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Develop a new MicroK8s addon

This document describes the process of developing a new addon for MicroK8s. As an example, we will create a simple addon demo-nginx, which creates a simple nginx deployment on our cluster. We will also show how to extend the microk8s CLI with new commands that work in tandem with the enabled addons.

Overview

Develop addon

1. Add entry in addons.yaml

Edit addons.yaml in the root of this repository and add an entry for your new addon. See the expected format and the list of supported fields in README.md.

For our nginx-test addon, this would look like this:

microk8s-addons:
  addons:
    # ... list of other addons...
    - name: "demo-nginx"
      description: "Demo addon that simply creates a deployment"
      version: "0.0.1"
      check_status: "deployment.apps/demo-nginx"
      supported_architectures:
        - amd64
        - arm64
        - s390x

2. Write the enable script

The enable script is called when running microk8s enable demo-nginx.

Create an empty directory addons/demo-nginx, then create addons/demo-nginx/enable. The enable script can be written in either Python or Bash, and even supports command-line arguments. It is highly recommended to avoid Bash if any non-trivial amount of work is required for your addon.

For our simple addon, we only need to create a deployment with nginx. We will support an optional command-line parameter --replicas, which will allow users to configure the number of replicas when enabling the addon.

In the example below, we use Click for simplicity.

#!/usr/bin/env python3
# addons/demo-nginx/enable

import os
import subprocess

import click

KUBECTL = os.path.expandvars("$SNAP/microk8s-kubectl.wrapper")

@click.command()
@click.option("--replicas", required=False, default=3, type=int)
def main(replicas):
    click.echo("Enabling demo-nginx")
    subprocess.check_call([
        KUBECTL, "create", "deploy", "demo-nginx", "--image", "nginx", "--replicas", str(replicas),
    ])
    click.echo("Enabled demo-nginx")

if __name__ == "__main__":
    main()

Make sure that the script is executable:

chmod +x ./addons/demo-nginx/enable

3. Write disable script

The disable script is called when running microk8s disable demo-nginx.

#!/usr/bin/env python3
# addons/demo-nginx/disable

import click
import os
import subprocess

KUBECTL = os.path.expandvars("$SNAP/microk8s-kubectl.wrapper")

@click.command()
def main():
    click.echo("Disabling demo-nginx")
    subprocess.check_call([
        KUBECTL, "delete", "deploy", "demo-nginx"
    ])
    click.echo("Disabled demo-nginx")

if __name__ == "__main__":
    main()

Like previously, make sure the script is executable:

chmod +x ./addons/demo-nginx/disable

4. Write unit tests

Add unit tests for the demo-nginx addon:

  • Create the tests/test_demo_nginx.py file
  • tests/test_demo_nginx.py: Create a class named TestDemoNginx
  • tests/test_demo_nginx.py: Write a validate_demo_nginx function under TestDemoNginx
  • tests/test_demo_nginx.py: Add a test_demo_nginx test case that enables, validates and then disables the addon under TestDemoNginx

The unit tests can be run against MicroK8s to verify that your addon is functional.

Use addon

Install MicroK8s, copy the addons/demo-nginx folder under the /var/snap/microk8s/common/addons/core/addons, and make sure to update /var/snap/microk8s/common/addons/core/addons.yaml as well.

Then, enable the addon with:

# simple ...
microk8s enable demo-nginx
# ... or, with command-line arguments
microk8s enable demo-nginx --replicas 5

You can check the status of the addon with the microk8s status command:

microk8s status --addon demo-nginx

And disable the addon:

microk8s disable demo-nginx

Custom commands

Addons may need to enhance the MicroK8s CLI with custom commands for management actions and day 2 operations. To this end the microk8s command can be extended via executable scripts or binaries called plugins. Plugins are found under $SNAP_COMMON/plugins, which is typically /var/snap/microk8s/common/plugins/. These plugins are executed within the MicroK8s snap environment, effectively running confined from the rest of the system.

Let's assume the demo-nginx addon needs to be paired with a microk8s nginxctl command to print a friendly message. In what follows we implement an nginxctl plugin to extend the microk8s command.

Create an example nginxctl plugin

The nginxctl plugin simply prints some information about the environment and lists the pods running in the cluster.

  1. Create /var/snap/microk8s/common/plugins/nginxctl as a simple bash script and mark it as executable:

    echo '#!/bin/bash

echo "Hello ${SNAP_NAME} plugins!"
echo "SNAP_DATA is at ${SNAP_DATA}"
echo "SNAP is at ${SNAP}"

$SNAP/microk8s-kubectl.wrapper get pods -A
' | sudo tee /var/snap/microk8s/common/plugins/nginxctl
sudo chmod +x /var/snap/microk8s/common/plugins/nginxctl

  2. Ensure MicroK8s knows about the plugin. Running microk8s should print the available commands, including a section looking like this:

    Available subcommands from addons are:
    nginxctl

  3. Execute the plugin by calling the microk8s nginxctl command:

    Hello microk8s plugins!
SNAP_DATA is at /var/snap/microk8s/x2
Snap is at /snap/microk8s/x2
NAMESPACE     NAME                                      READY   STATUS    RESTARTS   AGE
kube-system   calico-node-rbbpz                         1/1     Running   0          16h
kube-system   calico-kube-controllers-9969d55bb-7fdn9   1/1     Running   0          16h