add dependency between "userdata" and kubelet systemd service

Question

I have some experience with systemd, but haven't done anything beyond the basics with it. I have some scripts that I've added to (AWS) "userdata", which I think gets executed by the systemd service cloud-final.service (cloud-init), but not sure. I want to make sure that the contents of userdata get executed before kubelet.service starts. (Basically, I need to setup do some setup steps for a CSI-based filesystem before kubelet boots).

Both cloud-final.service and kubelet.service are not installed by me; they are managed by external install processes. So, I don't know if it would be a bad practice (or difficult) to modify those unit files.

I'm wondering if there's some way to set a dependency between those services without editing their unit files. Or if there's a better way to setup a dependency relationship between code that runs in userdata and kubelet. I'd appreciate any advice that you have to offer.

Answer 1

To set up a dependency between the userdata execution (which is typically managed by cloud-final.service) and kubelet.service without modifying their unit files directly, you can create a custom systemd service that explicitly sets up this dependency. This custom service will ensure that your userdata script runs and completes before kubelet.service starts.

Create a new unit file for your custom service, for example, /etc/systemd/system/userdata-setup.service:

[Unit] Description=Run userdata script and set up dependencies After=network.target cloud-final.service Before=kubelet.service Wants=cloud-final.service [Service] Type=oneshot ExecStart=/usr/local/bin/your-userdata-script.sh RemainAfterExit=true [Install] WantedBy=multi-user.target 

This unit file does the following:

Specifies that the service should run after cloud-final.service and before kubelet.service.

Ensures the cloud-final.service is started before this service.

Runs your userdata script located at /usr/local/bin/your-userdata-script.sh.

Ensures that the service is considered active after the script has run, preventing kubelet.service from starting before it finishes.

Make sure your userdata script is available at /usr/local/bin/your-userdata-script.sh and is executable:

sudo mv your-userdata-script.sh /usr/local/bin/your-userdata-script.sh sudo chmod +x /usr/local/bin/your-userdata-script.sh 

Then create a drop-in file /etc/systemd/system/kubelet.service.d/custom-dependencies.conf with the following content:

[Unit] After=userdata-setup.service Requires=userdata-setup.service 

This makes sure that kubelet.service will wait for userdata-setup.service to complete before starting.

Test it:

sudo systemctl status userdata-setup.service sudo systemctl status kubelet.service 

Enable the service to start at boot:

sudo systemctl enable userdata-setup.service sudo systemctl start userdata-setup.service 

By doing this, you create a clear dependency chain that ensures your userdata script runs and completes before kubelet.service starts, without modifying the unit files of the cloud-final.service or kubelet.service.

This approach keeps the system's managed services intact while allowing you to introduce the necessary dependency for your setup tasks.