I have a situation where I would like to run a package-provided service as multiple unit instances. I want to avoid creating my own template unit file, so I am trying to see if the package-provided unit file can be overriden to create the template unit file.

Since, per my understanding, the template unit file has a naming convention that is different from the regular unit file, I think I cannot override the package-provided unit file with a template file by placing it in /etc/systemd/system.

Is there any defined way to achieve what I am trying to do?

Specific scenario: The grafana package installs a grafana-server.service unit file. I want to run two instances of grafana on my machine - one each for DEV and STG. I have been able to do this:

  • modify the grafana-server.service file (using %I to set folder locations & file paths)
  • rename the modified grafana-server.service to grafana-server@.service
  • start instances of grafana by using:

    sudo systemctl start grafana-server@dev
    

    and

    sudo systemctl start grafana-server@stg
    

However, this breaks the link from the grafana provided service unit file, and if they enhance the service file when I upgrade, I will need to redo this activity again. My objective is to avoid this direct dependency, and instead convert it to an override dependency.

Any thoughts?

6

For both options below, first override the grafana-server.service (without the @) on /etc/systemd/system and suppress ExecStart (supposing it uses that) to make it starts nothing. On /etc/systemd/system/grafana-server.service.d/10-disable-execstart.conf:

[Service]
ExecStart=
WorkingDirectory=/path/to/your/confdir

Option 1 - Overriding with generic instances

Create a grafana-server@.service that corresponds to your settings with the following configuration to [Unit] and [Service]:

[Unit]
PartOf=grafana-server.service
ReloadPropagatedFrom=grafana-server.service

These should bind grafana-server start/stop/restart to all your instances together. The magic is not very well documented, but if you put <instance_name>.conf files on your /path/to/your/confdir, all those instances will be bound automagically!

Option 2 - Overriding specific instances to keep package config

If you want to keep all update goodness from package service file, but accept mainaining custom instances options, create a symbolic link for each instance name from generic

/lib/systemd/system/grafana-server.service

to

/etc/systemd/system/grafana-service@<instance>.service

and then override only the specific options of that instance using

/etc/systemd/system/grafana-server@<instance>.service.d/99-my-options.conf

Make sure to add the following configuration to [Unit] and [Service] to 99-my-options.conf:

[Unit]
PartOf=grafana-server.service
ReloadPropagatedFrom=grafana-server.service

This will assume for each instance all grafana-server.service options and will override them with all options on 99-my-options.conf file and also bind start/stop/restart actions to grafana-server.service.

On both options, if you run

systemctl start grafana-server.service

all your instances which have a /path/to/confdir/<instance>.conf file will be started. The same apply to stop and restart AND you can always manage them individually by using their grafana-server@<instance> service name.

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