Building a Basic DaemonSet

This page demonstrates how to build a basic DaemonSet that runs a Pod on every node in a Kubernetes cluster.It covers a simple use case of mounting a file from the host, logging its contents using an init container, and utilizing a pause container.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by usingminikube or you can use one of these Kubernetes playgrounds:

• iximiuz Labs
• Killercoda
• KodeKloud

A Kubernetes cluster with at least two nodes (one control plane node and one worker node)to demonstrate the behavior of DaemonSets.

Define the DaemonSet

In this task, a basic DaemonSet is created which ensures that the copy of a Pod is scheduled on every node.The Pod will use an init container to read and log the contents of /etc/machine-id from the host,while the main container will be a pause container, which keeps the Pod running.

application/basic-daemonset.yaml

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: example-daemonset
spec:
  selector:
    matchLabels:
      app.kubernetes.io/name: example
  template:
    metadata:
      labels:
        app.kubernetes.io/name: example
    spec:
      containers:
      - name: pause
        image: registry.k8s.io/pause
      initContainers:
      - name: log-machine-id
        image: busybox:1.37
        command: ['sh', '-c', 'cat /etc/machine-id > /var/log/machine-id.log']
        volumeMounts:
        - name: machine-id
          mountPath: /etc/machine-id
          readOnly: true
        - name: log-dir
          mountPath: /var/log
      volumes:
      - name: machine-id
        hostPath:
          path: /etc/machine-id
          type: File
      - name: log-dir
        hostPath:
          path: /var/log

1. Create a DaemonSet based on the (YAML) manifest:

   kubectl apply -f https://k8s.io/examples/application/basic-daemonset.yaml
   

2. Once applied, you can verify that the DaemonSet is running a Pod on every node in the cluster:

   kubectl get pods -o wide
   

   The output will list one Pod per node, similar to:

   NAME                                READY   STATUS    RESTARTS   AGE    IP       NODE
   example-daemonset-xxxxx             1/1     Running   0          5m     x.x.x.x  node-1
   example-daemonset-yyyyy             1/1     Running   0          5m     x.x.x.x  node-2
   

3. You can inspect the contents of the logged /etc/machine-id file by checking the log directory mounted from the host:

   kubectl exec <pod-name> -- cat /var/log/machine-id.log
   

   Where <pod-name> is the name of one of your Pods.

Cleaning up

To delete the DaemonSet, run this command:

kubectl delete --cascade=foreground --ignore-not-found --now daemonsets/example-daemonset

This simple DaemonSet example introduces key components like init containers and host path volumes,which can be expanded upon for more advanced use cases. For more details refer toDaemonSet.

What's next

• See Performing a rolling update on a DaemonSet
• See Creating a DaemonSet to adopt existing DaemonSet pods