Editing K3s - Kubernetes Simplified (section)

== Automated Upgrades:  System Upgrade Controller ==

Rancher has taken the simplicity of k3s to another level by providing a kubernetes controller that can update the cluster ``from within the cluster`` using the [https://rancher.com/docs/k3s/latest/en/upgrades/automated/ System Upgrade Controller].

Installing the controller is straightforward using a single manifest (also located in the git repo): 
 kubectl apply -f https://github.com/rancher/system-upgrade-controller/releases/download/v0.7.6/system-upgrade-controller.yaml

The controller looks for upgrade plans as custom resources (of type 'Plan', for some odd reason) to define its behavior.  Two 'Plan's are required ... one for server/master nodes and one for agent/worker nodes.  These plans can be set up to either upgrade to a specific k3s version or to monitor the release channel and automatically upgrade when a new release is available.  The manifest looks like this:

<pre># Server plan
apiVersion: upgrade.cattle.io/v1
kind: Plan
metadata:
  name: server-plan
  namespace: system-upgrade
spec:
  concurrency: 1
  cordon: true
  nodeSelector:
    matchExpressions:
    - key: node-role.kubernetes.io/master
      operator: In
      values:
      - "true"
  serviceAccountName: system-upgrade
  upgrade:
    image: rancher/k3s-upgrade
  #version: v1.20.4+k3s1
  channel: https://update.k3s.io/v1-release/channels/stable
---
# Agent plan
apiVersion: upgrade.cattle.io/v1
kind: Plan
metadata:
  name: agent-plan
  namespace: system-upgrade
spec:
  concurrency: 1
  cordon: true
  nodeSelector:
    matchExpressions:
    - key: node-role.kubernetes.io/master
      operator: DoesNotExist
  prepare:
    args:
    - prepare
    - server-plan
    image: rancher/k3s-upgrade #:v1.17.4-k3s1
  serviceAccountName: system-upgrade
  upgrade:
    image: rancher/k3s-upgrade
  #version: v1.20.4+k3s1
  channel: https://update.k3s.io/v1-release/channels/stable</pre>

Simply apply this manifest and the controller will identify nodes to upgrade and will take care of them as instructed.  It's as simple as that.  If you don't want to automatically upgrade, comment out the 'channel' parameter in each Plan, and uncomment the 'version' parameter and set it to the version to upgrade to and apply.  The controller will see the new plan and act accordingly.  If there is nothing to do, the controller just sits there waiting for a new plan with some work.