static.md

Static Pods

In kubernetes, regular pods are managed by the API Server, scheduled by the Scheduler and controlled by Controller Manager. At the opposite, static pods are managed directly by the kubelet running on a specific node, without the API server intervention. It does not have an associated controller and the kubelet itself controls it and restarts it when the pod crashes. Static pods are always bound to one kubelet daemon and always run on the same node with it.

For example, given a worker node, let's to create a nodejs application defined by nodejs-pod.yaml configuration file as a static pod running on that node

apiVersion: v1
kind: Pod
metadata:
  name: nodejs
  namespace: default
  labels:
    app:nodejs
spec:
  containers:
  - name: nodejs
    image: kalise/nodejs-web-app:latest
    ports:
    - containerPort: 8080

Copy the pod configuration file on the worker node

mkdir -p /etc/kubernetes/manifest
cp nodejs-pod.yaml /etc/kubernetes/manifest

Configure the kubelet on the worker node to use the above directory as place for static pods by editing the systemd configuration file /etc/systemd/system/kubelet.service for kubelet

...
[Service]
ExecStart=/usr/bin/kubelet \
...
  --pod-manifest-path=/etc/kubernetes/manifest/
  --v=2

Restart=on-failure
RestartSec=5
...

Restart the kubelet daemon

systemctl daemon-reload
systemctl restart kubelet

By default, kubelet automatically creates a mirror pod on the API server for each static pod, so the pods are visible from API server but cannot be controlled

kubectl get pods -o wide -n default
NAME             READY     STATUS    RESTARTS   AGE       IP            NODE
nodejs-kubew03   1/1       Running   0          30s       10.38.3.173   kubew03

Trying to delete that pod, the kubelet on the worker node automatically will recreate it

kubectl delete pod nodejs-kubew03 -n default

kubectl get pods -o wide -n default
NAME             READY     STATUS    RESTARTS   AGE       IP            NODE
nodejs-kubew03   1/1       Running   0          8s        10.38.3.173   kubew03

In some cases, it is useful to bound static pods to the worker node network address, as in the nodejs-pod-hostnet.yaml configuration file

apiVersion: v1
kind: Pod
metadata:
  name: nodejs-hostnet
  namespace: default
  labels:
    app: nodejs
spec:
  containers:
  - name: nodejs
    image: kalise/nodejs-web-app:latest
    ports:
    - containerPort: 8080
  hostNetwork: true

Move that file in the worker node's manifest directory and restart the kubelet

cp nodejs-pod-hostnet.yaml /etc/kubernetes/manifest
systemctl restart kubelet

and check the mirror pods from the API server

kubectl get pods -o wide -n default
NAME                     READY     STATUS    RESTARTS   AGE       IP            NODE
nodejs-hostnet-kubew03   1/1       Running   0          15s       10.10.10.83   kubew03
nodejs-kubew03           1/1       Running   0          12m       10.38.3.173   kubew03