Create static Pods

Static Pods are managed directly by the kubelet daemon on a specific node, without the API server observing them. Unlike Pods that are managed by the control plane (for example, a Deployment); instead, the kubelet watches each static Pod (and restarts it if it fails).

Static Pods are always bound to one Kubelet on a specific node.

The kubelet automatically tries to create a mirror Pod on the Kubernetes API server for each static Pod. This means that the Pods running on a node are visible on the API server, but cannot be controlled from there. The Pod names will be suffixed with the node hostname with a leading hyphen.

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 using minikube or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

This page assumes you're using CRI-O to run Pods, and that your nodes are running the Fedora operating system. Instructions for other distributions or Kubernetes installations may vary.

Create a static pod

You can configure a static Pod with either a file system hosted configuration file or a web hosted configuration file.

Filesystem-hosted static Pod manifest

Manifests are standard Pod definitions in JSON or YAML format in a specific directory. Use the staticPodPath: <the directory> field in the kubelet configuration file, which periodically scans the directory and creates/deletes static Pods as YAML/JSON files appear/disappear there. Note that the kubelet will ignore files starting with dots when scanning the specified directory.

For example, this is how to start a simple web server as a static Pod:

  1. Choose a node where you want to run the static Pod. In this example, it's my-node1.

    ssh my-node1

  2. Choose a directory, say /etc/kubernetes/manifests and place a web server Pod definition there, for example /etc/kubernetes/manifests/static-web.yaml:

    # Run this command on the node where kubelet is running
mkdir -p /etc/kubernetes/manifests/
cat <<EOF >/etc/kubernetes/manifests/static-web.yaml
apiVersion: v1
kind: Pod
metadata:
  name: static-web
  labels:
    role: myrole
spec:
  containers:
    - name: web
      image: nginx
      ports:
        - name: web
          containerPort: 80
          protocol: TCP
EOF

  3. Configure the kubelet on that node to set a staticPodPath value in the kubelet configuration file.
    See Set Kubelet Parameters Via A Configuration File for more information.

    An alternative and deprecated method is to configure the kubelet on that node to look for static Pod manifests locally, using a command line argument. To use the deprecated approach, start the kubelet with the
    --pod-manifest-path=/etc/kubernetes/manifests/ argument.

  4. Restart the kubelet. On Fedora, you would run:

    # Run this command on the node where the kubelet is running
systemctl restart kubelet

Web-hosted static pod manifest

Kubelet periodically downloads a file specified by --manifest-url=<URL> argument and interprets it as a JSON/YAML file that contains Pod definitions. Similar to how filesystem-hosted manifests work, the kubelet refetches the manifest on a schedule. If there are changes to the list of static Pods, the kubelet applies them.

To use this approach:

  1. Create a YAML file and store it on a web server so that you can pass the URL of that file to the kubelet.

    apiVersion: v1
kind: Pod
metadata:
  name: static-web
  labels:
    role: myrole
spec:
  containers:
    - name: web
      image: nginx
      ports:
        - name: web
          containerPort: 80
          protocol: TCP

  2. Configure the kubelet on your selected node to use this web manifest by running it with --manifest-url=<manifest-url>. On Fedora, edit /etc/kubernetes/kubelet to include this line:

    KUBELET_ARGS="--cluster-dns=10.254.0.10 --cluster-domain=kube.local --manifest-url=<manifest-url>"

  3. Restart the kubelet. On Fedora, you would run:

    # Run this command on the node where the kubelet is running
systemctl restart kubelet

Observe static pod behavior

When the kubelet starts, it automatically starts all defined static Pods. As you have defined a static Pod and restarted the kubelet, the new static Pod should already be running.

You can view running containers (including static Pods) by running (on the node):

# Run this command on the node where the kubelet is running
crictl ps

The output might be something like:

CONTAINER       IMAGE                                 CREATED           STATE      NAME    ATTEMPT    POD ID
129fd7d382018   docker.io/library/nginx@sha256:...    11 minutes ago    Running    web     0          34533c6729106

You can see the mirror Pod on the API server:

kubectl get pods

NAME                  READY   STATUS    RESTARTS        AGE
static-web-my-node1   1/1     Running   0               2m

Labels from the static Pod are propagated into the mirror Pod. You can use those labels as normal via selectors, etc.

If you try to use kubectl to delete the mirror Pod from the API server, the kubelet doesn't remove the static Pod:

kubectl delete pod static-web-my-node1

pod "static-web-my-node1" deleted

You can see that the Pod is still running:

kubectl get pods

NAME                  READY   STATUS    RESTARTS   AGE
static-web-my-node1   1/1     Running   0          4s

Back on your node where the kubelet is running, you can try to stop the container manually. You'll see that, after a time, the kubelet will notice and will restart the Pod automatically:

# Run these commands on the node where the kubelet is running
crictl stop 129fd7d382018 # replace with the ID of your container
sleep 20
crictl ps

CONTAINER       IMAGE                                 CREATED           STATE      NAME    ATTEMPT    POD ID
89db4553e1eeb   docker.io/library/nginx@sha256:...    19 seconds ago    Running    web     1          34533c6729106

Once you identify the right container, you can get the logs for that container with crictl:

# Run these commands on the node where the container is running
crictl logs <container_id>

10.240.0.48 - - [16/Nov/2022:12:45:49 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.47.0" "-"
10.240.0.48 - - [16/Nov/2022:12:45:50 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.47.0" "-"
10.240.0.48 - - [16/Nove/2022:12:45:51 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.47.0" "-"

To find more about how to debug using crictl, please visit Debugging Kubernetes nodes with crictl.

Dynamic addition and removal of static pods

The running kubelet periodically scans the configured directory (/etc/kubernetes/manifests in our example) for changes and adds/removes Pods as files appear/disappear in this directory.

# This assumes you are using filesystem-hosted static Pod configuration
# Run these commands on the node where the container is running
#
mv /etc/kubernetes/manifests/static-web.yaml /tmp
sleep 20
crictl ps
# You see that no nginx container is running
mv /tmp/static-web.yaml  /etc/kubernetes/manifests/
sleep 20
crictl ps

CONTAINER       IMAGE                                 CREATED           STATE      NAME    ATTEMPT    POD ID
f427638871c35   docker.io/library/nginx@sha256:...    19 seconds ago    Running    web     1          34533c6729106

What's next

Last modified June 20, 2024 at 12:44 PM PST: Sync changest from andygol/k8s-website (36d05bc8a1)