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Static Pods

If you are running clustered Kubernetes and are using static pods to run a pod on every node, you should probably be using a DaemonSet!

Static pods are managed directly by kubelet daemon on a specific node, without the API server observing it. It does not have an associated replication controller, and kubelet daemon itself watches it and restarts it when it crashes. There is no health check. Static pods are always bound to one kubelet daemon and always run on the same node with it.

Kubelet automatically creates so-called mirror pod on the Kubernetes API server for each static pod, so the pods are visible there, but they cannot be controlled from the API server.

Static pod creation

Static pod can be created in two ways: either by using configuration file(s) or by HTTP.

Configuration files

The configuration files are just standard pod definitions in json or yaml format in a specific directory. Use kubelet --pod-manifest-path=<the directory> to start kubelet daemon, which periodically scans the directory and creates/deletes static pods as yaml/json files appear/disappear there. Note that 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 we want to run the static pod. In this example, it’s my-node1.

   [joe@host ~] $ ssh my-node1
   

2. Choose a directory, say /etc/kubelet.d and place a web server pod definition there, e.g. /etc/kubelet.d/static-web.yaml:

[root@my-node1 ~] $ mkdir /etc/kubelet.d/
[root@my-node1 ~] $ cat <<EOF >/etc/kubelet.d/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

1. Configure your kubelet daemon on the node to use this directory by running it with --pod-manifest-path=/etc/kubelet.d/ argument. On Fedora edit /etc/kubernetes/kubelet to include this line:

   KUBELET_ARGS="--cluster-dns=10.254.0.10 --cluster-domain=kube.local --pod-manifest-path=/etc/kubelet.d/"
   

   Instructions for other distributions or Kubernetes installations may vary.

2. Restart kubelet. On Fedora, this is:

   [root@my-node1 ~] $ systemctl restart kubelet
   

Pods created via HTTP

Kubelet periodically downloads a file specified by --manifest-url=<URL> argument and interprets it as a json/yaml file with a pod definition. It works the same as --pod-manifest-path=<directory>, i.e. it’s reloaded every now and then and changes are applied to running static pods (see below).

Behavior of static pods

When kubelet starts, it automatically starts all pods defined in directory specified in --pod-manifest-path= or --manifest-url= arguments, i.e. our static-web. (It may take some time to pull nginx image, be patient…):

[joe@my-node1 ~] $ docker ps
CONTAINER ID IMAGE         COMMAND  CREATED        STATUS         PORTS     NAMES
f6d05272b57e nginx:latest  "nginx"  8 minutes ago  Up 8 minutes             k8s_web.6f802af4_static-web-fk-node1_default_67e24ed9466ba55986d120c867395f3c_378e5f3c

If we look at our Kubernetes API server (running on host my-master), we see that a new mirror-pod was created there too:

[joe@host ~] $ ssh my-master
[joe@my-master ~] $ 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 and can be used as usual for filtering.

Notice we cannot delete the pod with the API server (e.g. via kubectl command), kubelet simply won’t remove it.

[joe@my-master ~] $ kubectl delete pod static-web-my-node1
pod "static-web-my-node1" deleted
[joe@my-master ~] $ kubectl get pods
NAME                       READY     STATUS    RESTARTS   AGE
static-web-my-node1        1/1       Running   0          12s

Back to our my-node1 host, we can try to stop the container manually and see, that kubelet automatically restarts it in a while:

[joe@host ~] $ ssh my-node1
[joe@my-node1 ~] $ docker stop f6d05272b57e
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
CONTAINER ID        IMAGE         COMMAND                CREATED       ...
5b920cbaf8b1        nginx:latest  "nginx -g 'daemon of   2 seconds ago ...

Dynamic addition and removal of static pods

Running kubelet periodically scans the configured directory (/etc/kubelet.d in our example) for changes and adds/removes pods as files appear/disappear in this directory.

[joe@my-node1 ~] $ mv /etc/kubelet.d/static-web.yaml /tmp
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
// no nginx container is running
[joe@my-node1 ~] $ mv /tmp/static-web.yaml  /etc/kubelet.d/
[joe@my-node1 ~] $ sleep 20
[joe@my-node1 ~] $ docker ps
CONTAINER ID        IMAGE         COMMAND                CREATED           ...
e7a62e3427f1        nginx:latest  "nginx -g 'daemon of   27 seconds ago